"High-side" fall Vittore Cossalter

There is a maxim that expresses an inexorable truth: "there are riders who have fallen and riders that they will fall." Great truth, which rider can say to be never fallen?

Assen, 2013: great victory of Valentino but how many terrible falls for high-side.

What is the high-side? Let's try to provide an explanation scientifically correct.

The example shows the simulation of a ?high-side? fall, which is closely related to vehicle stability and the interdependence between lateral and longitudinal tyre forces.
Frame l In a curve the rider starts to brake the rear wheel, therefore the longitudinal braking force increases, as does the total friction force;
Frame 2 The total friction force reaches the limit value, the rear wheel loses grip and the rear of the motorcycle moves outward;
Frame 3 The rider makes a mistake, reducing the braking force suddenly, and the rear wheel takes grip again;
Frame 4 The large side slip, which is still present, generates a lateral force impulse; the impulse torque around the centre of mass produced by the lateral force is not balanced by the torque caused by the tire load;
Frame 5 The result is that the motorcycle is violently twisted and pushed upwards.

Tire behaviour during a "high-side" may be better understood by looking at the following figure, which shows the available lateral tyre force when a longitudinal tyre force is present.

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.
• When the rider releases the brake, the rear wheel takes grip again.
• The new condition is represented by point C, where there is still a large side-slip angle but where the longitudinal slip is negligible.
• therefore, the lateral force impulse takes place, because the lateral force increases suddenly from the value of point B to the value of point C.

This example shows the simulation of a "high-side" fall, due to the driving force.

Frame l In a curve the rider starts to thrust the rear wheel, therefore the longitudinal driving force increases, as does the total friction force;
Frame 2 The total friction force reaches the limit value, the rear wheel loses grip and the rear of the motorcycle moves outward;
Frame 3 The rider makes a mistake, reducing the thrusting force suddenly, and the rear wheel takes grip again;
Frame 4 The large side slip, which is still present, generates a lateral force impulse; the impulse torque around the centre of mass produced by the lateral force is not balanced by the torque caused by tyre load;
Frame 5 The result is that the motorcycle is violently twisted and pushed upwards.

Tire behaviour during a "high-side" may be better understood by looking at the following figure, which shows the available lateral tyre force when a longitudinal driving tyre force is present.

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 accelerate the motorcycle, 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.
• When the rider releases the accelerator, the rear wheel takes grip again.
• The new condition is represented by point C, where there is still a large side-slip angle but where the longitudinal slip is negligible.
• therefore, the lateral force impulse takes place, because the lateral force increases suddenly from the value of point B to the value of point C.

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