Front-end geometry is the term manufacturers use
suspension arm and the frame to obtain the desired
to describe steering and front wheel alignment. Front
caster. If the knuckle pivots (kingpins) are tilted
end geometry includes PIVOT INCLINATION,
forward, the caster is said to be negative (fig. 2-14).
WHEEL CASTER, WHEEL CAMBER, TOE-IN, and
The caster is said to be positive when the knuckle
TOE-OUT. These terms refer to angles in the front
pivots are tilted backward. Most vehicles have positive
wheel alignment that may change because of driving
caster, but some modern vehicles have negative caster.
over rough terrain, striking stationary objects, accident
Caster is measured in degrees and varies from
damage, and wear.
approximately 0.5 to approximately 3 on modern
vehicles. The technical manual for the vehicle should
P I VOT I N C L I NAT I O N ( s o m e t i m e s c a l l e d
list the correct settings.
kingpin angle) is the number of degrees that the
kingpin is tilted toward the center of the vehicle from a
WHEEL CAMBER is the number of degrees that
vertical position (fig. 2-13). Pivot inclination keeps the
the wheels are tilted in or out at the top (fig. 2-13).
wheel spindles pointed outward and in line with the
Wheels having camber are closer together at the
axle, and helps to make steering easier.
bottom than they are at the top. Camber, together with
pivot inclination, reduces side thrust on the kingpin
WHEEL CASTER is the number of degrees that
bearings in the steering knuckle and support, thus
the steering knuckle is tilted to the rear, or to the front.
permitting easier steering and less wear of parts.
Caster tends to keep the front wheels pointed straight
Camber angle, in today's vehicles, very seldom
ahead and brings them back to a straightforward
exceeds 1, and is obtained by tilting the wheel
position after a turn. The front wheel of a bicycle is
spindles slightly downward on the steering knuckles.
castered and permits the rider to steer without using his
Camber brings the wheels perpendicular to the surface
hands. When the castered wheel of a bicycle is turned
of the road, permitting better rolling contact. Wheels
from the straight-ahead position by leaning sideways,
that have camber must also have toe-in and toe-out.
the front end is slightly raised. After the turn is made,
the weight of the bicycle forces the front end down and
NOTE: In modern vehicle design, greater pivot
helps straighten the wheels.
inclination reduces the need for excessive camber.
Caster in automotive vehicles with leaf-type
TOE-IN is the number of inches that the front
springs is obtained by inserting wedges or shims
wheels point in toward the center of the vehicle (fig.
between the front axle and the spring so that the
2-15). When forced to follow a straight path by motion
steering knuckle pivots are tilted slightly backward
of the vehicle, cambered wheels tend to slip away from
from the vertical. However, most modern automotive
each other. But toe-in wheels tend to travel toward each
vehicles do not have leaf-type springs. Vehicles
other and, therefore, balance the effect of camber.
without leaf-type springs use shims between the upper
If your shop has floating turntables, use them to
check steering errors. Run the vehicle up on the tables,
CAMBER
CAMBER
and then turn the front wheels with the steering wheel.
Each floating table turns with the wheel on it and
registers the angle of the turn. When one wheel turns
PIVOT
INCLINATION
20, the other should turn about 23, or to the
specifications for your unit.
Before checking front wheel alignment, be sure
PIVOT AXIS
that the front tires are properly inflated and that
WHEEL
KING PIN
AXIS
AXLE
NEGATIVE
CASTER PIVOT
POSITIVE
PIVOT LINE
CASTER
ANGLE
LINE
WHEEL
ANGLE
VERTICAL
SPINDLE
VERTICAL
STEERING KNUCKLE
VERTICAL LINES
FORWARD
FORWARD
CENTER OF
TURNING RADIUS
TIRE CONTACT
ROAD CONTACT
ASf02014
POINTS
ASff02013
AS02013
Figure 2-13.--Pivot inclination and camber.
Figure 2-14.--A. Negative caster; B. positive caster.
2-11
