In the June article “Record Breaker”, Ron Ayers tells us that “Rear-wheel steering is theoretically unstable, partly because of the mathematics of it …”.
Does anyone know where such mathematics can be found? If it can be found, at best it would provide only a description for the supposed instability and not a reason for it.
The assumptions on which such mathematics might rest require close scrutiny, since there would not appear to be anything unstable about the low-speed operation of a standard fork-lift truck.
There are two standard approaches to vehicle lateral stability analysis. One presumes the steering system to be fixed, the other, that it is free. The conventional wisdom is that cars are steered primarily by control of the steering wheel angle; that is, by position control as opposed to force control.
If a rear-steered car has its steering wheel fixed and the steering linkage is perfectly stiff, the simplest assumption possible, the rear wheels do not steer. For a stability point of view, there is no difference at all between this vehicle and the corresponding one with front steering. If one is unstable, then so is the other. If the steering system is compliant, then, naturally, the design of the steering system must be accounted for in any stability analysis.
The topic is covered in some detail in the book “Vehicle Stability” by Dean Karnopp, Marcel Dekker, 2004, the writer’s review of which can be found in the Journal of Mechanical Engineering Science, Proc. I. Mech. E., 221(C3), 2007, 393-397.
Robin Sharp, Visiting Professor, University of Surrey