Jerry Kaidor tells us:
>My TR2 has ZERO CASTER, which means that the only thing making the car want
>to go straight, is the silentblock bushings in the steering gear ( GAAK ).
And John Taylor says:
>OK, so the Moke has positive camber. Why do racing cars have negative camber?
>Does this give better steering, or does it stop the wheels from being pulled
>under the car (| --> \ --> _) during high-speed turns, or is there some other
>reason?
In reverse order, here are some responses.
Negative camber is indeed desirable because of cornering forces. Given that
the sidewall of any tire isn't totally rigid (Is "totally rigid" redundant?),
the tire will always roll over in a turn. This deforms the tread by pulling
it toward the inside of the turn where it hits the road, thus lifting the
inside of the tread. To counter this, you want the tire to lean inward so
that the tread flattens down onto the road when it is laterally loaded.
Concerning zero-caster, there is another factor inducing self-centering of
the steering, one that is more significant, especially so at higher speeds.
In a turn the lateral deformation of the contact patch increases from front
to rear. The reason is that the tread comes into contact with the road on
the tangent along which the tire is pointing, and it is then locked onto the
road surface while the tire itself moves progressively further way from that
tangent. This lateral displacement is the result of the slip angle, i.e. the
fact that the tire isn't pointing in the direction it is actually moving.
When the tread finally lifts off the road, it snaps back to center. This
gives the deformation pattern the general shape of a sawtooth, ramping up to
its maximum value near the end of the contact patch.
Now, steering self-centering (or feel) is the sum of the torques around the
steering axis integrated over the contact patch. Zero caster nominally places
the steering axis at the center of the contact patch. Deformation in front of
this axis torques the steering into the turn and deformation behind this axis
torques the steering back to center. Since the deformation increases from
front to back, the total torque from the back half of the contact patch, i.e.
to re-center the steering, is always greater than that from the front half.
Hence, you get self-centering even when the caster is zero. Positive caster
just increases this effect.
Positive caster gives you some other effects, mostly a change in camber as
the wheels are steered. (I supposed we should call this vertical angle, since
camber is properly defined only when the wheels are pointed straight ahead.)
When the wheels with positive caster are steered, the outside wheel will lean
into the turn, effectively decreasing its camber. The inside wheel will lean
to the outside, increasing its camber. But since the inside wheel doesn't take
as much of the cornering load, this camber increase isn't such a loss. One bad
side-effect of positive camber is that the vertical positioning of each wheel
changes, with the outside wheel moving up and the inside wheel moving down,
increasing the chassis roll to the outside. This can be countered with the
pitch of the threads on the trunnions. Using right-hand threads on the left
and left-hand threads on the right, the wheel will move in the opposite
direction vertically, keeping their positioning more constant.
This comcludes today's lesson. There'll be a quiz next week. :-)
Jim Muller
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