Читать книгу Extreme and defensive driving - Dmitry Aleksandrovich Liskin - Страница 10

In the chapter “Drift: causes of origin and methods of fighting” it was shown, that when passing an S-like corner drift can occur when moving to the second half of the corner, which is related to the rocking of car body. Let us understand, what causes occurrence of drift and learn how to fight with rhythmic drift.

When passing a corner, centrifugal force causes the center of mass of car to move to the wheels, which are located on the outside of a corner. Suspension springs of the wheels, which are loaded with car’s mass, are compressed and store mechanical energy. When changing the direction of corner the energy is released, the springs are discharged and push the center of mass in the opposite direction, and other two springs begin to shrink. Definite frequency of changing direction of corners is able to increase amplitude of movement of the center of mass. Resonant increase in movement of the center of mass of machine is called rocking. Rocking can be longitudinal and transversal.

Longitudinal rocking

Longitudinal rocking – is alternately moving the center of mass along the direction of car body (forward-backward) with increasing amplitude. During longitudinal rocking the front and rear suspension springs are alternately compressed and stretched.

Compressions and stretches of suspension springs during driving can be caused by road roughness and resonant processes, which are related to characteristics of suspension. In virtue of loading of the front and rear axles alternately changes, grip on the road of the front and rear wheels changes sequentially. On the one hand, when the front axle is partially unloaded, grip of the front wheels on the road worsen. For example, if the front axle bounces on a hillock in a corner, car will move almost straightforward during the front axle is in unloading state. On the other hand, unloading the rear axle can cause vehicle to drift. Unloading of the rear axle is well felt, when a descent begins.

Longitudinal rocking is an undesirable effect, since

• behavior of machine becomes less predictable;

• the best grip on the road is achieved, if load between the axles distributed evenly; longitudinal displacement of the center of mass redistribute load of axles, the total tyre grip on the road is reduced;

• efficiency of acceleration and braking is reduced.

Longitudinal rocking can occur at small oscillation amplitudes and be invisible to a racer. When drive axle hits a hole, suspension contracts, and motor speed increases. When leaving a hole, suspension is released, motor speed decreases. To change engine speed, it is needful to make an effort. The engine connected to transmission is able to partially absorb longitudinal rocking. Suppression of longitudinal rocking – is one of the reasons not to disconnect the engine from transmission while driving.

Undesirable longitudinal vibrations on road roughness can be caused by incorrectly selected spring rates and resistances to bump and rebound of shock absorbers. Often longitudinal rocking occurs on a car with weak shock absorbers and very strong springs: such a suspension does not “swallow” roughness, but pushes away from them, while vibrations of the body increase.

Bump resistance of suspension is created by springs and shock absorbers. If a compressive force is applied to a suspension, a suspension movement, by which it is compressed, depends on springs. Shock absorbers are necessary to dampen compression-stretching vibrations of springs and create additional resistance to the movement. How quickly bump or rebound occurs, mainly depends on shock absorbers.

When suspension springs are compressed, shock absorber rod moves inside shock absorber body, the shock absorber creates bump resistance. When shock absorber rod is pulled out of the body, the shock absorber creates resistance to so-called rebound.