Wikipedia to the rescue again.

Go to “Bicycle and motorcyle dynamics.” Lots of fascinating stuff there, but the appropriate section copied from Wikipedia here:

Longitudinal stability A motorcycle performing a stoppie.

Mechanical analysis of the forces generated by a bike with a wheelbase *L* and a center of mass at height *h* and halfway between the wheels, with both wheels locked, reveals that the normal (vertical) forces at the wheels are: for the rear wheel and for the front wheel,

while the frictional (horizontal) forces are simply *F**r* = μ*N**r* for the rear wheel and *F**f* = μ*N**f* for the front wheel, where μ is the coefficient of friction, *m* is the mass, and *g* is the acceleration of gravity. Therefore, if

then the normal force of the rear wheel will be zero (at which point the equation no longer applies) and the bike will begin to flip forward over the front wheel.

The coefficient of friction of rubber on dry asphalt is between 0.5 and 0.8. Using the lower value of 0.5, and assuming the center of mass height is greater than or equal to the wheelbase, the front wheel can generate enough stopping force to flip the bike and rider forward over the front wheel.

On the other hand, if the center of mass height is less than half the wheelbase and at least halfway towards the rear wheel, as is true, for example on a tandem or a long-wheel-base recumbent, then, even if the coefficient of friction is 1.0, it is impossible for the front wheel to generate enough braking force to flip the bike. It will skid unless it hits some fixed obstacle, such as a curb.

In the case of a front suspension, especially telescoping fork tubes, this increase in downward force on the front end may cause the suspension to compress and the front end to lower. This is known as *brake diving*. A riding technique that takes advantage of how braking increases the downward force on the front wheel is known as *trail braking*.

edit] Front wheel braking

The limiting factors on the maximum deceleration in front wheel braking are: the maximum, limiting value of static friction between the tire and the ground,the kinetic friction between the brake pads and the rim or disk,pitching (of bike and rider) over the front wheel.

For an upright bicycle on dry asphalt with excellent brakes, pitching will probably be the limiting factor. The combined center of mass of a typical upright bicycle and rider will be about 60 cm (23.6 in) back from the front wheel contact patch and 120 cm (47.2 in) above, allowing a maximum deceleration of 0.5 g (4.9 m/s² or 16 ft/s²). If the rider modulates the brakes properly, however, pitching can be avoided. If the rider moves his weight back and down, even larger decelerations are possible.

Front brakes on many inexpensive bikes are not strong enough so, on the road, they are the limiting factor. Cheap cantilever brakes, especially with “power modulators”, and Raleigh-style side-pull brakes severely restrict the stopping force. In wet conditions they are even less effective.

Front wheel slides are more common off-road. Mud, water, and loose stones reduce the friction between the tire and trail, although knobby tires can mitigate this effect by grabbing the surface irregularities. Front wheel slides are also common on corners, whether on road or off. Centripetal acceleration adds to the forces on the tire-ground contact, and when the friction force is exceeded the wheel slides.

Of course, the angle of the terrain can influence all of the calculations above. All else remaining equal, the risk of pitching over the front end is reduced when riding up hill and increased when riding down hill.

GNU license to copy this text: http://en.wikipedia.org/wiki/Wikipedia:Text_of_the_GNU_Free_Documentation_License