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 rear braking

Vittore Cossalter

We will now consider a rider that brakes the rear 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 rear wheel to be insufficient; consequently the rear wheel begins to slip.

Frame 3 - In order to follow the desired trajectory, the driver turns the handle-bar.

Frame 4 - With regards to the braking action, the driver can decide to stop or to continue acting the rear wheel in order to get more control of the motorcycle. If the braking action is constant the rear tire continues slipping to external side.

Frame 5 - The 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 rear tyre
Fx = longitudinal force
Fy = lateral force
Fz = vertical load
The parameter of the curves is the side slip angle l of rear 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 rear 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 acting on the rear wheel during the motorcycle fall.

The torques around the centre of mass, produced by the lateral force, is not balanced by the torque caused by tyre load , consequently the rider falls.