Figure 2.1. The driven wheel with radius rw is exposed to the incoming torque Tw from
the drive shafts. This torque forces the wheel to rotate with angular velocity !w. N is
the normal force and Fres represents the resistance forces that act on the vehicle. Fa is
the adhesive force, that is the tractive force that drives the vehicle forward with velocity
vveh.
The adhesive force Fa is the driving force. According to (2.1) it must exceed
the resistance forces in order for the vehicle to accelerate. The adhesive force can
be written as a product of the friction coefficient μ and the normal force N as in
Fa = μN (2.3)
To get a fast and stable acceleration, the adhesive force should be maximised
throughout the acceleration. Since it is direct proportional to the friction coefficient,
the goal is reached by maximising the friction coefficient. The friction
coefficient depends on several factors, among others the wheel slip.