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From: (Jobst Brandt)
Subject: Re: rear steering, was Re: why ir's stable
Date: 6 Jan 1998 23:03:51 GMT

Matt O'Toole writes:

>> The reason to not have rear steering is the same for all vehicles,
>> even ones that do not balance.  A rear steering vehicle is inherently
>> dynamically unstable.  You might try this sometime with your car in a
>> large empty parking lot by driving backward at about 20mph and try to
>> execute a turn.

> In this particular situation, the primary reason for the instability
> is that the automobile's steering geometry is reversed when going
> backwards: you get negative caster and negative trail.  Instead of
> self centering, the steering wheel moves more off center.  With the
> slightest steering input, the car will try to fling itself around in
> a "bootlegger" turn.

I think you'll find that a forklift truck has no such feature in its
steering and is also dangerous to drive fast with the forks out front.
This is a safety rule that is violated now and then, but generally one
does not "travel" with a forklift, steering wheels to the rear, for the
same reason that one does not do so with other vehicles.

The classic is the fire engine ladder truck.  It is extremely hard to
control at speed and should be rear steered only at low speed in city
streets where curves could otherwise not be cleared with the tractor
alone.  Children learn this early with their toy wagon that dumps them
on the street if thy try to run it backwards.  Engineering starts early.

> What about the recent land speed record attempt with a rear steering
> car?  They thought it would be more stable with the weight at the
> front, and the steering at the rear, like an arrow.

I don't know anything about the steering of that vehicle, but if what
you say is true, it must be analogous to the trailing wheel on an
aircraft since this was a flightless aircraft with aerodynamic guidance
for stability (rudder).

Jobst Brandt      <>

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