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TIRE CARE
ALIGNMENT MEASURES
Wheelbase
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.
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).
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 aster 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.
Camber
Viewed from in front of the vehicle, camber describes tilt of the tire from
vertical. A tire 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 tire's contact with the road's surface under
given loaded cornering conditions. Because a tire'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 tires (heavily loaded) will need to have a statically set negative
camber. If driving is on highways where tires 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 tire wears across its treadface. A tire
wears most at the point(s) where the majority of the vehicle's load rests. A
properly set camber maximizes a tire'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.
Toe
If you were able to view the front tires 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 tires and the rear
edge is called toe. Toe describes how close to parallel the two tires are, and
whether they are toed-in (closer at the front of the tire) or toed-out (closer
at the rear of the tire). The goal of toe is to provide proper tire 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 tire
tend to push the front tires 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, hey 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 tires properly inflated, toe-in will result in additional understeer for
the car. In a corner the inside front tire will turn at less of an angle than
the outside tire. Additionally, excessive toe-in will result in premature tire
wear through feathering, and increased fuel consumption. Conversely, toe-out
will result in additional oversteer for the vehicle. This occurs as the inside
front tire turns at a greater angle than the outside tire. Thus, in a corner,
the inside tire is trying to turn even more than the heavily-loaded outside
tire. Excessive toe-out will also result in premature tire wear due to
feathering, and increased fuel consumption.
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