Re: [AMC-list] suspension 101

[tom jennings <tomj@xxxxxxx>]

Caster changes won't affect handling very much. Camber changes do though.

Looking at the suspension from the front towards the rear, the upper arm and
lower arm are horizontal (or should be) and parallel when static. At the
outer, knuckle (wheel) end, the arm joints (lower ball joint, upper
trunnion/ball joint) are in line vertically (or very close; the static
camber range around +1 to -1 degrees more or less).

However, the inner (chassis) end of the pivots are NOT vertically aligned;
the upper arm is shorter than the lower arm, so the pivot is closer to the
tire at the top and the lower arm is underneath, in the x-member.

When you see this, the rest is easy: when an arm moves, the longer arm draws
a bigger circle than the short arm. They're of course connected by the
steering knuckle, so they move together. So when the tire goes up (car goes
down) the TOP of the tire moves IN and the camber (tire verticality) goes
negative (tire tilts in towards the car).

Body roll makes things more complicated. On some cars (early American) the
soft springs and no anti-roll bars makes the body tilt comically to the left
when you make a right turn, but in that case the camber goes very negative
(tire tilts inward at the top outward at the bottom, it's like putting your
foot out) so it handles far better than you'd expect (roll center is crazy
high etc so it's no sports car).

But some cars (my Hornet) and esp. those with stiff springs, or with
sufficient torque, the roll geometry is such that the camber goes POSITIVE
or doesn't change and the car "snowplows" through turns.

Getting cars to handle right is a PITA.




The changing strut rod "length" (the radius it follows) also changes
(increases) toe-in, and that would have a bigger effect than camber in
turns. it also twists the lower arm, which is why the lower arm bushing is
pressed into ONE SIDE of the arm only, it floats in hte other half of the
arm. The forces there are fairly small and it's fail-safe, hardly anyone
knows about that detail and it never matters; it was John Elle who pointed
it out in this list some years back.
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