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Does anyone have any data on the stopping force of a modern road bike? IE when a road biker slams on the brakes the force applied to the ground is x.xxx Newtons (or lbf).

Another way to ask would be if anyone has any speed vs stopping distance vs rider weight data for road bikes.

TO CLARIFY : I'm asking if anyone has a data set of numbers, I'm fully capable of going out and testing my own bike and doing the physics / math to obtain the information I need, but I'd just rather not do the testing. I would expect some deviation based on the kind of brakes and rims involved, but I would expect any modern road bike to have similar stopping force.

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You might also try getting answers to this on the Physics site. –  Neil Fein Nov 30 '10 at 3:47
here or on the physics site, it would help if you could provide some estimate of the speed at time=0 and either how much time it takes to stop or how far it takes to stop. –  David Nov 30 '10 at 5:17
it is easier to write out the answer in on the physics site since it supports LaTeX, but you should be prepared to provide more details. –  David Nov 30 '10 at 5:18
@David: If you know those numbers, you already know the answer. I think the point of the question is "what are typical numbers". –  Jefromi Nov 30 '10 at 14:49
The direction is also important. In this case it's probably useful to consider the braking force and the downforce separately, but they are also related - the limit on friction is determined in part by the downforce. It's possible to induce a front wheel skid by getting far enough back over the saddle, so rider position will be critical. I wonder if it's possible the break the forks or frame by braking (rather than breaking, per the original question) –  Мסž Apr 11 '11 at 5:18
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3 Answers

up vote 5 down vote accepted

Beck Forensics have figures (pdf), peaking at about 0.5g for a MTB on flat concrete. I used this search to find that paper and some of the other results look relevant.

The number that springs to my mind is 0.3g, but that may be for cars. Bicycling Science or Human Power are where I would go for well-researched answers. Human Power doesn't seem to have anything, although it may just be missing from the index.

A great deal depends on the geometry of the bicycle, since that's the limiting factor (most bikes can throw the rider by overenthusiastic braking). A little thought experiment might help. Assume the CoG of the rider is in the hips, so about 10cm above the seat. A line from there through the front contact patch will be roughly 45 degrees above horizontal (give or take, say 15 degrees), so an upper limit of 1g is likely.

To further Feynman it, 10m/s is 36km/hr, so 1g would stop you from 36km/hr in one second. During which time you'd travel about 1/2at^2 or 5m (I cheated by making t=1). One simple test would therefore to sprint to 35kph then hit the brakes at a marked point and see what your stopping distance is.

On reflection, 1G or 10m/s/s sounds more plausible as an upper limit.

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Perhaps we should now introduce the confounding effect of recumbent bicycles :) Traction is often the limit because the rider can be placed lower down, thus giving the same stationary weight distribution but a smaller angle between the CoG and the contact patch. Especially on a recumbent tricycle, where the consequences of a front wheel skid are minor and thus it's safer to experiment. My velomobile can out-brake almost anything even though it's very heavy (by bicycle standards) for this reason. –  Мסž Apr 11 '11 at 22:13
A corollary is that a bike can stop in a tenth the distance of a car: much faster than a car driver might expect. So, when you're 'driving' in traffic, like at a roundabout, ... –  ChrisW Apr 12 '11 at 0:44
Definitely. It's not hard to outbrake a car, and that can be very embarrassing for the survivors. I've had the equivalent - an upright rider landing in my lap when I unexpectedly decided to give way to a motorist that was jumping a stop sign. Fortunately he didn't land on any of the really spiky bits. –  Мסž Apr 12 '11 at 1:07
Page 3 says, "Forester found that a pitch-over requires an acceleration of about -0.67g. While relying on the front brake, skilled bicyclists also will slide rearwards off the saddle and position themselves just above the rear tire." Whereas for cars, page 12 says, "asphalt roadways ... would normally have a drag factor of -0.70g between the roadway surface and a motor vehicle". –  ChrisW Apr 12 '11 at 1:49
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You can find out how to estimate this at wikihow; given a starting speed of 20 mph and a stopping distance of 30 ft, they provide an estimate of 14.6 ft/s^2 which is equivalent to 4.5 m/s^2.

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I'm going to guess that, with good tires and good brakes, if you slam on the brakes then you'll go over the handle bars.

If this is so then the limiting parameter isn't the weight of the rider, but rather the location of the rider's centre of mass relative to the front tire, which is more or less independent of (constant with respect to) the rider's weight.

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