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stopping distance measurements (20mph-0, etc.)?(5 posts)

stopping distance measurements (20mph-0, etc.)?kenyee
Aug 8, 2003 9:05 AM
Anyone know of a study that shows how many feet it takes to stop a bike going certain speeds (20, 30, 40, 50, etc.) in terms of feet like what's done in reviews of cars?

I know it's also a matter of the rider's skill, but I'm curious if anyone has tried measuring this... :-)
Just off the top of my head . . . .cory
Aug 8, 2003 9:19 AM
I saw a test years ago, maybe 12 or 15, in a bike magazine that's no longer published and the name of which I can't remember. Is THAT helpful?
Based on years of testing cars, though, I can make some guesses. Once you have enough braking power to lock the brakes (which you don't want to do, but traction is the limiting factor), it's a question of how precisely you can come ALMOST to that point and hold it there. That depends on rider skill, but also on the surface (a few grains of sand could get you sliding, then you'd have to release the brakes and re-apply), rubber compound, tire pressure and the brake material (some are easier to modulate than others).
I'm still pondering whether tire SIZE would make a difference--you'd think so, but the contact patch (part that touches the ground) varies more with pressure than with section width. And--as with motorcycles--testing to the limit is hard on a bike. If you screw up in a car, you just spin or squib off sideways. On a two-wheeled vehicle, you get an ambulance ride.
What I do remember is that the stopping distances were impressively short. The weight of the rider/bike is pretty low, and the effectiveness of the brakes was good. Wouldn't be hard to lay out a course in the street and run some tests yourself, if you want to bother.
Too many variablesSprint-Nick
Aug 8, 2003 10:12 AM
To answer your question I've never heard of a study to measure the stopping distance of bikes. I think theres far too many variables with skill, tires, rider weight, etc.. In a car one would think its a much more steady platform so these variables don't come into play as much in a dry test.

Anyone know of any reviews on stopping distance for motor bikes?

re: stopping distance measurements (20mph-0, etc.)?jamesau
Aug 8, 2003 10:13 AM
Here's a way to estimate stopping distances based on one stopping distance and simple physical arguments. Braking involves dissipating kinetic energy into heat energy. Kinetic energy is proportional to speed squared (speed times speed). Braking systems ideally have a maximum energy dissipation rate. So, figure it takes four times the distance to stop from 20 mph than it does from 10 mph, nine times the distance from 30 mph than 10 mph, etc. (this all assumes a level, uniform road surface and other things that I can't think of off the top of my head).
Note I've never tested this armchair hypothesis

Check this out on the numbers posted in car magazines; they typically post stopping distances from 60 and 80 mph. Since it takes 0 feet to stop from 0 mph you can draw a pretty straight line through the data (3 points) of a plot of stopping distance versus speed squared.
re: stopping distance measurements (20mph-0, etc.)?Seapig
Aug 8, 2003 8:32 PM
Not sure if this is helpful or not, but the way to measure stopping distance of cars is d (distance)= square root of 30x coefficient of friction of the roadway divided by the speed. This is a formula set by the NHTSA. It's assuming 100% braking efficiency in a straight line.