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#21
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Braking distance for bicycles with all relevant data explained
MrCheerful wrote:
On 22/02/2016 10:42, John Smith wrote: MrCheerful wrote: Then why can't they stop at traffic lights? or pedestrian crossings, Or before riding into the side of a bus? back of a car, pedestrian, other cyclist, toddler, lamp post , dog etc. etc. Please contract pancreatic cancer. What a witty response, you must be so proud of your intellectual put downs. Why should I - or anyone else for that matter - bother with 'intellectual putdowns' when faced with a cretinous ****hair like you? -- john smith |MA (Hons)|MPhil (Hons)|CAPES (mention très bien)|LLB (Hons) 'It never gets any easier. You just get faster' (Greg LeMond (1961 - )) |
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#22
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Braking distance for bicycles with all relevant data explained
MrCheerful wrote:
On 21/02/2016 21:20, Paul George wrote: On Sunday, February 21, 2016 at 7:06:35 PM UTC, Peter Parry wrote: Interesting - there isn't much published research on the subject. Perhaps because there are too many variables involved. Hophead fixie peddlars always claim they can stop instantly (perhaps in their heads they are) It must be so stressful to be a hater. but it is interesting to see that for most push bikes they can't even come close to the outdated 12m overall stopping distance (6m braking distance) for a car at 20MPH. But they can. Then why can't they stop at traffic lights? or pedestrian crossings, Or before riding into the side of a bus? back of a car, pedestrian, other cyclist, toddler, lamp post , dog etc. etc. 'Police installed first traffic camera at a sign south of Paris, and are now chasing 517 drivers who ignored it in just one day...' http://www.theguardian.com/world/2016/feb/22/stop-a-four-letter-word-for-french-drivers-traffic-camera-shows I should be very surprised if the figures were not higher in Psycholand. -- john smith |MA (Hons)|MPhil (Hons)|CAPES (mention très bien)|LLB (Hons) 'It never gets any easier. You just get faster' (Greg LeMond (1961 - )) |
#23
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Braking distance for bicycles with all relevant data explained
On Mon, 22 Feb 2016 00:10:35 -0000, "TMS320" wrote:
"Paul George" wrote On Sunday, February 21, 2016 at 7:06:35 PM UTC, Peter Parry wrote: but it is interesting to see that for most push bikes they can't even come close to the outdated 12m overall stopping distance (6m braking distance) for a car at 20MPH. It's not a like for like comparision. Driver reaction adds another 6m. There also needs to be an addition of 2m for distance of the front of the car ahead of the driver's eyes. If we simply consider braking distance and assume that the reaction time will not be dramatically different between cyclists and motorists then with a car the limiting factor is the rate of deceleration which can be achieved. All the figures below are for braking distances only. On a dry tarmac road driver skill is relatively unimportant as long as they brake hard. With a car deceleration at max braking is set by the adhesion of the tyres to the road surface and with modern cars and tyres on a tarmac surface is about 0.9g. A moderately degraded road surface will not significantly affect braking. However, in an emergency most drivers will not brake to the cars maximum capability (hence the introduction of Emergency Brake Assist). With a push bike rider skill is a far more important factor than with a car Most riders are unable to achieve anything like the best the cycle can brake at. The quality of the road surface also has a major impact. The average rider rarely manages more than 0.35g. However, if we assume the surface is dry and pristine and the pushbike is braking at the best the machine can achieve the limit for the pushbike is not the adhesion of the tyre on the road but weight transfer which can cause pitch over at about 0.6g (the subsequent adhesion of shredded flesh to tarmac is not usually considered to be part of stopping distance). It would appear that the calculator you used at exploratorium.edu is significantly wrong. For a speed of 24.85 MPH it gives a stopping distance of 7.14m. A test of the Shimano R785 Hydraulic road disc brakes at that speed for review produced stopping distances of 14m. The Campagnolo Chorus calipers and alloy rims produced similar results. At 15MPH a Dutch bicycle (new) fitted with disk brakes could stop in 5m, the web calculator claimed 2.6m. The latest Highway code braking distances have been reduced somewhat and appear to assume retardation of about 0.65g.This produces a braking distance figure from 60MPH of 54m, a test by Which? on some fairly average cars produced stopping distances at that speed between 34 and 44m. Time to brake to stop is the speed in metres per second divided by the deceleration rate. Cars can always decelerate more rapidly than bicycles. There doesn't appear to be any doubt that a car can stop more quickly than any bike at comparable speeds. In the rain the disparity increases considerably. |
#24
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Braking distance for bicycles with all relevant data explained
On Monday, February 22, 2016 at 8:37:24 AM UTC, MrCheerful wrote:
On 21/02/2016 23:45, Paul George wrote: On Sunday, February 21, 2016 at 11:40:21 PM UTC, MrCheerful wrote: I made no such claim, but in the absence of better qualified data I take the word of experts. What better data do you want than a real world video? Is there evidence of speed and distance ? It was Argyle St Birkenhead, near Birkenhead Central station. You can to to google maps and measure speed and distance for yourself. |
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Braking distance for bicycles with all relevant data explained
"Peter Parry" wrote
On Mon, 22 Feb 2016 00:10:35 -0000, "TMS320" wrote: "Paul George" wrote On Sunday, February 21, 2016 at 7:06:35 PM UTC, Peter Parry wrote: but it is interesting to see that for most push bikes they can't even come close to the outdated 12m overall stopping distance (6m braking distance) for a car at 20MPH. It's not a like for like comparision. Driver reaction adds another 6m. There also needs to be an addition of 2m for distance of the front of the car ahead of the driver's eyes. If we simply consider braking distance and assume that the reaction time will not be dramatically different between cyclists and motorists then with a car the limiting factor is the rate of deceleration which can be achieved. All the figures below are for braking distances only. On a dry tarmac road driver skill is relatively unimportant as long as they brake hard. With a car deceleration at max braking is set by the adhesion of the tyres to the road surface and with modern cars and tyres on a tarmac surface is about 0.9g. A moderately degraded road surface will not significantly affect braking. However, in an emergency most drivers will not brake to the cars maximum capability (hence the introduction of Emergency Brake Assist). With a push bike rider skill is a far more important factor than with a car Most riders are unable to achieve anything like the best the cycle can brake at. The quality of the road surface also has a major impact. The average rider rarely manages more than 0.35g. Do you mean that an average rider rarely does more than 0.35g by routine? Or has this figure been determined by putting a sample through controlled stress tests? Many countries allow bikes with rear brake only - are you sure you're not confusing two different things? If it is 'by routine' then drivers rarely do more than 0.35g. However, if we assume the surface is dry and pristine and the pushbike is braking at the best the machine can achieve the limit for the pushbike is not the adhesion of the tyre on the road but weight transfer which can cause pitch over at about 0.6g (the subsequent adhesion of shredded flesh to tarmac is not usually considered to be part of stopping distance). Pitchover is higher than 0.6g. Although there are techniques of improving weight distribution to stop pitchover during deep braking, I expect most cases are low speed when the rider puts feet down too soon. It would appear that the calculator you used at exploratorium.edu is significantly wrong. For a speed of 24.85 MPH it gives a stopping distance of 7.14m. A test of the Shimano R785 Hydraulic road disc brakes at that speed for review produced stopping distances of 14m. The Campagnolo Chorus calipers and alloy rims produced similar results. At 15MPH a Dutch bicycle (new) fitted with disk brakes could stop in 5m, the web calculator claimed 2.6m. The retardation for these distances is significantly below even your pitchover figure. The latest Highway code braking distances have been reduced somewhat and appear to assume retardation of about 0.65g.This produces a braking distance figure from 60MPH of 54m, a test by Which? on some fairly average cars produced stopping distances at that speed between 34 and 44m. The difference always looks dramatic on a side by side test but isn't particularly real world significant. In the distance a 50's car took from 30mph, the best modern car can only start from 36mph before using up that distance (including driver reaction). Time to brake to stop is the speed in metres per second divided by the deceleration rate. Cars can always decelerate more rapidly than bicycles. There doesn't appear to be any doubt that a car can stop more quickly than any bike at comparable speeds. In the rain the disparity increases considerably. If the dry limit for a bicycle is pitchover, in the wet it will be tyre adhesion. The disparity reduces. |
#26
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Braking distance for bicycles with all relevant data explained
On Tue, 23 Feb 2016 09:39:43 -0000, "TMS320" wrote:
"Peter Parry" wrote With a push bike rider skill is a far more important factor than with a car Most riders are unable to achieve anything like the best the cycle can brake at. The quality of the road surface also has a major impact. The average rider rarely manages more than 0.35g. Do you mean that an average rider rarely does more than 0.35g by routine? Or has this figure been determined by putting a sample through controlled stress tests? Controlled tests on well maintained bikes with brakes front and rear on dry surfaces. The riders were experienced "utility" cyclists. Nearly all applied insufficient braking force as they feared the front wheel would skid or they would be tipped over. Many countries allow bikes with rear brake only - are you sure you're not confusing two different things? Would a rear brake alone be capable of 0.35g retardation: I would have thought nearer 0.25g? If it is 'by routine' then drivers rarely do more than 0.35g. However, if we assume the surface is dry and pristine and the pushbike is braking at the best the machine can achieve the limit for the pushbike is not the adhesion of the tyre on the road but weight transfer which can cause pitch over at about 0.6g (the subsequent adhesion of shredded flesh to tarmac is not usually considered to be part of stopping distance). Pitchover is higher than 0.6g. Although there are techniques of improving weight distribution to stop pitchover during deep braking, I expect most cases are low speed when the rider puts feet down too soon. "For an upright bicycle on dry asphalt with excellent brakes, pitching will probably be the limiting factor. The combined center of mass of a typical upright bicycle and rider will be about 60 cm (24 in) back from the front wheel contact patch and 120 cm (47 in) above, allowing a maximum deceleration of 0.5 g (5 m/s2 or 16 ft/s2).[28] If the rider modulates the brakes properly, however, pitching can be avoided. If the rider moves his weight back and down, even larger decelerations are possible." (Wikipedia - Bicycle_and_motorcycle_dynamics). 0.67 seems to be about the absolute limit using extreme body positioning (chest on saddle) well beyond what most riders are capable of doing. The pitchover g force is reduced if the weight of the rider moves forward due to insufficient bracing against the handlebars (some authors consider this forward body movement to be the most common reason for riders going over the front of the bike). "High Speed Bicycling" (Wayne Pein, Bicycling Matters) says:- "Four-wheeled motor vehicles have much better emergency braking capabilities than bicycles, approximately 0.6 - 0.7 g (some cars can achieve more than 0.9 g), affording motorists a great margin for error beyond AASHTO’s roadway design specification. In contrast, a typical bicyclist can be expected to decelerate at 0.35 g on clean, dry, level pavement which, coincidentally, is AASHTO’s figure for roadway design purposes as previously noted. A conventional bicycle's theoretical maximum deceleration is limited to about 0.6 g on level pavement by weight transfer, which can cause pitch-over. However, only a highly skilled bicyclist using optimal technique may be able to achieve this 0.6 g; most will be far lower at about 0.35 g." It would appear that the calculator you used at exploratorium.edu is significantly wrong. For a speed of 24.85 MPH it gives a stopping distance of 7.14m. A test of the Shimano R785 Hydraulic road disc brakes at that speed for review produced stopping distances of 14m. The Campagnolo Chorus calipers and alloy rims produced similar results. At 15MPH a Dutch bicycle (new) fitted with disk brakes could stop in 5m, the web calculator claimed 2.6m. The retardation for these distances is significantly below even your pitchover figure. The latest Highway code braking distances have been reduced somewhat and appear to assume retardation of about 0.65g.This produces a braking distance figure from 60MPH of 54m, a test by Which? on some fairly average cars produced stopping distances at that speed between 34 and 44m. The difference always looks dramatic on a side by side test but isn't particularly real world significant. In the distance a 50's car took from 30mph, the best modern car can only start from 36mph before using up that distance (including driver reaction). The latest Highway Code figures reduced the braking distance but increased the thinking distance thus keeping the overall stopping distance more or less unchanged. I'm only considering braking distance here so the effectiveness of the brakes can be compared without confounding factors. There is no doubt that a car can stop more quickly than a pushbike. There is also no doubt once braking hard a pushbike riders skill (or not) is more of a factor than with the driver of a car who simply has to press the brake hard. Time to brake to stop is the speed in metres per second divided by the deceleration rate. Cars can always decelerate more rapidly than bicycles. There doesn't appear to be any doubt that a car can stop more quickly than any bike at comparable speeds. In the rain the disparity increases considerably. If the dry limit for a bicycle is pitchover, in the wet it will be tyre adhesion. The disparity reduces. It is usually brake effectiveness (or lack of) which creates the wet limit. A TRRL report from 1980 on rim brakes found that using synthetic brake blocks on chromed steel wheel rims in the dry a maximum retardation of 0.76g could be obtained, the same brake/wheel/rider combination managed only 0.21g in the wet. For alloy wheels the dry braking force was 0.46g dry and 0.39g wet.(TRRL supplementary report 619). Although the report is old most pushbikes still use caliper brakes and the results remain relevant. Moreover, fear of skidding means many riders do not brake effectively in the wet. |
#27
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Braking distance for bicycles with all relevant data explained
"Peter Parry" wrote in message On Tue, 23 Feb 2016 09:39:43 -0000, "TMS320" wrote: "Peter Parry" wrote With a push bike rider skill is a far more important factor than with a car Most riders are unable to achieve anything like the best the cycle can brake at. The quality of the road surface also has a major impact. The average rider rarely manages more than 0.35g. Do you mean that an average rider rarely does more than 0.35g by routine? Or has this figure been determined by putting a sample through controlled stress tests? Controlled tests on well maintained bikes with brakes front and rear on dry surfaces. The riders were experienced "utility" cyclists. Nearly all applied insufficient braking force as they feared the front wheel would skid or they would be tipped over. It's still hard to tell whether it was an actual stress test or an instruction "in your own time come to a stop as quickly as you can". Many countries allow bikes with rear brake only - are you sure you're not confusing two different things? Would a rear brake alone be capable of 0.35g retardation: I would have thought nearer 0.25g? Maybe. ... It would appear that the calculator you used at exploratorium.edu is significantly wrong. For a speed of 24.85 MPH it gives a stopping distance of 7.14m. A test of the Shimano R785 Hydraulic road disc brakes at that speed for review produced stopping distances of 14m. The Campagnolo Chorus calipers and alloy rims produced similar results. At 15MPH a Dutch bicycle (new) fitted with disk brakes could stop in 5m, the web calculator claimed 2.6m. The retardation for these distances is significantly below even your pitchover figure. The latest Highway code braking distances have been reduced somewhat and appear to assume retardation of about 0.65g.This produces a braking distance figure from 60MPH of 54m, a test by Which? on some fairly average cars produced stopping distances at that speed between 34 and 44m. The difference always looks dramatic on a side by side test but isn't particularly real world significant. In the distance a 50's car took from 30mph, the best modern car can only start from 36mph before using up that distance (including driver reaction). The latest Highway Code figures reduced the braking distance but increased the thinking distance thus keeping the overall stopping distance more or less unchanged. I'm only considering braking distance here so the effectiveness of the brakes can be compared without confounding factors. There is no doubt that a car can stop more quickly than a pushbike. But you are introducing confounding factors when you talk about cyclists only achieving 0.35g. And you haven't offered an explanation for why the brakes you quote above fall well short of 0.6g. Is the limitation on bicycle braking the actual brake power or pitchover? It can't be both. There is also no doubt once braking hard a pushbike riders skill (or not) is more of a factor than with the driver of a car who simply has to press the brake hard. There is also no doubt that cars are driven faster than bicycles are ridden and a car might have to stop when there would be a gap for a bicycle to go through. Time to brake to stop is the speed in metres per second divided by the deceleration rate. Cars can always decelerate more rapidly than bicycles. There doesn't appear to be any doubt that a car can stop more quickly than any bike at comparable speeds. In the rain the disparity increases considerably. If the dry limit for a bicycle is pitchover, in the wet it will be tyre adhesion. The disparity reduces. It is usually brake effectiveness (or lack of) which creates the wet limit. A TRRL report from 1980 on rim brakes found that using synthetic brake blocks on chromed steel wheel rims in the dry a maximum retardation of 0.76g could be obtained, the same brake/wheel/rider combination managed only 0.21g in the wet. For alloy wheels the dry braking force was 0.46g dry and 0.39g wet.(TRRL supplementary report 619). Although the report is old most pushbikes still use caliper brakes and the results remain relevant. Moreover, fear of skidding means many riders do not brake effectively in the wet. Riding in the wet does not necessarily mean the rims get wet. And the numbers above are inconsistent:- dry braking for steel 0.76g, alloy 0.46g. Eh? Add that to all the above and we are no better informed than before we started. |
#28
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Braking distance for bicycles with all relevant data explained
On Sun, 21 Feb 2016 11:14:23 -0800 (PST), Alycidon
wrote: If the figures above are correct, the bike has enough braking power to brake in half the distance of a 'standard' car." a. The figures are not correct b. It can't |
#29
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Braking distance for bicycles with all relevant data explained
On Sunday, 21 February 2016 13:48:12 UTC, MrCheerful wrote:
Just out of interest I found probably the same info. that the Collision investigators use to calculate braking distances for bicycles. The distance from perception to stop at 30kph (just under 20mph) is 37m on a level road. Metric and imperial versionss are both shown. http://www.muggaccinos.com/Liability...opFormulae.htm For a car the distance is 12.6m ( thinking plus braking ) |
#30
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Braking distance for bicycles with all relevant data explained
On Thursday, 14 May 2020 13:37:43 UTC+1, wrote:
On Sunday, 21 February 2016 13:48:12 UTC, MrCheerful wrote: Just out of interest I found probably the same info. that the Collision investigators use to calculate braking distances for bicycles. The distance from perception to stop at 30kph (just under 20mph) is 37m on a level road. Metric and imperial versionss are both shown. http://www.muggaccinos.com/Liability...opFormulae.htm For a car the distance is 12.6m ( thinking plus braking ) OMGods how did this resurface. |
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