Originally Posted by
Wilfred Laurier
The limit you are approaching is zero. The maximum deceleration when the wheel is six inches off the ground is less than the maximum deceleration when the wheel is two inches off the ground, and the maximum deceleration when the centre of mass is two inches behind the front axle is much less than when the wheel is six inches off the ground. And the maximum deceleration when the centre of mass is directly over the front wheel is zero.
Try again. Why would the maximum deceleration be
less when the wheel is 6" off the ground as opposed to 2" off the ground? There is no physical reason for this to occur. If you think it is, show some calculations to that effect.
The maximum deceleration would only be zero when the bike is directly over the contact patch if the bike has stopped. You don't necessarily have to stop when the bike is directly over the contact patch because you could continue forward of the contact patch if your momentum is greater than the maximum possible deceleration. If you are going too fast and try to stop too quickly, the rider is carried up and over the contact patch and falls to the ground.
That is what Wilson is saying. For a given weight and wheelbase used in his example, you can only develop about 0.56
g (5.5 m/s^2) of deceleration on a bicycle in a crouched position. If you need 0.57
g (5.6 m/s^2) of deceleration, you will continue past the contact patch and be thrown from the bicycle. The bicycle won't stop when the rider reachs the pitch over point.
Originally Posted by
Wilfred Laurier
Maximum deceleration occurs when the front brake is applied with such force that the rear wheel is unweighted but has not yet lifted off the ground. Nothing I have said contradicts this. Every millimeter that the rear wheel, and by extension the centre of mass, lifts off the ground, the lower the maximum braking.
Not according to Wilson nor many others. I used to think the same until it was demonstrated to me that maximum deceleration occurs when the rear wheel is completely unweighted.
As to your statement that "when the rear wheel is lifted, the rider has begun going over the handlebars", consider what happens if the rider uses only a rear brake. Weight shift occurs and the rear wheel is unweighted to the point where it may not have enough friction to keep turning...in other words, it skids. The ride is no where near "going over the handlebars" and, without something else stopping the rear wheel,
can't go over the handlebars. This is exactly the same thing that is happening when the rear wheel just starts to lift while uses a front brake. The rider isn't at risk of going over the handlebars because the center of gravity isn't far enough forward yet.