Readers letters
Belatedly I should like to join the discussion about rear-wheel steering. In the June issue of PE, Ron Ayers mentions his consultation with my much missed friend, the late Professor Dave Crolla. Dave discussed this issue with me at some length. Dave’s initial view echoed that of Robin Sharp’s (Letters, PE July), that there is no theoretical difference between front and rear-wheel steering, but I had (still have) misgivings. We concluded that there was one important difference in route negotiation between a road-going car and Thrust SSC; a road-going car has to negotiate bends whereas, apart from during low-speed manoeuvring, Thrust was intended to move only in straight lines. Thus Thrust’s steering was intended only to ever provide small trimming corrections, and besides, I guess the aerodynamics of the vehicle provided a powerful straight heading tendency.
Incidentally, it may be misleading to infer the stability of a vehicle designed for rear steering from the behaviour of a conventional car driven in reverse. This is because the steering system of a front-steered car is designed (primarily with castor angles) to cause the steering to self-centre during forward motion. In reverse the steering may have a tendency to turn away from centre and this alone may give a feeling of instability.
However, there is a fundamental difference between front and rear steer in the way in which a turn is initiated. For a front-steered vehicle, the front wheels are steered in the direction of the desired turn. This causes the front tyres to generate lateral forces towards the centre of turn. As the turning motion develops, the body of the vehicle “noses” into the turn causing the rear wheels, after a short delay, to be steered into the turn, whereupon the rear tyres develop forces towards the centre of turn. Neglecting issues of understeer/oversteer, this behaviour is essentially stable, since all tyre forces are directed towards the centre of turn.
For a rear-steered vehicle, a turn is initiated by steering the rear wheels away from the centre of turn, causing the rear tyres to generate lateral forces away from the centre of turn. As the turning motion develops, the body of the vehicle “noses” into the turn causing the front wheels, after a short delay, to be steered into the turn, whereupon the front tyres develop forces towards the centre of turn. At once one can see that there is now a couple turning the vehicle, and this is an unstable situation. One may argue that the driver can adjust the steering angle of the rear wheels to modify the turning behaviour. But notice that there is a small delay between any change of rear-wheel steer and any change in front-wheel steer angle. It is difficult to see how a driver could establish, except at low speed, a steady-state turn where all tyre forces act towards the centre of turn. I submit that as vehicle speed increases this situation becomes increasingly difficult for a driver to control. Because of the delay in vehicle reaction, the driver will almost certainly input too-large steering changes to which the delayed vehicle-responses will become increasingly large. It is likely that the vehicle response will be cyclically divergent around the intended mean path, until eventually loss of adhesion is experienced at one end, or the vehicle overturns, or both. Practical experience tends to support this view.
I look forward to Robin Sharp’s response!
John Whitehead
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