Chris Jeggo wrote:If your COG is height H above a level road and horizontal distance D behind the front tyre point of contact, then excessive braking will send you over the handlebar if the coefficient of friction is greater than D/H and cause a skid otherwise.
exactly!
pwa wrote:A couple of decades ago I was regularly going to the Alps on cycling holidays and I had a couple of inner tube explosions due to overheated rims. So I did some reading to find out what I was doing wrong. It turned out that I was making the mistake, on long fast descents, of retarding the speed more or less constantly, feathering the brakes. That is a recipe for very hot rims and disaster. What I should have been doing, and have done ever since, is have spells with no braking, letting the speed rise, and doing all the braking really hard at the last possible moment. That heats the rims less than constant feathering.
But if you are braking hard you need to know the most effective way to do it. And the technique I picked up was this. Apply both brakes at once, not too firmly, then as the deceleration puts more force down through the front tyre, increase front wheel braking. All this transition in a couple of seconds. Obviously you also want your body weight low down and you bum far back, and doing whatever leaning you need for the corner. And it works. I'm not saying we should all be doing this to the extreme on average UK roads, but it illustrates how once deceleration is under way it is the front wheel that can handle most braking, with the rear just helping.
exactly #2!
BTW 'hard braking' is often a lot harder than some folk imagine. If you are able to stop at about 1G (H = or < D and a suitably high coefficient of friction prevails ) then stopping distances can be thus (approximately) ;
at ~10mph you can stop in just over 0.5s and travel ~1.25m in the process
at ~20mph you can stop in just over 1s and travel ~5m in the process
at ~30mph you can stop in just over 1.5s and travel ~11.25m in the process
It will normally take over 0.5s for a rider to sense the braking effort and then adjust it, which is why achieving these stopping distances (or doing a controlled stoppie) is actually rather difficult; unless you are going quite fast the stop is short duration (or could be) and there just isn't time to react and modulate the brake effectively. Most folk don't brake hard enough to get near these stopping distances, or only manage to apply the brake hard enough to lift the rear wheel near the end of the stop.
Note that the most effective time (during a hard stop) to scrub off speed is initially, not latterly. If you manage to (say) scrub off 10% of your speed right away (at 1G), you are going 10% slower for the whole stop, whereas if you only manage 1G near the end of the stop, this has relatively little impact on your stopping distance.
Obviously there is reaction time to add on to the above distances; this can soon outweigh the actual stopping time/distance if you cannot react quickly enough, which is why anticipation is so important.
cheers
