Tire behaviour during a ?high-side? may be better understood by looking at the following figure, which shows the available late

"Low-side" fall due to front braking

Vittore Cossalter

We will now consider a rider that brakes the front wheel while in a curve.

Frame 1 - Due to the braking longitudinal slip, the side force, necessary for maintaining equilibrium, is obtained with a slip angle greater than the one necessary in curve without the presence of the braking force.

Frame 2 - In these conditions it is quite possible for the side force produced by the front wheel to be insufficient; consequently the front wheel begins to slip.

If the braking action is constant the front tire continues slipping to external side.

Frame 3 - If the front tire loses adherence he motorcycle tilts and falls laterally. In the fall the vehicle also drags the driver down. If the driver has not been hit by other vehicles, the fall might not be dangerous, in the sense that the motorcycle does not fall against the driver.

Low fall due to rear braking may be better understood by looking at the following figure, which shows the available lateral tyre force when a longitudinal tyre force is present.

The parameter of the curves is the longitudinal slip k of front tyre
Fx = longitudinal force
Fy = lateral force
Fz = vertical load
The parameter of the curves is the side slip angle l of front tyre

In both figures, the envelope of the families of curves is the friction ellipse.

The initial condition is represented by point A in which a lateral force is present and the side-slip angle is about 1.5°.
When the driver starts to brake the front wheel, the point moves in the horizontal direction and the side-slip angle increases in order to keep the lateral force constant in the presence of an increasing longitudinal force.
The loss of grip takes place at point B when the friction ellipse is reached; a large side-slip angle (of about 3°) is present.
If the rider continues to brake, the loss of grip causes the motorcycle fall ( point C).

Equilibrium of motorcycle in steady turning. Forces not in equilibrium acting on the front wheel during the motorcycle fall.

The following figure shows a simulation of a motorcycle fall due to the front braking.