Originally Posted by Olebiker
Perhaps some engineering or physics student can produce an equation to answer this. If I use both brakes, will the rear wheel unweight the same amount as it would if I use only the front brake?
Newton's second and third laws of motion fully describe the behavior.
You can only unload the rear wheel with the rear brake to the point at which the friction from the brake overcomes the friction from the pavement and the wheel locks up. After that point, the only remaining force is the friction of the locked wheel being dragged over the asphalt. The coefficient of kinetic friction (dragging a locked wheel across the pavement) is much smaller than the coefficient of static friction (normal rolling motion), and thus the skidding, locked rear wheel provides very little stopping power.
You can't go over the handlebars (something which we might be able to call "negative loading," the rear wheel actually coming off the ground) this way, but you can reduce the weight on the rear wheel to almost zero. The front brake can be used to unload the rear wheel even further, until it goes over your head, but that's irrelevant. The bottom line is that, given a specific deceleration, caused by either brake, the rear wheel will be unloaded to the same extent. The difference is that the rear brake cannot provide a deceleration sufficient to throw you over the handlebars, while the front brake can.
- Warren