Alan,
Bump steer on the front end is caused by the steering linkage to wheel
tie rod not being parallel to the car horizontal plane. The inner tie
rod point moves with the car ride height in the vertical plane but as
the wheel travels up & down, the outer tie rod end moves through a
slight arc in the opposite direction. This combination of changes causes
the effective tie rod length to change & causes slight toe in or toe out
changes which make the wheels steer a different path than the one
selected by the driver. Eg. If the right spring compresses on a bump the
body mounted end of the rod moves down & the wheel end moves up. Since
the rod is a fixed length the wheel can only change steer direction when
this happens. Whether this causes the wheel to toe in or toe out depends
on whether the suspension is designed to have the tie rods attached in
front or in back of the wheel pivots. If the rods are horizontal at the
car's neutral ride height the arc change & resultant effective tie rod
length change will be minimal. Longer tie rods have less effective
length change than shorter rods but we are usually stuck with the
factory design.
This is not usually a problem with factory designs. An exception would
be a car that had been seriously lowered in the front. This lowers the
inner tie rod fixing points while leaving the outer points at the
original height. This causes the wheel to self (bump) steer more than
the normal amount. A remedy is to bend the outer arms down a bit to get
the outer rod anchor points back to horizontal alignment. If the
steering arms are purposely bent they must be reheat treated & checked
for cracks.
Another remedy is to replace the tie rod ends with "Heim type" joints
which can be adjusted for height with spacers between the joint ends &
the steering arms.
Similar things can happen at the rear for different reasons. A whole
different part of the game.
As far as I know, this is not a closely guarded secret, but usually,
only racers who are after the last little bit of handling perfection or
have made large suspension geometry changes go to the lengths involved
in measuring & correcting bump steer. Pro racers are very fussy about
handling quirks that less serious drivers would never notice.
Dave Russell
BN2
Alan Bromfield wrote:
> Oh learned team - I am intrigued.
>
> In the June issue (Vol 36, No.5) of AHC-USA magazine, there is an
> interesting article from Chris Kellner describing the build of 'Annie'
> the Red Racer.
>
> Within the pictures there is a shot of Eddie Miller bending a steering
> arm to "eliminate bump steer". Now, I had got used to the excitement
> generated when pushing hard through the twisties, but never really come
> to terms with the unpredictable reactions in the bumpy bits. I hadn't
> considered bump steer, figuring that the handling was a feature of the
> steering box or the 'period' suspension.
>
> Is bump steer a real factor in these cars? Can it be reduced in the way
> that Eddie has adopted? Is it a closely guarded secret held only by
> those racers that want to keep an edge, or can the arcane wizardry be
> shared in public without shame?
>
> Go on - I can take it.
>
> Cheers.............AlanB
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