TERMINOLOGY
CAMBER
 Viewed
from in front of the vehicle, camber describes tilt of
the tyre from vertical. A tyre has negative camber when
its top inclines toward the vehicle. Positive camber
occurs when its top tilts away from the vehicle. Camber
is measured in degrees, and varies by car model and
year. A wheel's camber angle should be adjusted to
maximize a tyre's contact with the road's surface under
given loaded cornering conditions. Because a tyre's
camber changes slightly as its suspension moves during
travel, the static angle at which the camber is set will
depend on driving habits. If a driving style entails
hard cornering, outside tyres (heavily loaded) will need
to have a statically set negative camber. If driving is
on highways where tyres are mainly subjected to lightly
loaded cornering conditions, the static camber setting
should be zero or slightly positive. Camber plays a
large role in determining both the overall handling feel
of a vehicle and how a tyre wears across its treadface.
A tyre wears most at the point(s) where the majority of
the vehicle's load rests. A properly set camber
maximizes a tyre's contact patch, leading to even wear.
Excessive negative or positive camber has an adverse
effect on treadlife by causing premature outer or inner
shoulder wear. Q: Should I try adjusting caster or
camber first to improve handling?
A: Caster, and here's why:
1. Camber doesn't improve turn-in, positive
caster does.
2. Camber is not good for tyre wear.
3. Camber doesn't improve directional stability.
4. Camber adversely effects braking and
acceleration.
CASTER
 To
determine caster, first draw an imaginary line through
the upper and lower ball joints. The angle made by this
line (the steering axis) with another imaginary line
drawn perpendicular to the ground (the centerline) is
the caster. If the angle between the steering axis and
centerline is toward the front of the car, caster is
negative. If toward the rear of the car, caster is
positive. Measured in degrees, caster plays a large role
in determining both steering feel and high-speed
stability. The goal of proper caster alignment is to
achieve optimal balance between low-speed steering
effort and high-speed stability.
An increasingly positive caster enhances high-speed
stability, but increases low-speed steering effort. An
increasingly negative caster decreases low-speed
steering effort and high-speed stability. For cars with
power steering, an increase in low-speed steering effort
increases the rate of wear in the power steering system.
With most suspension designs, there is a trade-off
between caster and camber angles at the extreme limits.
Q: Is there such a thing as too much caster?
A: No, and here's why:
1. Maximize tyre contact patch during roll.
2. Improve turn-in response.
3. Increase directional stability.
4. Maximize tyre contact patch during braking and
acceleration.
5. Improved steering feel and self-center increases
dynamic negative camber (on turn).
TOE
 If
you were able to view the front tyres of a vehicle from
above the car, you would expect them to look exactly
parallel to each other. In fact, they rarely are. The
difference in distance between the front edge of the
tyres and the rear edge is called toe. Toe describes how
close to parallel the two tyres are, and whether they
are toed-in (closer at the front of the tyre) or
toed-out (closer at the rear of the tyre). The goal of
toe is to provide proper tyre wear through various
driving conditions. The amount of toe your suspension is
set to varies by the drive layout of your vehicle,
driving preference, and car's handling characteristics.
On a rear-wheel-driven car, acceleration forces on
the tyre tend t o push the front tyres back slightly in
the wheel well. Static toe-in will result in a zero-toe
situation at speed. For a front-wheel-driven vehicle,
the front wheels will pull themselves forward in the
wheel wells under acceleration. This happens because as
the (driven) front wheels claw for traction, they pull
themselves forward, dragging the rest of the car along.
For this situation, static toe-out will result in a
zero-toe condition at speed. Assuming that the rest of
the suspension is correctly aligned and maintained, and
the tyres properly inflated, toe-in will result in
additional understeer for the car. In a corner the
inside front tyre will turn at less of an angle than the
outside tyre. Additionally, excessive toe-in will result
in premature tyre wear through feathering, and increased
fuel consumption. Conversely, toe-out will result in
additional oversteer for the vehicle. This occurs as the
inside front tyre turns at a greater angle than the
outside tyre. Thus, in a corner, the inside tyre is
trying to turn even more than the heavily-loaded outside
tyre. Excessive toe-out will also result in premature
tyre wear due to feathering, and increased fuel
consumption. TRACKING
Relates to the distance of each wheel to the vehicle's
centerline. Each wheel should be equidistant from this
centerline so that, as the vehicle moves straight ahead,
wheel tracks are parallel to the vehicle's centerline
(e.g., the axle should not be cocked).
WHEEL BASE
Refers to the distance between the front and rear axles
measured at the hub centers. This distance should be
equal on both sides of the car. If not, some suspension
components are worn, bent, or damaged.
DYNAMIC Vs STATIC
Typically used when discussing wheel alignment and
geometry. By this, we are referring to the difference in
alignment angles between a stationary and a moving
vehicle. That is, the same vehicle will typically have
different caster, camber and toe readings when it is
moving compared to when the alignment was done in static
form in the workshop. In an ideal world, all wheel
alignments would be done on a dynamic wheel aligner but
these are expensive and quite rare. This concept is very
important, as the only suspension angles that really
matter are those present while the vehicle is moving
(dynamic). What is done to the vehicle's alignment while
the vehicle is stationary, (static) is a process of
trying to predict the levels of change while the vehicle
is moving and setting the angles according to these
predictions.
 STEERING
AXIS INCLINATION Steering Axis Inclination (SAI)
is the angle formed by the rotational axis of the front
suspension in relation to a vertical angle through the
center of the wheel spindle. SAI causes the wheels to
return to the straight ahead position after cornering,
and is therefore a steering control angle. It is not
adjustable, and if incorrect, requires replacement of
suspension parts to correct it.
THRUST ANGLE
 Thrust
Angle is the angle formed by the two rear wheels in
relation to the geometric center line of the vehicle. If
the angle points to the right (the driver's side), it is
+ (positive). If it points to the left, it is -
(negative) . This is a steering control angle, because
the car will always travel in the direction that the
rear wheels are pointed. This may be adjustable if the
rear Toe is adjustable. On a fixed axle, it indicates
that the rear axle is out of line and may be
correctable. |
