<VV> Bump Steer

[Brent Covey]

Hi Andrew

> To avoid bump steer you would want to have a
> backspacing of 5.25 to keep the wheel centered on the
> same plane as the original but you will have
> interference with the suspension components?

Strong anti-bump-steer is engineered into the Corvair front suspension in
its stock configuration, the geometry of the Corvair was adopted by other GM
lines starting with the 1968 Buicks. Buick trumpeted the change as
'Accu-Drive" in thier advertising for a few years after '68 introductions.

Pretty much any suspension ride height decrease or tire width increase will
kill this good geometry and add to the disturbing forces. Losing the
benefits of the original geometry isnt the end of the world, they still
drive fine but it makes them less than *perfectly* stable and free of twitch
and makes straight line driving require slight attention where a stock car
on factory tires could run for literally miles with your hand off the wheel
originally if the alignment and road was good. Corvairs were reknown for
this attribute when they were new.

The factors that kill bump steer are related to the changes that happen in
the suspension alignment as the wheel moves up and down in the body, mostly
and to a lesser degree to the narrow original tires and the charachetristic
of a bias ply tire carcass to distort and sort of smother any obstruction
instead of slamming right into it like a belted tire. Bias ply tires have
some interesting characteristics but one of them is they will tolerate
unbeleivable impacts which tend to be absorbed fairly effectively in the
tire body itself, without imparting extreme forces into the steering linkage
and suspension, you could drive over a standard city square cornered curb in
the region of 50 mph without very much potential for loss of control, you'd
literally be likely to knock the spindle off before there was a problem from
the drivers seat. The spindle geometry intersects the pavement at the road
surface in line with the brake backing plate on stock height tires,
approximately.

The usual aim of anto bump steer geometry is to maintain driver control in
emergencies such as whenn you have launched the car right off the ground,
and reduce fatigue from twitches on one wheel bumps. Speaking for a stock,
new Corvair on the original tires, on small one wheel bumps, the tire
striking the bump will move upwards slightly from normal ride height, and
will alter its geometry slightly- it will steer outboard slightly on that
wheel, and gain positive camber which reduces the track on that side
nominally- this helps lessen the leverage of the disturbance, and reduces
both the twitch in the wheel and the need to correct the course of the car,
when the bump passes you're still on your original course and seldom feel
much thru the wheel. In contrast to FWD cars, most RWD GM cars hang the tire
cantilever past the spindle, which means the entire tread is outboard of the
spindle axis which runs about 8 degrees tipped inwards to the car center.
This was done to impart strong self centering for the steering from the
forces trying to splay the tires from the car motion, but at parking speeds
the tires roll around the spindle which significantly reduces parking
effort. On FWD vehicles and many European makes, the steering axis
intersects the road somewhere around the center of the tire tread, which
kills torque steer impulses in FWD and is inherently less effected by any
road disturbance or power/braking force but requires greater steering effort
at low speeds, and requires things like greater steering axis inclination or
high caster for straight line stability. One upside of the centered axis is
rack and pinion steering isnt a problem on these vehicles as they have very
small camber changes and there isnt much feedback via the steering under any
circumstance, so the poor isolation of the rack and pinion style linkage
isnt an issue even in quite heavy vehicles. The worm and setor recirculating
ball type like Corvairs and most older cars use provides an added degree of
isolation and driver control as the sector is damped because it has a hard
time driving the worm, where the worm can turn the sector very easily. On
power assisted steering the fluid around the assist piston functions well as
a hydraulic damper as well.

If the Corvair is launched right off the ground and does a full compression
slam to the stops, the tires go to extremely negative camber, and very toed
out to carry the weight on the inboard side of the tread and dissipate the
energy via tire scrub to assist the driver in maintaing control over the
car. GM puts thier energy into making sure the driver has the maximum
leverage over steering at all times and that the cars natural geometry
changes do what they can to impart stability in a straight line in every
case of disturbance by bumps of any intensity.

Suspensions are designed for the loaded height of the car -'design height'.
Design height is the trim angle and suspension compression of a fully loaded
car, which is level and quite low in a Corvair, the tops of the original
tires are about even with the tops of the wheel openings at design height.
If a car is not loaded heavily normally, you will still have good geometry
from a anti-bump-steer perspective if the springs are changed to lower the
car as you usually operate it to around the design height the factory
intended for a loaded car. The softer the original suspension, the higher
the unladen car sits of course. Late Corvairs have the same wheel rates as
Cadillac, somewhere in the region of 75 lbs/in front and 160 in/lbs rear.
These are increased to around 140/200 on F41/Z17 equipped cars with the
suspension option. As a result, the F41 type cars sit about an inch lower at
the back and an inch and a half lower at the front unladen. Loaded to max
weight at design height, both cars sit pretty much the same.

The wider, shorter or more radially stiff the tire the greater its
propensity to transfer road disturbances into the suspension and steering.
The moment you drop below design height, the car starts losing its self
correcting ability designed into the suspension. There are some other
changes like steering effort increases as well.

Generally the differences are fairly minor and most go unnoticed but you do
have to add any extra tire width outboard of the original and this in tandem
with a large drop in suspension height could create some bump steer. Trying
to maintain tire height, maintaining trim at at least original design
height, and slight positive camber on the front tires will recover most of
the original inhibiting factors against bump steer, and usually you could
get satisfying results. Theres no real absolutes aside from if you drop ride
height far enough you'll lose the original geometry against bump steer. This
could be compensated for by raising the inner pivot of the upper control arm
upwards or using a shorter front suspension spindle, as well. The
track/camber change that induces self correcting for bump steer is basically
the upper arm swinging thru its maximum arc away from the car jast as the
wheel strikes the bump. This was the basis of Buick's Big Change in '68.

If you have any road test pictures of a normal Detroit '60's car (Fords are
especially hilarious, try to find a Fairlane) at full stretch in a corner
you'll see from the wild angles of the front tires that the Corvair looks
much more like a modern car and has significantly different angles from the
usual.

Anyhow hope thats some background, a little bump steer isnt a real problem
for a car doing autocross but if you just are after the 'look' of diferent
wheels and tires and ride height, try to be senstive to the disturbances
you're causing and select your new tires and springs with minimizing the
geometry changes if you'd like to retain the majority of the original
excellent geometry.

Brent Covey
vancouver BC