Lesson 1/Learning Event 2
tilted forward so a straight line drawn through it hits behind the point
where the tire contacts the ground, the wheel is said to have negative ()
caster.)
On a vehicle with axle suspension, caster is obtained by the axle being
mounted so that the top of the steering knuckle or kingpin is tilted to the
rear. On independent suspension, the upper pivot point is set to the rear
of the lower pivot point.
The caster angle is measured in degrees. The angle is shown by drawing one
line straight up and down and then drawing a second line through the center
of the kingpin or pivot points. The caster angle is the angle formed at the
point where the two lines cross as viewed from the side of the vehicle.
From the above description of caster, we can say that caster or positive
caster is the backward tilt of the kingpin at the top. Negative caster is
just the reverse, with the kingpin tilted forward at the top.
Positive caster causes the vehicle to steer in the direction that it tends
to go. This is called an automatic steering effect. For instance, the
forward momentum of a vehicle tends to keep wheels with positive caster in
the straightahead position. After rounding a turn, this causes the wheels
to return to a straightahead position if the driver releases the steering
wheel. This automatic steering effect is also called selfrighting action
or selfcentering action.
The automatic steering effect of caster can be shown by picturing a bicycle
with an excess amount of caster. As the wheel is pushed forward, it resists
movement; so it pulls back at the point where it contacts the road. Since
the kingpin is pointed in front of the tire contact, the wheel pulling back
tends to keep it in the straightahead position.
Other forces besides forward momentum react with caster so that the
automatic steering is not always perfectly straight ahead. Any force that
is pushing on the side of the vehicle tends to pivot the wheels in the
direction of the force. For this reason, positive caster tends to cause a
vehicle to steer down off a crowned road and in the direction of a
crosswind. Some passenger cars are designed to have negative caster so that
just the opposite will happen; that is, they will tend to steer up a crowned
road and against or into a crosswind.
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