Re: bump steer? what is it?

[DeWitt Harrison <de@aztek.com>]

On Wed, 18 Jun 1997 23:00:22 -0500, Joe Rae wrote:

>Can someone tell me what bumpsteer is? 

The front wheels are free to move vertically through their suspension
travel but are still tied to the chassis by the steering linkage. This
linkage has fixed dimensions and cannot accommodate the relative
motion of wheel to chassis. Something must give and it is the steering
angle of the wheels. This is analogous to a tethered balloon being blown
to one side by a cross wind. Since the tether ("linkage") has a fixed length,
the balloon cannot move laterally ("bump") without also being drawn
toward the ground ("steer").

Note that if both front wheels move up and down exactly together,
their respective steering errors are in opposing directions and
will cancel. You will feel the bump steer at the steering wheel when one
front wheel moves much more than the other. The effect is most
severe during cornering when one wheel is in bump and the other
in droop making the bump steer geometry most unfavorable; so
unfavorable, in fact, that the canceling effect can be reversed
and become a reenforcing effect. In hard cornering, the saving grace
is that most of the vehicle weight is on only the outside wheels
and undesirable steering angle movements at the inside, unweighted
wheel have little effect.

Geometric games are played by the experts to minimize this effect
or possibly to use it to advantage. In the case of our cars, the
Audi engineers minimized bump steer at the stock ride height.
Much discussion has taken place here about whether lowering
the suspension has sufficient negative impact on bump steer (and
other handling parameters) to be of concern. Trouble is, each
of us has a different set of criteria so disagreement abounds.

DeWitt Harrison    de@aztek.com
Boulder, CO
88 5kcstq