<VV> Bump Steer

[Andrew Allen]

Thanks Brent, I will get as close to original ride
height as I can by using a low profile tire on 15x7
rims with 5 inch backspacing. Looking around the
various websites to find the right radius tire. Most
seem to be using 205 width in 50 or 60 profile. I
might go to 14x7 after reading your post...Thanks!

Andy
'66 A/C Corsa

--- Brent Covey <brentcovey at hotmail.com> wrote:

> 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 
=== message truncated ===


__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around 
http://mail.yahoo.com