Steering Synopsis
Konstruktion 7BS
Revised 13.12.05

Steering at high speeds is different from steering at low speeds.  In low speed turns, the radial acceleration is low because the car velocity is low.  In high speed turns, the radial (sideways) acceleration is appreciable.  The tires behave differently in a high speed turn.  The springiness of the tires comes into play.

Tire Slip Angle, a

As this tire springiness comes into play, its effect is seen in that the tire track (its direction of travel) is not the same as its heading.  The angle between track and heading is called the slip angle.  The slip angle, a, is related to the sideways force by

Ca is called the cornering stiffness.

Steering Angle, d

The actual angle of the front tires to the car axis, is d.  It is related to the slip angles of the front and rear by

a_f = a_r
a_f < a_r

Static Margin

The static margin is the distance the neutral steer point falls behind the vehicle center of mass, expressed in percentage of wheelbase (L).  The neutral steer point is the point along the car centerline where a sideways force would produce equal slip angles on both wheels.  If the neutral steer point is behind the center of mass, the car has understeer.  If the neutral steer point is at the center of mass, the car has neutral steer.  If the neutral steer point is forward of the center of mass, the car has oversteer.

Understeer is generally desirable because it produces a car that is directionally stable.  I.e. it tries to correct, not exacerbate, sideways upset forces that occur in straight-ahead travel.