Back to the bicycle example, what happened when you turned the handlebars
backwards and let go? The wheel returned violently to the straight-ahead
position and sent you flying over the handlebars because of the extremely
positive caster angle created by turning the handlebars backwards.
Excessively positive or negative caster causes the vehicle to track right or left.
Failure of the steering wheel to return to the straight-ahead position after making
a turn is another indication of caster misalignment. Also, maladjusted caster
causes the vehicle to wander while driving down the road. In extreme cases on
front-wheel-drive vehicles, excessively negative caster on one side will cause a
condition called “torque steer.” In this condition, when you accelerate quickly
from a stopped position, the vehicle will pull hard in the direction of the negative
caster angle. Quite often adjusting the caster angle will eliminate “torque steer.”
Caster angle is corrected by either mechanical adjustment or by bending a
suspension part.
Four-Wheel Alignment
For most of today’s cars, the alignment angles can be adjusted on all four wheels
to gain maximum vehicle control and tire longevity. In a four-wheel alignment,
the angles on all four wheels are measured and then compared to manufacturer’s
specifications. Adjustments are made by mechanical adjustment. When a car
capable of four-wheel alignment is adjusted properly, the vehicle tracks straight,
handles better on the road, and the tires last longer. On vehicles with a rear
straight axle, the adjustment is limited to caster and camber. On vehicles with
independent suspensions, often the toe angle is also adjustable via adjustable
trailing arms.