Stability of rear-steered vehicles

The big question is “how fast must they travel before they cannot be controlled by a human pilot?"

In common with Ron Ayers, Jeff Bishop seems convinced that rear-steered vehicles are unstable. He promotes the idea that they become more unstable as they travel faster. In the context of very-high-speed vehicles, the big question is “how fast must they travel before they cannot be controlled by a human pilot?”.

Bishop’s case rests on the dynamics of a supermarket trolley with freely-castoring wheels only at the rear. Ignoring the inertia of such wheels, they will align perfectly with the direction of travel and will generate no lateral forces. Standard theory (and intuition) then verifies that such a vehicle will spin round in a turn – the front can manage it but the rear cannot.

However, an on-board pilot would have no control, since he (she) has no way of influencing the steering wheels. It should be clear that the supermarket trolley is not a proper model for vehicles that must be steered by someone travelling in the vehicle, as opposed to someone outside the vehicle, pushing it along.

If a non-compliant steering linkage is added to the trolley and the pilot fixes the steering wheel position, is there any reason to think that the trolley will not turn like any other vehicle? Fork-lift truck drivers may well have a view.

Robin Sharp, Visiting Professor, University of Surrey

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