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Dum Wheel Aerodynamics Q
All else being equal, what is it about fluid dynamics which makes a low
spoke wheel more aerodynamic than a high spoke count wheel? Is this truly the case when rotating? Before you blurt out an answer, consider this: A disc wheel has infinite spokes (or one spoke depending on how you look at it) and is always considered the most aerodynamic of wheels. Obviously air can flow between the spokes of a spoked wheel and can't in a disk wheel. But, when rotating, wouldn't a high spoke wheel still be more like a disc than a low spoke count? -Andy B. |
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Dum Wheel Aerodynamics Q
Andy Birko wrote:
All else being equal, what is it about fluid dynamics which makes a low spoke wheel more aerodynamic than a high spoke count wheel? Is this truly the case when rotating? Before you blurt out an answer, consider this: A disc wheel has infinite spokes (or one spoke depending on how you look at it) and is always considered the most aerodynamic of wheels. A disk wheel is like a flat plate, a spoked wheel is a collection of tiny cylinders. Compare the drag characteristics of cylinders and flat plates he http://www.princeton.edu/~asmits/Bicycle_web/blunt.html -- terry morse Palo Alto, CA http://bike.terrymorse.com/ |
#3
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Dum Wheel Aerodynamics Q
On Thu, 17 Jun 2004 19:22:08 -0400, Andy Birko wrote:
All else being equal, what is it about fluid dynamics which makes a low spoke wheel more aerodynamic than a high spoke count wheel? Is this truly the case when rotating? Before you blurt out an answer, consider this: A disc wheel has infinite spokes (or one spoke depending on how you look at it) and is always considered the most aerodynamic of wheels. Obviously air can flow between the spokes of a spoked wheel and can't in a disk wheel. But, when rotating, wouldn't a high spoke wheel still be more like a disc than a low spoke count? -Andy B. Well, if you want to use your own arguments, you already won! You see a disc wheel as having only one spoke, and you say in your first paragraph that low spoke wheels are more aerodynamic....you do the math. 32, 36, 40 whatever number of spokes has that many low pressure zones behind each spoke and nipple as it rotates. A disc wheel only has one (or zero), itself. The 40 spoke also has that many frontal areas. IIRC, the first generation Specialized tri spoke carbon wheels actually create lift in certain crosswind conditions, like a sail on a boat. That would seem to me to have propulsion, not drag. But hey, what do I know? I'm just a dumb ass machinist. -- Skuke Reverse the domain name to send email |
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Dum Wheel Aerodynamics Q
On Thu, 17 Jun 2004 19:22:08 -0400, "Andy Birko"
wrote: All else being equal, what is it about fluid dynamics which makes a low spoke wheel more aerodynamic than a high spoke count wheel? Is this truly the case when rotating? Before you blurt out an answer, consider this: A disc wheel has infinite spokes (or one spoke depending on how you look at it) and is always considered the most aerodynamic of wheels. Obviously air can flow between the spokes of a spoked wheel and can't in a disk wheel. But, when rotating, wouldn't a high spoke wheel still be more like a disc than a low spoke count? -Andy B. Dear Andy, The smooth disk [he blurted out] is a different kettle of fish than wire rods whipping through the air like an egg whisk. There's a thin layer of turbulent air on a continuous disk surface that to some degree "greases" the surface. Each spoke, on the other hand, flies through the air with the delicacy of a tumbling brick, its thinness being its saving grace, and whips up its own horrifying mess of turbulence. Another way to think of it is by comparing wheels to fans. More blades (spokes) will whip up more air, but a smooth disk attached to a fan motor is almost useless for stirring up the air. It's the interruption of the ideal surface that causes the trouble. Another example would be circular saw blades. Until you interrupt the rim with teeth or carbide grit, they won't bite into the wood. Carl Fogel |
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Dum Wheel Aerodynamics Q
wrote in message ... On Thu, 17 Jun 2004 19:22:08 -0400, "Andy Birko" wrote: All else being equal, what is it about fluid dynamics which makes a low spoke wheel more aerodynamic than a high spoke count wheel? Is this truly the case when rotating? There's a thin layer of turbulent air on a continuous disk surface that to some degree "greases" the surface. Each spoke, on the other hand, flies through the air with the delicacy of a tumbling brick, its thinness being its saving grace, and whips up its own horrifying mess of turbulence. I understand what you're getting at, but why doesn't this turbulant air turn into an "air disk" so to speak? I.e., as you've mentiones, there is less friction between laminar air and turbulant air than there is between laminar air and a solid (hence the dimples on golf balls and some of the Zipps), so why doesn't this "disk" of turbulent air created by the spokes act in a similar fashion? Mind you I am talking about longitudinal drag (i.e. headwind etc.), not the drag of the wheel spinning itself. Are there any wind tunnel tests on this? |
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Dum Wheel Aerodynamics Q
carlfogel wrote:
On Thu, 17 Jun 2004 19:22:08 -0400, "Andy Birko" wrote: All else being equal, what is it about fluid dynamics which makes a low spoke wheel more aerodynamic than a high spoke count wheel? Is this truly the case when rotating? Before you blurt out an answer, consider this: A disc wheel has infinite spokes (or one spoke depending on how you look at it) and is always considered the most aerodynamic of wheels. Obviously air can flow between the spokes of a spoked wheel and can't in a disk wheel. But, when rotating, wouldn't a high spoke wheel still be more like a disc than a low spoke count? Carl Fogel Not quite the same. There are two components to drag, skin friction and frontal area. In the disk wheel, there is many times more skin friction than a spoked wheel while the spoke wheel's drag is from the frontal area of the spokes. When you ask which one gives less drag, you have to further specify in what conditions. While a disk wheel would be the best for high speed, this high speed on a bike is close enough to prevailing winds and the resultant apparant wind, if not in line with the bicycle's motion, can induce far more drag than the disk wheel affords. The best compromise is a moderate height rim (20-30mm), 16-24 bladed spokes (butted and bladed are best like CX-Rays) and a smooth tire to rim profile (advantage to clinchers). Disk wheels only offer advantages in absolute still conditions, like indoor track. -- |
#7
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Dum Wheel Aerodynamics Q
Andy Birko wrote:
I understand what you're getting at, but why doesn't this turbulant air turn into an "air disk" so to speak? I.e., as you've mentiones, there is less friction between laminar air and turbulant air than there is between laminar air and a solid (hence the dimples on golf balls and some of the Zipps), so why doesn't this "disk" of turbulent air created by the spokes act in a similar fashion? There is no "disk" of turbulent air created by the spokes. There is a lot of airspace between each spoke and the next, so each generates a little turbulence behind it and gets its own independent drag. A long thin streamlined shape like a full disk or the spoke of a Trispoke induces an airflow with a very different behavior. IOW, as long as the airspace between spokes is larger than the spoke diameter, you are in a different regime than the disk and thinking of the disk as the limit of a large number of spokes is not correct. The dimples have something to do with delaying boundary layer separation. I think "less friction between laminar and turbulent than laminar and solid" is oversimplifying, see e.g. http://www.physlink.com/Education/AskExperts/ae423.cfm Mind you I am talking about longitudinal drag (i.e. headwind etc.), not the drag of the wheel spinning itself. Are there any wind tunnel tests on this? Yes. |
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Dum Wheel Aerodynamics Q
On Thu, 17 Jun 2004 20:51:30 -0400, "Andy Birko"
wrote: wrote in message .. . On Thu, 17 Jun 2004 19:22:08 -0400, "Andy Birko" wrote: All else being equal, what is it about fluid dynamics which makes a low spoke wheel more aerodynamic than a high spoke count wheel? Is this truly the case when rotating? There's a thin layer of turbulent air on a continuous disk surface that to some degree "greases" the surface. Each spoke, on the other hand, flies through the air with the delicacy of a tumbling brick, its thinness being its saving grace, and whips up its own horrifying mess of turbulence. I understand what you're getting at, but why doesn't this turbulant air turn into an "air disk" so to speak? I.e., as you've mentiones, there is less friction between laminar air and turbulant air than there is between laminar air and a solid (hence the dimples on golf balls and some of the Zipps), so why doesn't this "disk" of turbulent air created by the spokes act in a similar fashion? Mind you I am talking about longitudinal drag (i.e. headwind etc.), not the drag of the wheel spinning itself. Are there any wind tunnel tests on this? Dear Andy, The turbulent horror whipped up by widely spaced spokes is a swirling, billowing mess, with little to constrain it. The leading edge of the spoke is directly compressing and displacing helpless air in its path and also dragging innocent-bystander-style air on either side into the partial void behind it, where it gets slammed by the next spoke. (Of course, the air was all knocked silly by the comparatively massive tire and rim.) In contrast, a smooth disk just rubs gently against the air. It doesn't slam into it, the displacement is much gentler, and the tiny layer of turbulent air greases things along nicely. (For all I know, minute dimpling or dolphin-style corrugations might improve things, but the leading edge of the tire and rim complicate things.) Remember, it's fluid dynamics. Think of trying to spin a flat disk in water (a wet grinding wheel moves easily through its water trough, stirring up little.) Now think of of normal bicycle wheel with its spokes splashing through the same water trough, each spoke slamming into the already stirred-up water. Perhaps at an outlandishly high speed, a crude "air disk" might form, but the speed of the spokes would have to be higher than the speed at which the air under normal atmospheric pressure rushes into the low pressure area behind the spoke and starts swirling madly. With about 14-15 psi at sea level. a triple cylinder 50 cc motorcycle engine can happily fill its tiny combustion chambers at 22,500 rpm, which gives you some idea of how fast air rushes back in behind moving objects. With a solid disk, there's no rushing in and billowing out. Keep in mind that aerodynamics is a beastly tricky business. The Bernoulli pressure-velocity effect, for example, might lead you to predict baseballs curving the wrong way, so you have to reach for the Magnus-Robins drag effect or the pitchers will snicker at you. Similarly, the erratic flight of the almost spinless knuckleball is often mistakenly invoked to explain the inaccuracy of smooth-bore muskets, which actually are inaccurate because they're practically guaranteed to put a wicked (but unpredictable) spin on any lead ball--just not the special spin of a rifled slug that is the only spin that allows accuracy. The unrifled musket is just a device for demonstrating the golfer's slice through 360 degrees. The only parallel to the almost spinless knuckleball in firearms is the special case of shotgun pellets, which are contained in a plastic cup until they leave the barrel and therefore acquire no spin. The spinless shotgun pellets are remarkably accurate, as waterfowl and clay pigeons know to their sorrow, since unlike baseballs with heavy raised stitching and seams, the pellets are as smooth as the ammunition makers can manage. Modern smoothbore tank cannons-- I beg your pardon, We were discussing disk wheels and spokes, not the outrageous price of Benjamin Robins' "New Principles of Gunnery." If those used-book dealers think that I'm going to pay $300 or even more for a lapsed Quaker's 1742 ballistics experiments . . . Carl Fogel |
#9
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Dum Wheel Aerodynamics Q
"Benjamin Weiner" wrote in message
news:40d24556$1@darkstar... The dimples have something to do with delaying boundary layer separation. I think "less friction between laminar and turbulent than laminar and solid" is oversimplifying, see e.g. When dimpled spokes appear on the market, I will know to whom the blame shuld be assigned! -- Mark South: World Citizen, Net Denizen |
#10
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Dum Wheel Aerodynamics Q
Also, would tread on a road tire break up the laminar flow and "dimpleize"
the air flow around the tire? -- Phil, Squid-in-Training |
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