Robin Sharp's request (Letters, July 2012) for the location of certain mathematics can be found here!
Assuming the centre of gravity of a vehicle lies within the polygon of road-wheel contact and that castor angles of steered wheels are such that the centre of the road-wheel contact point trails the projection of the road-wheel-steering axis intersect with the ground in the primary direction of travel, then front wheel steered vehicles will return to straight-line travel if the steering wheel is released (stable) whereas rear wheel steered vehicles under the same circumstances will cause the steering angle of the wheels to be increased to the maximum that the steering system will allow (unstable) due to centripetal forces acting on the force-moment created by the trail of the wheel contact point behind the wheel-steering axis intersect with the ground. Thus the faster the rear-wheel steered vehicle travels the more unstable it becomes. Simples!
Jeff Bishop, Derby