The diagram illustrates a four-wheel vehicle, in which the front axle is located a distance a ahead of the centre of gravity and the rear axle is a distance b aft of the cg. The body of the car is pointing in a direction θ θ (theta) whilst it is travelling in a direction ψ ψ (psi). In general, these are not the same. The tyre treads at the region of contact point in the direction of travel, but the hubs are aligned with the vehicle body, with the steering held central. The tyres distort as they rotate to accommodate this mis-alignment, and generate side forces as a consequence.