Figure 7: The effect of increasing the accelerating force on the weight distribution of a racing car. At rest the weight of the car is biased to the rear. Upper panel: the effect of acceleration. Solid lines show the reaction forces at front and rear wheels, and how these change with acceleration. The dashed line shows the traction limit; ie., the weight that must be placed on front the wheels for them to grip. Lower panel: the effect of braking, assuming that 70% of the braking force is directed to the front wheels. There are now two dashed lines showing the weight that must be placed on the front and rear wheels respectively.
As gets larger, the reaction force at the front gets smaller while that at the rear gets larger. Since the maximum traction force depends on the weight on the tyre, this increases as the car accelerates harder. The limit to the acceleration is set either by the point at which the traction limit is reached, or the point at which the front wheels lift off the ground and directional control is lost. Figure 7 (upper panel) shows the situation for a typical race car.