Re: spider alignment

[James Montebello <jamesm@domain.elided>]

On Wed, 16 Oct 2002, Wayne Abbott wrote:
> Camber is the number of degrees from true vertical.  Positive camber is when
> the top of the tire is further from the center of the vehicle than the bottom,
> reminds my of my uncle's old tractor, and negative camber is the opposite, the
> top of the tire is closer inboard toward the center of the vehicle and the
> bottom is further from the center of the vehicle.

Negative camber is typically dialled in to compensate for body roll,
and because tire distortion from cornering forces.  The ideal is to
have the outside front tire, when loaded by cornering, to have equal
"pressure" across the contact patch, thus spreading the load as much
as possible.  This is usually done with zero to very slight negative
camber (tire distortion).  On a car with a live-axle or DeDion tube,
like most Alfas, camber can't be changed at the rear, and the tires are
always at zero camber.  Cars with struts, like the 164, have to have
a bit more negative camber to compensate for camber changed caused by
roll and suspension compression, than do cars with A-arms, like nearly
every other Alfa sold in the US.


> Toe angle is the number of degrees from straight ahead, toe in and the front
> of the tires point toward the center line of the vehicle, toe out would be
> when the fronts of the tires point away from the center line of the vehicle.
> Rear driven vehicles usually have toe in because the rear wheels push the car
> and the car drags the front wheels along.  This drag causes the wheels to move
> rearward slightly and ending up pretty much pointing straight ahead.

Toe has all kinds of interesting dynamic effects, but for most non-racers,
the chief effect is straight-line stability.  A car with zero to toe-in
will be directionally stable.  A car with toe-out will not.  However,
in a hillclimb or autocross, a bit of toe-out may reduce understeer
and generally make the car turn in better.  In a road-racer, the same
setup would probably be dangerously unstable.  On a street car, it would
definitely not be a good idea.  Too much toe (in or out) causes excessive
tire wear, so street cars are usually set up to have a touch of toe-in
statically, which will give zero toe dynamically.  The exception is front
drivers, which tend to "toe in" the driven wheels, so setting them up
statically with zero toe is better.

Toe at the rear is, again, not adjustable on most US Alfas.  Toe out at
the back is a recipe for sudden snap oversteer, so it's usually avoided.

james montebello
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