Originally Posted by
cyccommute
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.
Because the rider's center of gravity is higher and further forward. The force of braking acts at the front wheel contact patch, parallel to the ground, creating a moment around the center of gravity that tends to tip the bike forward. Under normal braking that moment is countered by the shifting of weight from the rear wheel to the front, which generates an opposing moment.
When the rear wheel lifts off the ground (assuming the rider stays in the same position relative to the bike) the moment from braking force gets stronger because the C of G is higher (as long as braking force stays the same) and the opposing moment gets weaker because the C of G is horizontally closer to the front wheel contact patch (assuming the rider doesn't spontaneously take on or jettison some form of ballast).
If braking force doesn't change, the only way a rider
isn't going over the handlebars at this point is if they are going so slowly that they don't have enough kinetic energy to lift their center of gravity past the front wheel contact patch (or if the front wheel skids for some reason, but I'm not arguing about that).
Originally Posted by
cyccommute
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.
You seem to be assuming that when someone brakes hard enough to lift their rear wheel they release the brake, and arguing against people who are assuming that braking remains constant throughout.
I can draw the force diagrams, but I don't have enough time before work. Maybe tonight.