andrew wrote regarding 'Re: [OT] Calculating torque' on Wed, Jan 18 at
22:38:
> Well, wouldn't a scaled down version of a car's rack and pinion
> steering w/ tie rods accomplish this?  It would allow you to set
> toe-in (as in FWD cars), which I believe allows them to seek center
> by themselves..

There are a few minor things wrong with this. :)  First, toe-in is
adjustable on almost all RWD cars too (I'd say all, but then there'd
be *something* obscure which isn't).  Second, toe-in affects stability
at speed, and to an extent, steering effort.  It has little effect
(within a reasonable range of adjustment) on self-centering.  The
centering is mostly controlled by the caster angle, which isn't
adjustable (well, not easily adjustable) on most cars.

> Also, the angle to the center would not need to be straight inline
> because the inner and outer tie rod ends provide for some amount of
> angular difference out of a straight line. (think, a wheel turns
> around the ball joints and the tie rod is at the end of the rim
> inside, and the angle the tie rod makes to the straight-line of the
> axis of the rack is not always constant)

Here again, the angle between the tie rod ends and the steering rack
is pretty important when turning, as the thing being pushed/pulled by
the tie rods is moving in an arc.  When you change the angle of the
tie rod arms with respect to the steering knuckle, you mess with the
rate of change across the arc, and can greatly affect steering effort
as you approach the extremes.  If you don't get the rack in the right
place with the proper length tie rods, you'll screw up the ackerman
angles of the front wheels and end up dragging the inside or outside
tire while turning, which results in jerky, imprecise steering.  It's
like bump steer, but different (though they're both caused by the rack
positioning and tie rod length relativel to the steering knuckes).
There's some freedom to play around, but not a lot.

Basically, if the vehicle handles acceptably now, it would probably be
best to try to maintain as much of the original steering geometry as
possible - maybe by attaching a linear motion device to the existing
pitman arm / idler arm / steering rack.

--Danny
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