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THE LOGIC OF TRIKES an outsider's viewpoint by Andre Jute
THE LOGIC OF TRIKES
an outsider's viewpoint by Andre Jute When an uncommitted cyclist comes to look at recumbents, he sees bicycles in which everything has been sacrificed for aerodynamic advantage. Recumbents were originally presented as solving the problem of the bike saddle, the last pressure point (pun intended) of diamond frame design, by getting the rider's weight off his butt and onto his back, and by lowering the seat on the bike to make it easier for older people and the less nimble to mount and dismount. So much for good intentions. TWO WHEELERS Beyond aero advantage -- and its resultant of speed -- two-wheel recumbents have no advantage over a well-designed bicycle of traditional geometry, even over the standard diamond frame. They do however have a great many disadvantages. The greatest of these is near-invisibility to drivers, symbolized by a flag on a flexible antenna carried on many recumbents. And designers have now taken the designs to such extremes that, far from solving the problems of mounting and dismounting, recumbents require distortions to mount and dismount, and a flexible spine even to see the road from the extreme recumbent positions. The mainstream of recumbents are in such a mess both as design and marketing exercises that a whole new concept of so-called Compact Long Wheelbase Semi-Recumbent bicycles was created to solve the same old saddle and mounting problems all over again, of which the Giant Revive is already off the market again, the USED Scooterbike isn't doing too well, and the Utopia Phoenix sells well enough into its upmarket niche recently to have been further developed. RECUMBENT TRIKES At first recumbent trikes seem to have an advantage: the stability of three wheels. We can dismiss the socalled delta tricycles with the single wheel at the front: they are inherently unstable when compared like for like (wheel size and seat height) with the socalled tadpole tricyles which carry the single wheel at the rear. We can also dismiss the novelty of rear wheel steering as unsolved and very likely inherently unstable. Unfortunately, even on three wheels, designers have again gone to the extremes for a sporting advantage and thereby frittered away the advantage of the topology for everyone heyond the speedfreak minority. In particular, they have gone for small wheels and consequently were forced to install seats a very few inches off the ground, suitable only for the young and flexible to drop into, from which no one can rise gracefully. They have also, to gain the least frontal area, made trikes pretty narrow (which again forces a very low seat to avoid the thing overturning on the least taxing corner taken at low speed), so that the nominal advantage of static stability doesn't carry over very far into the dynamics of the trike in motion. Because of the way trikes sacrifice everything for aerodynamics, I have severe doubts whether any trike on the market will be faster around a downhill corner than a recumbent bicycle with the same frontal area. The road holding and handling of a two-wheeler will in all respects be superior on a dry road and on clean wet roads too, leaving the exceptional case of slippery roads (cow dung, mud) as the natural milieu of the recumbent trike -- and who wants his face inches above that ****? OTHER COMMON PROBLEMS Common recumbent two-wheelers and trikes place the rider's feet vulnerably out in front of the wheels; you can forget riding these things in mixed automobile and pedestrian traffic. You can't use them for a dash across a piece of pavement as you can an upright bike. Given that the rider's height on them is conducive to a feeling of insecurity in traffic, that limits their use severely. I define a utility bicycle as one the owner rides from his door on every occasion; a bicycle that must be carried on an automobile to where one can ride it is a novelty item, not a useful bike. Because standard types of recumbents carry the feet way out in front of the front wheels, the chainlines are grossly inefficient and ugly and expensive to maintain. That alone is enough to condemn recumbents for any thoughtful engineer. CAN A COMFORTABLE AND STABLE TRIKE BE BUILT? It may be possible. Posit a 29er trike with 700 wheels and balloon tyres to reduce the spinging excursion at the rear wheel, and thereby reduce damping and other control requirements, some of the less obvious ones to be discussed below. Of course it is a tadpole, with two wheels forward which steer and one at the rear which is driven. The bottom bracket is on the swing arm carrying the rear wheel and rear suspension, and the arm is pivoted in front of the bottom bracket or concentrically with it so that the chain line is straight and without idlers when an internal gear hub is used. Practically then the crank must be behind the front axle which is in any event desirable from a safety and psychological viewpoint. The seat can be put at the same height as an office chair so that anyone can sit down gracefully on it and rise equally gracefully from it. At this point the recumbent faithful will start screeching hysterically that the thing will fall over. But that is because they haven't put their minds in gear and are simply assuming that such a trike will be built to the same dimensions and perverted principles of current offerings. WHY DO TRIKES TURN OVER? Vehicles need a point to flip themselves over sideways: something must dig in to cause the flip. Bicycles and motorbikes cannot reach the sticking point that leads to the flip because their weight and payload lies directly in line with the wheels under virtually all conditions of tilt; when traction is lost, the wheels slide away. The roll centre is defined on the centre line of a multiwheel vehicle by the suspensions linkages. The centre of gravity is defined by the distribution of masses in the vehicle; think of it as the pivot of the scales (a three-dimensional definition is below). On a fourwheel vehicle the sticking-point is the line between the centres of the contact patches of the tires of the front and rear wheels on the inside of the corner. When the weight of the vehicle reacts with the centripetal force through the roll centre to shift the centre of gravity of the vehicle and payload outside this line, it flips: the scales are no longer balanced. It's a little more complicated than that but for bicycle tires with tiny contact patches and in an application where zero-scrub radius is in any event de riguer, we can take the tire centreline as the datum point. In real life the centre of gravity of the vehicle (including its occupant) is defined in three dimensions. On a tricycle for one occupant it very likely falls on the longitudinal centreline, so we need only to know where the CoG resides at standstill on the wheelbase and what its static height is. We will of course design suspension linkages that control dive under braking and squat under acceleration, so that all that concerns us now is the sideways movement of the CoG in a turn. The flipover line on a tricyle runs between the centres of the contact patches of the front inside tyre in the corner and the single rear tyre. Thus, to make the thing corner well, it is necessary to move the CoG as near to over the front axle as is possible in order to get it as far as possible from the flipover barrier. This is impossible to do if the rider's feet are to be inside the wheelbase or at least not too far in front of the front axle line. But there are many ways to skin a cat if the designer doesn't allow current practice to handcuff him to fashionable stupidities. FIRST SOLUTION TO A COMFORTABLE UTILITY TRIKE We're still assuming a trike with the seat at office height and common 29er balloon wheels. The seat height determines the hub height and therefore the size of the wheel/tyre combination. We need the large wheels because, though in theory we can attach control arms anywhere along the height of the wheel, in practice (and most especially on a bicycle where ounces count!), the control arms, which determine the start position of the centre of gravity and its sideways motion, are best disposed around the vertical centre of the wheel. 36 inch monocycle wheels, for which Croker can supply tyres, would be even better because they would put the seat below the hubs, but such wheels/tyres are less common so let's stick with the common 29er which puts the bottom of the seat at hub height. Now, the wider the front track, and the longer the wheelbase (on a tricycle), the further the centre of gravity has to travel to cross the flipover line between the front and rear contact patches. Having the bottom bracket inside the wheelbase together with a lowish seat already makes for a long wheelbase, so that is taken care of. For stability the track should be as wide as possible. The question is, will the resulting vehicle be viewed as an invalid carriage, in which case it should be narrow enough to fit on pavements, or will it be a general utility vehicle with stability for fast corners on the open road? If stability on the open road is wanted, the track should be around five feet, which would make the bicycle as wide as a small car. Voila, we now have a tricycle with the seat at a comfortable height and with stability around corners. COMPARISON WITH EXISTING RECUMBENT TRIKE DESIGNS The contortions of the current crop of recumbent trike designers are seen to be injurious to their sales for no great advantage except that in some extreme cases their tricycles are narrow enough to go on pavements -- and in those cases my high but widebase design will kill them around fast corners. Being aerodynamically fast is no use if the aero advantage is brought about by being so narrow that the trike flips in corners taken at more than moderate speed. In short, current ultra-recumbent trikes are claimed to be extreme but in fact are compromised on every aspect of performance. COMPARING THE COMFORT-TRIKE WITH A TWO-WHEELER At this point my comfortable trike suffers the same *competitive* problem vis a vis two-wheelers around really fast corners as the standard ultra-low trike available off the shelf right now from every run-of-the-mill bent maker: a two-wheel bike is much faster around corners under all but the least likely circumstances. A skilled and brave rider on a two-wheeler on a dry road will be hanging in there long after any tricycle, no matter how low and how wide, has flipped over: this conclusion is inherent in the angling of the flipover line between the front and rear wheels. You can see this conclusion easily when you consider that a four-wheel human powered vehicle of the same track as the tricycle will give the two-wheeler a much closer run for its money -- and will still lose unless the track is made grotesquely wide and the test is conducted on a very wide, uncambered surface (an airport, a closed multilane road? -- see, we're talking about extremes). SECOND SOLUTION: MAKING THE COMFORTABLE TRICYCLE FAST There is a way to make a trike or a quadricycle hang on to the road after the two-wheeler has lost traction and balance and slid away in the ditch. It is, historically, an accident of incompetent suspension design, in which equal length, parallel wishbones (and other older suspensions), failed to stop the body of the car tilting, and failed to hold the wheel upright (the two prime desiderata of automobile suspension design). However, with narrow bicycle tyres on a human powered tricycle (or quadricycle) it is desirable for the wheels and the body to tilt, because in that way the vehicle can be made to emulate cycle leanover, and thus hang on to traction longer and, most of all, avoid flipping over longer, instead sliding. Such a vehicle is generally referred to as "leaning" or "tilting". The design requirements of a tilting tricycle or quadricycle are for the most part simple to anyone with automobile experience: whatever you learned is totally undesirable in a good racing car will make a wonderful tilting vehicle! A tilting vehicle must have its roll centre at ground level and suspension linkages that allow the body to tilt in roll and the wheel camber to change proportionally to the roll. That's easily taken care of parallel, equal length wishbones. Tilting to 35 degrees from horizontal seems reasonable. A tilting vehicle must have zero scrub radius and this is easily taken care of by a somewhat extreme kingpin inclination. Ackermann steering arm angles must be chosen with some care to avoid the desire for reasonably light steering interfering with the tilting. Suitable castor and trail to give a tilting vehicle steering the correct self-centring and weight can be discovered on a drawing or on a model or on the road by using adjustable links in the suspension. Adjustable links would be desireable anyway to regulate the degree of proportionality between roll and camber, that is, to adjust the suspension ever so slightly away from equal length. Progressive springing and damping at the front wheels, especially if the progression is adjustable (a row of mounting points will do fine), will help control the tilt for various amounts of steer angle. A common disc brake on any swinging arm can be used to lock the tilting at standstill. It all sounds like a great deal of work with parameters which fight each other, and it is, but the computer makes what was impossible well within living memory not only easy but comparatively fast. At this point we have a comfortable and practical tilting tricycle which, unless it is grossly badly executed, will give a two-wheeler real competion around a corner even in the dry, and which will shrug off bad roads. CAN A TILTING TRI/QUADRICYCLE BE PERFECT? In theory, yes, if it has electricity for computing power and for driving stepper motors; it might be possible to run electronic controls for active tilting suspension off a hub dynamo. (Shimano has long since commercialized active Di2 suspension forks and gear changing run off a hub dynamo.) In practice, even powered tilting devices do not yet respond perfectly to imperfect roads. Strictly human-powered tilting vehicles with more than two wheels are far from a solved problem. The difficulty is not tilting: on roads cambered perfectly appropriately in turns, tilting can be automatic, look ma, no hands; if such roads between turns were furthermore table- flat, the perfect tilting trike would need no steering whatsoever. Read that again: no steering. The problem is that no road is perfectly flat, nor ever perfectly cambered, and no trike is perfectly built, nor can the human passenger ever be perfectly symmetrical, sit perfectly still on the straights or dispose of his weight perfectly optimally in corners. Steering is essential and will always be. But steering fights tilting. In addition, one might wish the vehicle to tilt more or less than dictated by the real-life camber on real-life roads (as distinct from the ideal roads in the computer), for instance for something as simple as a bumpy road or for cutting an apex, so a manual tilt control (in addition to the stops built into the suspension to limit suspension arm travel to a tilt of 35 degrees) is desirable. Complications and weight start to mount, and even the simplest system has a learning curve. Riding a tilting tri/quadricycle will never be as intuitive as riding a bicycle (for a start, the rider needs to set it up for the curve like a bicycle by first momentarily turning the wrong way, which isn't what happens in a normal multi-wheel vehicle like a car). The tilting tri/quadricycle, which seemed simple in conception, has now been mechanically complicated and weighted up quite a bit, and we see that to make it work perfectly not only counterintuitive techniques but also a dangerous new control (the tilt control) will have to be learned. It is a dangerous control because overenthusiastic or clumsy or ignorant use can achieve what the tilting mechanism is intended to prevent, flip the vehicle over. Or the designer can throw up his hands and say that for safe operation and the least mechanical complication and light weight, he will sacrifice theoretical perfections by building the tilting tri/ quadricycle with tilt (directly or indirectly) proportional to the steering angle and thus controlled through the control already familiar to riders, the steering. IN SUMMARY Current recumbent bicycles have betrayed their original impulse of a butt-saver on which it was easy to sit down and get up. The same applies to current recumbent trikes, whose single advantage of static stability doesn't even apply dynamically to trikes with narrow tracks (virtually all) for notional aero efficiency -- for what good is speed if it flips the rider over on corners? Current recumbents are so extreme (small wheels, groundhugging seats) that they are totally impractical for everyday use. It is possible to build a tricycle which is more practical by starting with an office chair seat height and standard 29er wheels, and by giving it a much wider track than is now common to make it faster around corners than the current offerings. It will also seat the rider high enough to make him feel more secure in traffic. While this comfortable, practical tricycle by virtue of its wide track will be faster around corners than the current recumbent trike offerings, it will never be faster than two-wheelers. Another way of putting it is that even this good and secure recumbent will always have a lower cornering limit on good roads than a bicycle; it will only shine in fast work in really bad conditions. The good and practical trike can be made faster and more secure with simple mechanical tilting. There is a learning requirement because turn-in is different from other multi-wheeled vehicles the rider may be familiar with. But a simple tilting trike or quadricycle should be able to approach the cornering abilities of a bicycle on good roads and exceed it on slippery roads (which means cow dung or mud or oil, not just water on clean tarmac -- even balloon tyres have too small a contact patch to hydroplane easily). For more complication, weight and cost, variable tilting under the control of the pilote is possible but there will be a steep learning curve, and clumsy use could turn the controlled-tilting trike into a more dangerous vehicle than the non-tilting or simple-tilting one. CONCLUSION On the whole then, current recumbents are simply fashion, not much chop even for their stated purpose, perverted beyond any practical use by their originally intended consumers, and even a good tricycle has so few advantages that it is probably best limited to those with balance problems or truly awful roads or for special purposes like sand-sailing. If speed is required, simple tilting mechanisms on the tricycle could move the roll-over speed in any corner upwards appreciably. Recumbents (two and three wheelers) are an unnecessary niche, nothing but an extreme fashion accessory. People (the old and the handicapped) who can truly benefit even from a more practical tricycle as described above are likely to ride too slowly to discover the speed advantages of a wide-track fast tadpole and so should have high-seat tricycles with tracks narrow enough for versatility on pavements and in doorways. That makes even my practical, speedy trike design concept superfluous, an interesting mental exercise of the type: "Well, we have trikes but they don't deliver on their promises. Let's see if we can design one that does." We can. So what? Looks like I've wasted several days considering how recumbents can be improved... For the rest of us, it is not surprising that the diamond frame still dominates. For those who want or need to put their feet flat on the road without leaving the saddle or seat, the only small surprise is that the Giant Revive did not survive, but it is no surprise that its more traditional-appearing spiritual and geometric soul-sisters from Electra and Trek and RANS are doing well, even becoming trendy. Nor is it surprising that their makers eschew calling them what they are (semi-recumbents) because the name "recumbent" is so discredited, instead preferring "crank forward" or even the somewhat disturbing "flatfoot". IN THE END.... I conclude that the upright and the semi-recumbent bicycles and the narrow wheelbase invalid carriage and the child's tricycle are necessary human-power formats, and the rest (recumbents regardless of number of wheels, plus my fast wide-track tadpole) are the unnecessary jewelry of an excessive society. Copyright © 2009 Andre Jute Free to use on not-for-profit netsites as longs the entire article, including this notice, is reprinted intact. Any other use contact author |
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THE LOGIC OF TRIKES an outsider's viewpoint by Andre Jute
ANYONE KNOW JUTE'S COORDINATES ?
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THE LOGIC OF TRIKES an outsider's viewpoint by Andre Jute
datakoll wrote:
ANYONE KNOW JUTE'S COORDINATES ? Those combined with Kim Jong Il's email address might have some potential. -- Andrew Muzi www.yellowjersey.org/ Open every day since 1 April, 1971 |
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THE LOGIC OF TRIKES an outsider's viewpoint by Andre Jute
"Andre Jute" wrote in message ... THE LOGIC OF TRIKES an outsider's viewpoint by Andre Jute snippage of large volumes of trite verbiage Andre. Andre, Andre, I'm speaking as someone who has an insider's viewpoint and I must say that you've typed a term paper full of nonsense. You have an intense prejudice against recumbent bicycles and have gone to great length to show that, and have not made a single substantial argument that could stand up the careful scrutiny of a peer review. gotbent aka FRT rider |
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THE LOGIC OF TRIKES an outsider's viewpoint by Andre Jute
"Andre Jute" wrote in message ... THE LOGIC OF TRIKES an outsider's viewpoint by Andre Jute When an uncommitted cyclist comes to look at recumbents, he sees bicycles in which everything has been sacrificed for aerodynamic advantage. Recumbents were originally presented as solving the problem of the bike saddle, the last pressure point (pun intended) of diamond frame design, by getting the rider's weight off his butt and onto his back, and by lowering the seat on the bike to make it easier for older people and the less nimble to mount and dismount. So much for good intentions. [...] Blah, Blah, Blah ... You must be an idiot to think anyone in his right mind would read your exceedingly long, rambling, nonsensical post. Who do you think inhabits these newsgroups anyway? A message to a Usenet newsgroup is for making a single point only, and doing it with as few words as possible. Because you do not have a clue about anything, I can assure you that no one will read your post, let alone answer any of it. Regards, Ed Dolan the Great - Minnesota aka Saint Edward the Great - Order of the Perpetual Sorrows - Minnesota |
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THE LOGIC OF TRIKES an outsider's viewpoint by Andre Jute
On 3 June, 00:35, Andre Jute wrote:
THE LOGIC OF TRIKES They tend to be a bit bigger than you first expect, and then some. Wondered when the picnic basket would be mentioned along with 5 star fuel and a linear engine, and where would it all fit? |
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THE LOGIC OF TRIKES an outsider's viewpoint by Andre Jute
On Jun 3, 7:30*am, someone wrote:
On 3 June, 00:35, Andre Jute wrote: THE LOGIC OF TRIKES They tend to be a bit bigger than you first expect, and then some. Wondered when the picnic basket would be mentioned along with 5 star fuel and a linear engine, and where would it all fit? Never a truer word spoken in jest. I was lying in bed this afternoon -- I sleep during the day and work at night -- thinking that the tilting quadricycle I took as far as a dimensioned sketch in order to work out the article below would make a wonderful cyclecar. Think about it. With the suspension I sketched out at the front front and triangulated parallel arms locating the rear hubs both transversely longitudinally, the entire chassis could be two rails of one to one- and-a-half inch pipe disposed vertically one above the other, with a few triangulating crosses, and not very long either even as a tandem. Two riders in tandem sitting on top of this frame, some exiguous bodywork a la Bugatti T35, a motorbike engine centrally disposed under the rear seat, and the whole thing could weigh well under 400 pounds complete with bicycle fenders and a full fuel tank. Be a blast in my lanes -- and we already have just the picnic basket too. Actually, I've always thought it odd that my book "Designing and Building Special Cars" (Batsford, London) is so popular with Ultralight builders, as it was written on hand of the Bentleys I turned into sports cars, my Panther Royale chassis, and my 155mph desert cars -- the lightest thing in there is 120in wheelbase 4-seat convertible with an ali chassis constructed of rectangular sections large enough to double as body sides! I didn't become interested in lightweight construction (meaning for road use, outside racing, lest some idiot publishes an old photo of me working on a racing frame and calls me a liar) until I had already given up the car to become a cyclist. It would be ironic if via bicycles I were reborn as an ultralight motorist... Andre Jute Visit Andre's books at http://www.audio-talk.co.uk/fiultra/THE%20WRITER'S%20HOUSE.html Original post: THE LOGIC OF TRIKES an outsider's viewpoint by Andre Jute When an uncommitted cyclist comes to look at recumbents, he sees bicycles in which everything has been sacrificed for aerodynamic advantage. Recumbents were originally presented as solving the problem of the bike saddle, the last pressure point (pun intended) of diamond frame design, by getting the rider's weight off his butt and onto his back, and by lowering the seat on the bike to make it easier for older people and the less nimble to mount and dismount. So much for good intentions. TWO WHEELERS Beyond aero advantage -- and its resultant of speed -- two-wheel recumbents have no advantage over a well-designed bicycle of traditional geometry, even over the standard diamond frame. They do however have a great many disadvantages. The greatest of these is near-invisibility to drivers, symbolized by a flag on a flexible antenna carried on many recumbents. And designers have now taken the designs to such extremes that, far from solving the problems of mounting and dismounting, recumbents require distortions to mount and dismount, and a flexible spine even to see the road from the extreme recumbent positions. The mainstream of recumbents are in such a mess both as design and marketing exercises that a whole new concept of so-called Compact Long Wheelbase Semi-Recumbent bicycles was created to solve the same old saddle and mounting problems all over again, of which the Giant Revive is already off the market again, the USED Scooterbike isn't doing too well, and the Utopia Phoenix sells well enough into its upmarket niche recently to have been further developed. RECUMBENT TRIKES At first recumbent trikes seem to have an advantage: the stability of three wheels. We can dismiss the socalled delta tricycles with the single wheel at the front: they are inherently unstable when compared like for like (wheel size and seat height) with the socalled tadpole tricyles which carry the single wheel at the rear. We can also dismiss the novelty of rear wheel steering as unsolved and very likely inherently unstable. Unfortunately, even on three wheels, designers have again gone to the extremes for a sporting advantage and thereby frittered away the advantage of the topology for everyone heyond the speedfreak minority. In particular, they have gone for small wheels and consequently were forced to install seats a very few inches off the ground, suitable only for the young and flexible to drop into, from which no one can rise gracefully. They have also, to gain the least frontal area, made trikes pretty narrow (which again forces a very low seat to avoid the thing overturning on the least taxing corner taken at low speed), so that the nominal advantage of static stability doesn't carry over very far into the dynamics of the trike in motion. Because of the way trikes sacrifice everything for aerodynamics, I have severe doubts whether any trike on the market will be faster around a downhill corner than a recumbent bicycle with the same frontal area. The road holding and handling of a two-wheeler will in all respects be superior on a dry road and on clean wet roads too, leaving the exceptional case of slippery roads (cow dung, mud) as the natural milieu of the recumbent trike -- and who wants his face inches above that ****? OTHER COMMON PROBLEMS Common recumbent two-wheelers and trikes place the rider's feet vulnerably out in front of the wheels; you can forget riding these things in mixed automobile and pedestrian traffic. You can't use them for a dash across a piece of pavement as you can an upright bike. Given that the rider's height on them is conducive to a feeling of insecurity in traffic, that limits their use severely. I define a utility bicycle as one the owner rides from his door on every occasion; a bicycle that must be carried on an automobile to where one can ride it is a novelty item, not a useful bike. Because standard types of recumbents carry the feet way out in front of the front wheels, the chainlines are grossly inefficient and ugly and expensive to maintain. That alone is enough to condemn recumbents for any thoughtful engineer. CAN A COMFORTABLE AND STABLE TRIKE BE BUILT? It may be possible. Posit a 29er trike with 700 wheels and balloon tyres to reduce the spinging excursion at the rear wheel, and thereby reduce damping and other control requirements, some of the less obvious ones to be discussed below. Of course it is a tadpole, with two wheels forward which steer and one at the rear which is driven. The bottom bracket is on the swing arm carrying the rear wheel and rear suspension, and the arm is pivoted in front of the bottom bracket or concentrically with it so that the chain line is straight and without idlers when an internal gear hub is used. Practically then the crank must be behind the front axle which is in any event desirable from a safety and psychological viewpoint. The seat can be put at the same height as an office chair so that anyone can sit down gracefully on it and rise equally gracefully from it. At this point the recumbent faithful will start screeching hysterically that the thing will fall over. But that is because they haven't put their minds in gear and are simply assuming that such a trike will be built to the same dimensions and perverted principles of current offerings. WHY DO TRIKES TURN OVER? Vehicles need a point to flip themselves over sideways: something must dig in to cause the flip. Bicycles and motorbikes cannot reach the sticking point that leads to the flip because their weight and payload lies directly in line with the wheels under virtually all conditions of tilt; when traction is lost, the wheels slide away. The roll centre is defined on the centre line of a multiwheel vehicle by the suspensions linkages. The centre of gravity is defined by the distribution of masses in the vehicle; think of it as the pivot of the scales (a three-dimensional definition is below). On a fourwheel vehicle the sticking-point is the line between the centres of the contact patches of the tires of the front and rear wheels on the inside of the corner. When the weight of the vehicle reacts with the centripetal force through the roll centre to shift the centre of gravity of the vehicle and payload outside this line, it flips: the scales are no longer balanced. It's a little more complicated than that but for bicycle tires with tiny contact patches and in an application where zero-scrub radius is in any event de riguer, we can take the tire centreline as the datum point. In real life the centre of gravity of the vehicle (including its occupant) is defined in three dimensions. On a tricycle for one occupant it very likely falls on the longitudinal centreline, so we need only to know where the CoG resides at standstill on the wheelbase and what its static height is. We will of course design suspension linkages that control dive under braking and squat under acceleration, so that all that concerns us now is the sideways movement of the CoG in a turn. The flipover line on a tricyle runs between the centres of the contact patches of the front inside tyre in the corner and the single rear tyre. Thus, to make the thing corner well, it is necessary to move the CoG as near to over the front axle as is possible in order to get it as far as possible from the flipover barrier. This is impossible to do if the rider's feet are to be inside the wheelbase or at least not too far in front of the front axle line. But there are many ways to skin a cat if the designer doesn't allow current practice to handcuff him to fashionable stupidities. FIRST SOLUTION TO A COMFORTABLE UTILITY TRIKE We're still assuming a trike with the seat at office height and common 29er balloon wheels. The seat height determines the hub height and therefore the size of the wheel/tyre combination. We need the large wheels because, though in theory we can attach control arms anywhere along the height of the wheel, in practice (and most especially on a bicycle where ounces count!), the control arms, which determine the start position of the centre of gravity and its sideways motion, are best disposed around the vertical centre of the wheel. 36 inch monocycle wheels, for which Croker can supply tyres, would be even better because they would put the seat below the hubs, but such wheels/tyres are less common so let's stick with the common 29er which puts the bottom of the seat at hub height. Now, the wider the front track, and the longer the wheelbase (on a tricycle), the further the centre of gravity has to travel to cross the flipover line between the front and rear contact patches. Having the bottom bracket inside the wheelbase together with a lowish seat already makes for a long wheelbase, so that is taken care of. For stability the track should be as wide as possible. The question is, will the resulting vehicle be viewed as an invalid carriage, in which case it should be narrow enough to fit on pavements, or will it be a general utility vehicle with stability for fast corners on the open road? If stability on the open road is wanted, the track should be around five feet, which would make the bicycle as wide as a small car. Voila, we now have a tricycle with the seat at a comfortable height and with stability around corners. COMPARISON WITH EXISTING RECUMBENT TRIKE DESIGNS The contortions of the current crop of recumbent trike designers are seen to be injurious to their sales for no great advantage except that in some extreme cases their tricycles are narrow enough to go on pavements -- and in those cases my high but widebase design will kill them around fast corners. Being aerodynamically fast is no use if the aero advantage is brought about by being so narrow that the trike flips in corners taken at more than moderate speed. In short, current ultra-recumbent trikes are claimed to be extreme but in fact are compromised on every aspect of performance. COMPARING THE COMFORT-TRIKE WITH A TWO-WHEELER At this point my comfortable trike suffers the same *competitive* problem vis a vis two-wheelers around really fast corners as the standard ultra-low trike available off the shelf right now from every run-of-the-mill bent maker: a two-wheel bike is much faster around corners under all but the least likely circumstances. A skilled and brave rider on a two-wheeler on a dry road will be hanging in there long after any tricycle, no matter how low and how wide, has flipped over: this conclusion is inherent in the angling of the flipover line between the front and rear wheels. You can see this conclusion easily when you consider that a four-wheel human powered vehicle of the same track as the tricycle will give the two-wheeler a much closer run for its money -- and will still lose unless the track is made grotesquely wide and the test is conducted on a very wide, uncambered surface (an airport, a closed multilane road? -- see, we're talking about extremes). SECOND SOLUTION: MAKING THE COMFORTABLE TRICYCLE FAST There is a way to make a trike or a quadricycle hang on to the road after the two-wheeler has lost traction and balance and slid away in the ditch. It is, historically, an accident of incompetent suspension design, in which equal length, parallel wishbones (and other older suspensions), failed to stop the body of the car tilting, and failed to hold the wheel upright (the two prime desiderata of automobile suspension design). However, with narrow bicycle tyres on a human powered tricycle (or quadricycle) it is desirable for the wheels and the body to tilt, because in that way the vehicle can be made to emulate cycle leanover, and thus hang on to traction longer and, most of all, avoid flipping over longer, instead sliding. Such a vehicle is generally referred to as "leaning" or "tilting". The design requirements of a tilting tricycle or quadricycle are for the most part simple to anyone with automobile experience: whatever you learned is totally undesirable in a good racing car will make a wonderful tilting vehicle! A tilting vehicle must have its roll centre at ground level and suspension linkages that allow the body to tilt in roll and the wheel camber to change proportionally to the roll. That's easily taken care of parallel, equal length wishbones. Tilting to 35 degrees from horizontal seems reasonable. A tilting vehicle must have zero scrub radius and this is easily taken care of by a somewhat extreme kingpin inclination. Ackermann steering arm angles must be chosen with some care to avoid the desire for reasonably light steering interfering with the tilting. Suitable castor and trail to give a tilting vehicle steering the correct self-centring and weight can be discovered on a drawing or on a model or on the road by using adjustable links in the suspension. Adjustable links would be desireable anyway to regulate the degree of proportionality between roll and camber, that is, to adjust the suspension ever so slightly away from equal length. Progressive springing and damping at the front wheels, especially if the progression is adjustable (a row of mounting points will do fine), will help control the tilt for various amounts of steer angle. A common disc brake on any swinging arm can be used to lock the tilting at standstill. It all sounds like a great deal of work with parameters which fight each other, and it is, but the computer makes what was impossible well within living memory not only easy but comparatively fast. At this point we have a comfortable and practical tilting tricycle which, unless it is grossly badly executed, will give a two-wheeler real competion around a corner even in the dry, and which will shrug off bad roads. CAN A TILTING TRI/QUADRICYCLE BE PERFECT? In theory, yes, if it has electricity for computing power and for driving stepper motors; it might be possible to run electronic controls for active tilting suspension off a hub dynamo. (Shimano has long since commercialized active Di2 suspension forks and gear changing run off a hub dynamo.) In practice, even powered tilting devices do not yet respond perfectly to imperfect roads. Strictly human-powered tilting vehicles with more than two wheels are far from a solved problem. The difficulty is not tilting: on roads cambered perfectly appropriately in turns, tilting can be automatic, look ma, no hands; if such roads between turns were furthermore table- flat, the perfect tilting trike would need no steering whatsoever. Read that again: no steering. The problem is that no road is perfectly flat, nor ever perfectly cambered, and no trike is perfectly built, nor can the human passenger ever be perfectly symmetrical, sit perfectly still on the straights or dispose of his weight perfectly optimally in corners. Steering is essential and will always be. But steering fights tilting. In addition, one might wish the vehicle to tilt more or less than dictated by the real-life camber on real-life roads (as distinct from the ideal roads in the computer), for instance for something as simple as a bumpy road or for cutting an apex, so a manual tilt control (in addition to the stops built into the suspension to limit suspension arm travel to a tilt of 35 degrees) is desirable. Complications and weight start to mount, and even the simplest system has a learning curve. Riding a tilting tri/quadricycle will never be as intuitive as riding a bicycle (for a start, the rider needs to set it up for the curve like a bicycle by first momentarily turning the wrong way, which isn't what happens in a normal multi-wheel vehicle like a car). The tilting tri/quadricycle, which seemed simple in conception, has now been mechanically complicated and weighted up quite a bit, and we see that to make it work perfectly not only counterintuitive techniques but also a dangerous new control (the tilt control) will have to be learned. It is a dangerous control because overenthusiastic or clumsy or ignorant use can achieve what the tilting mechanism is intended to prevent, flip the vehicle over. Or the designer can throw up his hands and say that for safe operation and the least mechanical complication and light weight, he will sacrifice theoretical perfections by building the tilting tri/ quadricycle with tilt (directly or indirectly) proportional to the steering angle and thus controlled through the control already familiar to riders, the steering. IN SUMMARY Current recumbent bicycles have betrayed their original impulse of a butt-saver on which it was easy to sit down and get up. The same applies to current recumbent trikes, whose single advantage of static stability doesn't even apply dynamically to trikes with narrow tracks (virtually all) for notional aero efficiency -- for what good is speed if it flips the rider over on corners? Current recumbents are so extreme (small wheels, groundhugging seats) that they are totally impractical for everyday use. It is possible to build a tricycle which is more practical by starting with an office chair seat height and standard 29er wheels, and by giving it a much wider track than is now common to make it faster around corners than the current offerings. It will also seat the rider high enough to make him feel more secure in traffic. While this comfortable, practical tricycle by virtue of its wide track will be faster around corners than the current recumbent trike offerings, it will never be faster than two-wheelers. Another way of putting it is that even this good and secure recumbent will always have a lower cornering limit on good roads than a bicycle; it will only shine in fast work in really bad conditions. The good and practical trike can be made faster and more secure with simple mechanical tilting. There is a learning requirement because turn-in is different from other multi-wheeled vehicles the rider may be familiar with. But a simple tilting trike or quadricycle should be able to approach the cornering abilities of a bicycle on good roads and exceed it on slippery roads (which means cow dung or mud or oil, not just water on clean tarmac -- even balloon tyres have too small a contact patch to hydroplane easily). For more complication, weight and cost, variable tilting under the control of the pilote is possible but there will be a steep learning curve, and clumsy use could turn the controlled-tilting trike into a more dangerous vehicle than the non-tilting or simple-tilting one. CONCLUSION On the whole then, current recumbents are simply fashion, not much chop even for their stated purpose, perverted beyond any practical use by their originally intended consumers, and even a good tricycle has so few advantages that it is probably best limited to those with balance problems or truly awful roads or for special purposes like sand-sailing. If speed is required, simple tilting mechanisms on the tricycle could move the roll-over speed in any corner upwards appreciably. Recumbents (two and three wheelers) are an unnecessary niche, nothing but an extreme fashion accessory. People (the old and the handicapped) who can truly benefit even from a more practical tricycle as described above are likely to ride too slowly to discover the speed advantages of a wide-track fast tadpole and so should have high-seat tricycles with tracks narrow enough for versatility on pavements and in doorways. That makes even my practical, speedy trike design concept superfluous, an interesting mental exercise of the type: "Well, we have trikes but they don't deliver on their promises. Let's see if we can design one that does." We can. So what? Looks like I've wasted several days considering how recumbents can be improved... For the rest of us, it is not surprising that the diamond frame still dominates. For those who want or need to put their feet flat on the road without leaving the saddle or seat, the only small surprise is that the Giant Revive did not survive, but it is no surprise that its more traditional-appearing spiritual and geometric soul-sisters from Electra and Trek and RANS are doing well, even becoming trendy. Nor is it surprising that their makers eschew calling them what they are (semi-recumbents) because the name "recumbent" is so discredited, instead preferring "crank forward" or even the somewhat disturbing "flatfoot". IN THE END.... I conclude that the upright and the semi-recumbent bicycles and the narrow wheelbase invalid carriage and the child's tricycle are necessary human-power formats, and the rest (recumbents regardless of number of wheels, plus my fast wide-track tadpole) are the unnecessary jewelry of an excessive society. Copyright © 2009 Andre Jute Free to use on not-for-profit netsites as longs the entire article, including this notice, is reprinted intact. Any other use contact author |
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THE LOGIC OF TRIKES an outsider's viewpoint by Andre Jute
On Jun 2, 11:35 pm, Andre Jute wrote:
THE LOGIC OF TRIKES an outsider's viewpoint by Andre Jute snip We can also dismiss the novelty of rear wheel steering as unsolved and very likely inherently unstable. Actually the rear-steered tadpole design has been studied to death since the late 1920s and was in wide use for decades as the landing gear for airplanes. All it takes is the willingness to look outside a particular application to other fields that use a similar layout. In aircraft the parameters if CG height, distance of CG behind the front wheels, maximum lateral acceleration, and what happens when the limits of adhesion are exceeded at any of the three wheels have been in the hot-rodder's parlance "scienced out". By staying within certain ratios of CG height to track width, and front/rear balance a rear steer tadpole can be a stable machine with quirks that can be manage with operator experience, much like a single-track machine. snip Because standard types of recumbents carry the feet way out in front of the front wheels, the chainlines are grossly inefficient and ugly and expensive to maintain. That alone is enough to condemn recumbents for any thoughtful engineer. I don't see recumbent chains as having any more problems than chains for DF bikes aside from the length needed to get from cranks to drive wheels and the only additional expense I have seen has been the requirement for frequent lubrication and cleaning to avoid having to replace the chain, and the expense of buying multiple pre-packaged chains to service a single machine. CAN A COMFORTABLE AND STABLE TRIKE BE BUILT? It may be possible. Posit a 29er trike with 700 wheels and balloon tyres to reduce the spinging excursion at the rear wheel, and thereby reduce damping and other control requirements, some of the less obvious ones to be discussed below. Of course it is a tadpole, with two wheels forward which steer and one at the rear which is driven. The bottom bracket is on the swing arm carrying the rear wheel and rear suspension, and the arm is pivoted in front of the bottom bracket or concentrically with it so that the chain line is straight and without idlers when an internal gear hub is used. Practically then the crank must be behind the front axle which is in any event desirable from a safety and psychological viewpoint. The seat can be put at the same height as an office chair so that anyone can sit down gracefully on it and rise equally gracefully from it. At this point the recumbent faithful will start screeching hysterically that the thing will fall over. But that is because they haven't put their minds in gear and are simply assuming that such a trike will be built to the same dimensions and perverted principles of current offerings. WHY DO TRIKES TURN OVER? Vehicles need a point to flip themselves over sideways: something must dig in to cause the flip. Bicycles and motorbikes cannot reach the sticking point that leads to the flip because their weight and payload lies directly in line with the wheels under virtually all conditions of tilt; when traction is lost, the wheels slide away. The roll centre is defined on the centre line of a multiwheel vehicle by the suspensions linkages. The centre of gravity is defined by the distribution of masses in the vehicle; think of it as the pivot of the scales (a three-dimensional definition is below). On a fourwheel vehicle the sticking-point is the line between the centres of the contact patches of the tires of the front and rear wheels on the inside of the corner. When the weight of the vehicle reacts with the centripetal force through the roll centre to shift the centre of gravity of the vehicle and payload outside this line, it flips: the scales are no longer balanced. It's a little more complicated than that but for bicycle tires with tiny contact patches and in an application where zero-scrub radius is in any event de riguer, we can take the tire centreline as the datum point. OK that is just a bunch of hogwash. The tipover point for a multi- track vehicle is determined by the ratio of the CG height to the track width, or in the case of a delta or tadpole trike the effective track width at the CG location (that would be the CG distance from the paired wheels over the wheelbase times the track width, or in graphic terms the width of a pair of lines drawn between the contact points of the front and rear wheels at the CG). When the lateral acceleration causes the force on the CG times the CG height divided by half the track to exceed the static load on the inside wheel of a turn or the uphill wheel of a side hill vehicle then the wheel will lift and if traction is maintained the vehicle will turn over. That is it. Suspension linkages can only make this happen sooner rather than later by causing the sideways motion of the vehicle load away from the inside wheel. This is why Karts are some of the best handling vehicles in racing. No suspension no messing up the pure relationship between speed and tip over, which allows a driver to develop the feel of when a vehicle is going to either slide or tip over. Opus |
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THE LOGIC OF TRIKES an outsider's viewpoint by Andre Jute
On Jun 3, 6:13*pm, Opus wrote:
On Jun 2, 11:35 pm, Andre Jute wrote: THE LOGIC OF TRIKES an outsider's viewpoint by Andre Jute snip We can also dismiss the novelty of rear wheel steering as unsolved and very likely inherently unstable. Actually the rear-steered tadpole design has been studied to death since the late 1920s and was in wide use for decades as the landing gear for airplanes. All it takes is the willingness to look outside a particular application to other fields that use a similar layout. In aircraft the parameters if CG height, distance of CG behind the front wheels, maximum lateral acceleration, and what happens when the limits of adhesion are exceeded at any of the three wheels have been in the hot-rodder's parlance "scienced out". By staying within certain ratios of CG height to track width, and front/rear balance a rear steer tadpole can be a stable machine with quirks that can be manage with operator experience, much like a single-track machine.snip Because standard types of recumbents carry the feet way out in front of the front wheels, the chainlines are grossly inefficient and ugly and expensive to maintain. That alone is enough to condemn recumbents for any thoughtful engineer. I don't see recumbent chains as having any more problems than chains for DF bikes aside from the length needed to get from cranks to drive wheels and the only additional expense I have seen has been the requirement for frequent lubrication and cleaning to avoid having to replace the chain, and the expense of buying multiple pre-packaged chains to service a single machine. You don't consider those disadvantages? It's your list, pal, not mine, but I must tell you that my bikes have chains serviced twice a year that last almost forever. Anything less is an imposition on my time and patience by an incompetent designer. CAN A COMFORTABLE AND STABLE TRIKE BE BUILT? It may be possible. Posit a 29er trike with 700 wheels and balloon tyres to reduce the spinging excursion at the rear wheel, and thereby reduce damping and other control requirements, some of the less obvious ones to be discussed below. Of course it is a tadpole, with two wheels forward which steer and one at the rear which is driven. The bottom bracket is on the swing arm carrying the rear wheel and rear suspension, and the arm is pivoted in front of the bottom bracket or concentrically with it so that the chain line is straight and without idlers when an internal gear hub is used. Practically then the crank must be behind the front axle which is in any event desirable from a safety and psychological viewpoint. The seat can be put at the same height as an office chair so that anyone can sit down gracefully on it and rise equally gracefully from it. At this point the recumbent faithful will start screeching hysterically that the thing will fall over. But that is because they haven't put their minds in gear and are simply assuming that such a trike will be built to the same dimensions and perverted principles of current offerings. WHY DO TRIKES TURN OVER? Vehicles need a point to flip themselves over sideways: something must dig in to cause the flip. Bicycles and motorbikes cannot reach the sticking point that leads to the flip because their weight and payload lies directly in line with the wheels under virtually all conditions of tilt; when traction is lost, the wheels slide away. The roll centre is defined on the centre line of a multiwheel vehicle by the suspensions linkages. The centre of gravity is defined by the distribution of masses in the vehicle; think of it as the pivot of the scales (a three-dimensional definition is below). On a fourwheel vehicle the sticking-point is the line between the centres of the contact patches of the tires of the front and rear wheels on the inside of the corner. When the weight of the vehicle reacts with the centripetal force through the roll centre to shift the centre of gravity of the vehicle and payload outside this line, it flips: the scales are no longer balanced. *It's a little more complicated than that but for bicycle tires with tiny contact patches and in an application where zero-scrub radius is in any event de riguer, *we can take the tire centreline as the datum point. OK that is just a bunch of hogwash. Okay, let's see you do better. The tipover point for a multi- track vehicle is determined by the ratio of the CG height to the track width, or in the case of a delta or tadpole trike the effective track width at the CG location (that would be the CG distance from the paired wheels over the wheelbase times the track width, or in graphic terms the width of a pair of lines drawn between the contact points of the front and rear wheels at the CG). Yes, that's what I said, but you cut it away. Here it is again: "The flipover line on a tricyle runs between the centres of the contact patches of the front inside tyre in the corner and the single rear tyre. Thus, to make the thing corner well, it is necessary to move the CoG as near to over the front axle as is possible in order to get it as far as possible from the flipover barrier. This is impossible to do if the rider's feet are to be inside the wheelbase or at least not too far in front of the front axle line. But there are many ways to skin a cat if the designer doesn't allow current practice to handcuff him to fashionable stupidities." When the lateral acceleration causes the force on the CG times the CG height divided by half the track to exceed the static load on the inside wheel of a turn or the uphill wheel of a side hill vehicle then the wheel will lift and if traction is maintained the vehicle will turn over. That is it. Yes, that's what I said, except I applied it to tricycles, if one puts the kindest possible interpretation on the hogwash you actually wrote: "or the uphill wheel of a side hill vehicle". What, for heaven's sake, is a "side hill vehicle"? Suspension linkages can only make this happen sooner rather than later by causing the sideways motion of the vehicle load away from the inside wheel. This is why Karts are some of the best handling vehicles in racing. No suspension no messing up the pure relationship between speed and tip over, which allows a driver to develop the feel of when a vehicle is going to either slide or tip over. This is wrong, probably arising from your expectation that any suspension applied will result in a roll centre above ground. In the text you cut away I explain carefully that an older suspension, equal length parallel arms, put the roll centre at ground level, thus making it relatively harder for the vehicle to overturn. You have fallen into the same trap you warn me about above, where you talk of rear wheel steering on aircraft. Note that it is even possible to move the roll centre below ground. Here is my full post again; it is internally and externally coherent and when you snip it carelessly, you make mistakes like the ones above. **** THE LOGIC OF TRIKES an outsider's viewpoint by Andre Jute When an uncommitted cyclist comes to look at recumbents, he sees bicycles in which everything has been sacrificed for aerodynamic advantage. Recumbents were originally presented as solving the problem of the bike saddle, the last pressure point (pun intended) of diamond frame design, by getting the rider's weight off his butt and onto his back, and by lowering the seat on the bike to make it easier for older people and the less nimble to mount and dismount. So much for good intentions. TWO WHEELERS Beyond aero advantage -- and its resultant of speed -- two-wheel recumbents have no advantage over a well-designed bicycle of traditional geometry, even over the standard diamond frame. They do however have a great many disadvantages. The greatest of these is near-invisibility to drivers, symbolized by a flag on a flexible antenna carried on many recumbents. And designers have now taken the designs to such extremes that, far from solving the problems of mounting and dismounting, recumbents require distortions to mount and dismount, and a flexible spine even to see the road from the extreme recumbent positions. The mainstream of recumbents are in such a mess both as design and marketing exercises that a whole new concept of so-called Compact Long Wheelbase Semi-Recumbent bicycles was created to solve the same old saddle and mounting problems all over again, of which the Giant Revive is already off the market again, the USED Scooterbike isn't doing too well, and the Utopia Phoenix sells well enough into its upmarket niche recently to have been further developed. RECUMBENT TRIKES At first recumbent trikes seem to have an advantage: the stability of three wheels. We can dismiss the socalled delta tricycles with the single wheel at the front: they are inherently unstable when compared like for like (wheel size and seat height) with the socalled tadpole tricyles which carry the single wheel at the rear. We can also dismiss the novelty of rear wheel steering as unsolved and very likely inherently unstable. Unfortunately, even on three wheels, designers have again gone to the extremes for a sporting advantage and thereby frittered away the advantage of the topology for everyone heyond the speedfreak minority. In particular, they have gone for small wheels and consequently were forced to install seats a very few inches off the ground, suitable only for the young and flexible to drop into, from which no one can rise gracefully. They have also, to gain the least frontal area, made trikes pretty narrow (which again forces a very low seat to avoid the thing overturning on the least taxing corner taken at low speed), so that the nominal advantage of static stability doesn't carry over very far into the dynamics of the trike in motion. Because of the way trikes sacrifice everything for aerodynamics, I have severe doubts whether any trike on the market will be faster around a downhill corner than a recumbent bicycle with the same frontal area. The road holding and handling of a two-wheeler will in all respects be superior on a dry road and on clean wet roads too, leaving the exceptional case of slippery roads (cow dung, mud) as the natural milieu of the recumbent trike -- and who wants his face inches above that ****? OTHER COMMON PROBLEMS Common recumbent two-wheelers and trikes place the rider's feet vulnerably out in front of the wheels; you can forget riding these things in mixed automobile and pedestrian traffic. You can't use them for a dash across a piece of pavement as you can an upright bike. Given that the rider's height on them is conducive to a feeling of insecurity in traffic, that limits their use severely. I define a utility bicycle as one the owner rides from his door on every occasion; a bicycle that must be carried on an automobile to where one can ride it is a novelty item, not a useful bike. Because standard types of recumbents carry the feet way out in front of the front wheels, the chainlines are grossly inefficient and ugly and expensive to maintain. That alone is enough to condemn recumbents for any thoughtful engineer. CAN A COMFORTABLE AND STABLE TRIKE BE BUILT? It may be possible. Posit a 29er trike with 700 wheels and balloon tyres to reduce the spinging excursion at the rear wheel, and thereby reduce damping and other control requirements, some of the less obvious ones to be discussed below. Of course it is a tadpole, with two wheels forward which steer and one at the rear which is driven. The bottom bracket is on the swing arm carrying the rear wheel and rear suspension, and the arm is pivoted in front of the bottom bracket or concentrically with it so that the chain line is straight and without idlers when an internal gear hub is used. Practically then the crank must be behind the front axle which is in any event desirable from a safety and psychological viewpoint. The seat can be put at the same height as an office chair so that anyone can sit down gracefully on it and rise equally gracefully from it. At this point the recumbent faithful will start screeching hysterically that the thing will fall over. But that is because they haven't put their minds in gear and are simply assuming that such a trike will be built to the same dimensions and perverted principles of current offerings. WHY DO TRIKES TURN OVER? Vehicles need a point to flip themselves over sideways: something must dig in to cause the flip. Bicycles and motorbikes cannot reach the sticking point that leads to the flip because their weight and payload lies directly in line with the wheels under virtually all conditions of tilt; when traction is lost, the wheels slide away. The roll centre is defined on the centre line of a multiwheel vehicle by the suspensions linkages. The centre of gravity is defined by the distribution of masses in the vehicle; think of it as the pivot of the scales (a three-dimensional definition is below). On a fourwheel vehicle the sticking-point is the line between the centres of the contact patches of the tires of the front and rear wheels on the inside of the corner. When the weight of the vehicle reacts with the centripetal force through the roll centre to shift the centre of gravity of the vehicle and payload outside this line, it flips: the scales are no longer balanced. It's a little more complicated than that but for bicycle tires with tiny contact patches and in an application where zero-scrub radius is in any event de riguer, we can take the tire centreline as the datum point. In real life the centre of gravity of the vehicle (including its occupant) is defined in three dimensions. On a tricycle for one occupant it very likely falls on the longitudinal centreline, so we need only to know where the CoG resides at standstill on the wheelbase and what its static height is. We will of course design suspension linkages that control dive under braking and squat under acceleration, so that all that concerns us now is the sideways movement of the CoG in a turn. The flipover line on a tricyle runs between the centres of the contact patches of the front inside tyre in the corner and the single rear tyre. Thus, to make the thing corner well, it is necessary to move the CoG as near to over the front axle as is possible in order to get it as far as possible from the flipover barrier. This is impossible to do if the rider's feet are to be inside the wheelbase or at least not too far in front of the front axle line. But there are many ways to skin a cat if the designer doesn't allow current practice to handcuff him to fashionable stupidities. FIRST SOLUTION TO A COMFORTABLE UTILITY TRIKE We're still assuming a trike with the seat at office height and common 29er balloon wheels. The seat height determines the hub height and therefore the size of the wheel/tyre combination. We need the large wheels because, though in theory we can attach control arms anywhere along the height of the wheel, in practice (and most especially on a bicycle where ounces count!), the control arms, which determine the start position of the centre of gravity and its sideways motion, are best disposed around the vertical centre of the wheel. 36 inch monocycle wheels, for which Croker can supply tyres, would be even better because they would put the seat below the hubs, but such wheels/tyres are less common so let's stick with the common 29er which puts the bottom of the seat at hub height. Now, the wider the front track, and the longer the wheelbase (on a tricycle), the further the centre of gravity has to travel to cross the flipover line between the front and rear contact patches. Having the bottom bracket inside the wheelbase together with a lowish seat already makes for a long wheelbase, so that is taken care of. For stability the track should be as wide as possible. The question is, will the resulting vehicle be viewed as an invalid carriage, in which case it should be narrow enough to fit on pavements, or will it be a general utility vehicle with stability for fast corners on the open road? If stability on the open road is wanted, the track should be around five feet, which would make the bicycle as wide as a small car. Voila, we now have a tricycle with the seat at a comfortable height and with stability around corners. COMPARISON WITH EXISTING RECUMBENT TRIKE DESIGNS The contortions of the current crop of recumbent trike designers are seen to be injurious to their sales for no great advantage except that in some extreme cases their tricycles are narrow enough to go on pavements -- and in those cases my high but widebase design will kill them around fast corners. Being aerodynamically fast is no use if the aero advantage is brought about by being so narrow that the trike flips in corners taken at more than moderate speed. In short, current ultra-recumbent trikes are claimed to be extreme but in fact are compromised on every aspect of performance. COMPARING THE COMFORT-TRIKE WITH A TWO-WHEELER At this point my comfortable trike suffers the same *competitive* problem vis a vis two-wheelers around really fast corners as the standard ultra-low trike available off the shelf right now from every run-of-the-mill bent maker: a two-wheel bike is much faster around corners under all but the least likely circumstances. A skilled and brave rider on a two-wheeler on a dry road will be hanging in there long after any tricycle, no matter how low and how wide, has flipped over: this conclusion is inherent in the angling of the flipover line between the front and rear wheels. You can see this conclusion easily when you consider that a four-wheel human powered vehicle of the same track as the tricycle will give the two-wheeler a much closer run for its money -- and will still lose unless the track is made grotesquely wide and the test is conducted on a very wide, uncambered surface (an airport, a closed multilane road? -- see, we're talking about extremes). SECOND SOLUTION: MAKING THE COMFORTABLE TRICYCLE FAST There is a way to make a trike or a quadricycle hang on to the road after the two-wheeler has lost traction and balance and slid away in the ditch. It is, historically, an accident of incompetent suspension design, in which equal length, parallel wishbones (and other older suspensions), failed to stop the body of the car tilting, and failed to hold the wheel upright (the two prime desiderata of automobile suspension design). However, with narrow bicycle tyres on a human powered tricycle (or quadricycle) it is desirable for the wheels and the body to tilt, because in that way the vehicle can be made to emulate cycle leanover, and thus hang on to traction longer and, most of all, avoid flipping over longer, instead sliding. Such a vehicle is generally referred to as "leaning" or "tilting". The design requirements of a tilting tricycle or quadricycle are for the most part simple to anyone with automobile experience: whatever you learned is totally undesirable in a good racing car will make a wonderful tilting vehicle! A tilting vehicle must have its roll centre at ground level and suspension linkages that allow the body to tilt in roll and the wheel camber to change proportionally to the roll. That's easily taken care of parallel, equal length wishbones. Tilting to 35 degrees from horizontal seems reasonable. A tilting vehicle must have zero scrub radius and this is easily taken care of by a somewhat extreme kingpin inclination. Ackermann steering arm angles must be chosen with some care to avoid the desire for reasonably light steering interfering with the tilting. Suitable castor and trail to give a tilting vehicle steering the correct self-centring and weight can be discovered on a drawing or on a model or on the road by using adjustable links in the suspension. Adjustable links would be desireable anyway to regulate the degree of proportionality between roll and camber, that is, to adjust the suspension ever so slightly away from equal length. Progressive springing and damping at the front wheels, especially if the progression is adjustable (a row of mounting points will do fine), will help control the tilt for various amounts of steer angle. A common disc brake on any swinging arm can be used to lock the tilting at standstill. It all sounds like a great deal of work with parameters which fight each other, and it is, but the computer makes what was impossible well within living memory not only easy but comparatively fast. At this point we have a comfortable and practical tilting tricycle which, unless it is grossly badly executed, will give a two-wheeler real competion around a corner even in the dry, and which will shrug off bad roads. CAN A TILTING TRI/QUADRICYCLE BE PERFECT? In theory, yes, if it has electricity for computing power and for driving stepper motors; it might be possible to run electronic controls for active tilting suspension off a hub dynamo. (Shimano has long since commercialized active Di2 suspension forks and gear changing run off a hub dynamo.) In practice, even powered tilting devices do not yet respond perfectly to imperfect roads. Strictly human-powered tilting vehicles with more than two wheels are far from a solved problem. The difficulty is not tilting: on roads cambered perfectly appropriately in turns, tilting can be automatic, look ma, no hands; if such roads between turns were furthermore table- flat, the perfect tilting trike would need no steering whatsoever. Read that again: no steering. The problem is that no road is perfectly flat, nor ever perfectly cambered, and no trike is perfectly built, nor can the human passenger ever be perfectly symmetrical, sit perfectly still on the straights or dispose of his weight perfectly optimally in corners. Steering is essential and will always be. But steering fights tilting. In addition, one might wish the vehicle to tilt more or less than dictated by the real-life camber on real-life roads (as distinct from the ideal roads in the computer), for instance for something as simple as a bumpy road or for cutting an apex, so a manual tilt control (in addition to the stops built into the suspension to limit suspension arm travel to a tilt of 35 degrees) is desirable. Complications and weight start to mount, and even the simplest system has a learning curve. Riding a tilting tri/quadricycle will never be as intuitive as riding a bicycle (for a start, the rider needs to set it up for the curve like a bicycle by first momentarily turning the wrong way, which isn't what happens in a normal multi-wheel vehicle like a car). The tilting tri/quadricycle, which seemed simple in conception, has now been mechanically complicated and weighted up quite a bit, and we see that to make it work perfectly not only counterintuitive techniques but also a dangerous new control (the tilt control) will have to be learned. It is a dangerous control because overenthusiastic or clumsy or ignorant use can achieve what the tilting mechanism is intended to prevent, flip the vehicle over. Or the designer can throw up his hands and say that for safe operation and the least mechanical complication and light weight, he will sacrifice theoretical perfections by building the tilting tri/ quadricycle with tilt (directly or indirectly) proportional to the steering angle and thus controlled through the control already familiar to riders, the steering. IN SUMMARY Current recumbent bicycles have betrayed their original impulse of a butt-saver on which it was easy to sit down and get up. The same applies to current recumbent trikes, whose single advantage of static stability doesn't even apply dynamically to trikes with narrow tracks (virtually all) for notional aero efficiency -- for what good is speed if it flips the rider over on corners? Current recumbents are so extreme (small wheels, groundhugging seats) that they are totally impractical for everyday use. It is possible to build a tricycle which is more practical by starting with an office chair seat height and standard 29er wheels, and by giving it a much wider track than is now common to make it faster around corners than the current offerings. It will also seat the rider high enough to make him feel more secure in traffic. While this comfortable, practical tricycle by virtue of its wide track will be faster around corners than the current recumbent trike offerings, it will never be faster than two-wheelers. Another way of putting it is that even this good and secure recumbent will always have a lower cornering limit on good roads than a bicycle; it will only shine in fast work in really bad conditions. The good and practical trike can be made faster and more secure with simple mechanical tilting. There is a learning requirement because turn-in is different from other multi-wheeled vehicles the rider may be familiar with. But a simple tilting trike or quadricycle should be able to approach the cornering abilities of a bicycle on good roads and exceed it on slippery roads (which means cow dung or mud or oil, not just water on clean tarmac -- even balloon tyres have too small a contact patch to hydroplane easily). For more complication, weight and cost, variable tilting under the control of the pilote is possible but there will be a steep learning curve, and clumsy use could turn the controlled-tilting trike into a more dangerous vehicle than the non-tilting or simple-tilting one. CONCLUSION On the whole then, current recumbents are simply fashion, not much chop even for their stated purpose, perverted beyond any practical use by their originally intended consumers, and even a good tricycle has so few advantages that it is probably best limited to those with balance problems or truly awful roads or for special purposes like sand-sailing. If speed is required, simple tilting mechanisms on the tricycle could move the roll-over speed in any corner upwards appreciably. Recumbents (two and three wheelers) are an unnecessary niche, nothing but an extreme fashion accessory. People (the old and the handicapped) who can truly benefit even from a more practical tricycle as described above are likely to ride too slowly to discover the speed advantages of a wide-track fast tadpole and so should have high-seat tricycles with tracks narrow enough for versatility on pavements and in doorways. That makes even my practical, speedy trike design concept superfluous, an interesting mental exercise of the type: "Well, we have trikes but they don't deliver on their promises. Let's see if we can design one that does." We can. So what? Looks like I've wasted several days considering how recumbents can be improved... For the rest of us, it is not surprising that the diamond frame still dominates. For those who want or need to put their feet flat on the road without leaving the saddle or seat, the only small surprise is that the Giant Revive did not survive, but it is no surprise that its more traditional-appearing spiritual and geometric soul-sisters from Electra and Trek and RANS are doing well, even becoming trendy. Nor is it surprising that their makers eschew calling them what they are (semi-recumbents) because the name "recumbent" is so discredited, instead preferring "crank forward" or even the somewhat disturbing "flatfoot". IN THE END.... I conclude that the upright and the semi-recumbent bicycles and the narrow wheelbase invalid carriage and the child's tricycle are necessary human-power formats, and the rest (recumbents regardless of number of wheels, plus my fast wide-track tadpole) are the unnecessary jewelry of an excessive society. Copyright © 2009 Andre Jute Free to use on not-for-profit netsites as longs the entire article, including this notice, is reprinted intact. Any other use contact author |
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THE LOGIC OF TRIKES an outsider's viewpoint by Andre Jute
"Andre Jute" wrote in message ... among a whole lot of guff " I didn't become interested in lightweight construction (meaning for road use, outside racing, lest some idiot publishes an old photo of me working on a racing frame and calls me a liar) until I had already given up the car to become a cyclist Andre Jute" There ARE no old photos of you working on a racing frame nor any independent proof, either photographic or textual that you have ever designed or built any vehicle with any number of wheels. PH |
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