The force that "centers" the steering is "caster". Caster is the vertical
angle of the upper and lower ball joints/pivots. The more the upper pivot
point is back from the lower, the more caster angle. The force that keeps
the wheel centered (and returning to center) is actually the fact that when
the wheel is turned, the tire/wheel has to lift that side of the car
slightly. That's why you want both sides at the same angle when doing an
alignment, otherwise it will pull to the side with the higher angle. Think
of the angle of the fork tube on a bike, same principal.
Gary
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