What are the significant causes of excessive tire wear?
Uneven or accelerated tire wear can be caused by wheel misalignment,
worn suspension and steering components, and/or improper tire
inflation.
The most critical alignment angle with respect to tire wear is toe.
Camber is also important and can be affected by caster. Misalignment
is usually caused by worn, loose or bent suspension and steering
parts (bad tie rod ends, idler arms, control arm bushings, ball
joints, struts, etc.), but can also be due to spring sag or improper
ride height.
With tire inflation, underinflation is just as bad as overinflation.
Both can cause uneven tire wear. Underinflation can also make a tire
run dangerously hot.
For a tire to roll down the road with the least amount of resistance
and maximum directional stability, it must be aligned with the road,
parallel to the other three wheels on the vehicle, square with the
chassis, and properly inflated.
Rolling resistance is affected by a number of things, one of which
is the straightness of the tire with respect to the direction of
travel. If the tire is skewed slightly to one side or the other
(toed in or out), it will scrub as it rolls. Scrub increases rolling
resistance and also wears away the tread, leaving a feathered wear
pattern.
Toe refers to the parallelism of a front or rear wheel to its
companion on the opposite side. From a bird's-eye view, all four
wheels should be parallel to one another and pointing straight
ahead.
Toe is measured by comparing the distance between the front edges of
both tires on an axle to the distance between the trailing edges. If
the distance between the front edge is further apart than that at
the rear, wheels have toe-out.
If front is closer together than rear, wheels have toe-in. Toe
settings may be specified in inches, millimeters or degrees. Toe is
most affected by worn tie rod ends, a worn or loose idle arm or
center link, or a bent steering arm.
To minimize tire wear, rolling toe should be kept as near zero as
possible. To achieve this, the compliance or amount of give in the
vehicle's steering linkage and suspension must be taken into
consideration.
As a vehicle starts to move, rolling resistance pushes the tires
back. This compresses the couplings and rubber fittings in the
steering linkage and causes the wheels to toe-out slightly. It isn't
much of a change, but most rearwheel drive vehicles call for static
toe-in settings of around 1/16" to 1/8" to compensate.
With front-wheel drive, the situation is different because the front
wheels pull the vehicle down the road. Engine torque causes the
wheels to toe-in under load. Most FWD suspensions have a negative
scrub radius built into the steering geometry.
The tire pivots slightly to the outside of its centerline. This
helps to offset the tendency to toe-in. Even so, most front-wheel
drive vehicles still require toe settings of zero to 1/8" toe out.
Toe also changes as the front wheels turn. Because the inner and
outer wheels don't follow the same path (the inner one follows a
smaller circle than the outer one), the inner wheel must toe-out to
compensate. This is called the "Ackerman Principle of Steering"
after the engineer who invented the idea.
Toe-out depends on the angle of the steering arms with respect to
the steering knuckles and chassis. The amount by which toe changes
is called "toe-out on turns." This angle is also listed in alignment
specs and is checked by turning the wheels on the alignment rack.
If toe-out on turns is off because of a bent steering arm, the tires
will squeal and scrub whenever the wheels are steered.
Camber also affects tire wear. For directional stability, the tires
must be perpendicular (straight up and down) with respect to the
road. Any tilt to the inside or outside will cause a tire to turn in
that direction like a bicycle leaning into a turn. This causes
steering to pull to one side and creates uneven wear across the
tread face.
Camber is the inward or outward tilt of the wheels as viewed from
either front or rear of the car. If wheels lean in, they have
negative camber. If they lean out, they have positive camber.
The amount of camber is always expressed in degrees and is found by
measuring the tilt of the wheels on an alignment rack or by using a
level that attaches to the wheel.
Because camber affects tire wear, the ideal situation is to have
zero running camber on all four wheels to maintain full tread
contact with the road. Like toe, camber changes as the suspension
moves up and down. To keep the wheels vertical once the vehicle is
loaded, a small amount of static positive camber may be required.
Camber should generally be within 1/2 degree side-to-side.
Otherwise, the vehicle will lean toward the side with the most
positive camber.
Camber misalignment can be caused by a sagging spring, a bent strut,
bent spindle, worn control arm bushing, worn ball joint, or
mislocated strut tower (too far in or out).
Camber corrections are made by replacing worn parts and/or by
realigning the suspension using either the factory adjustments
(cams, shims or elongated holes) or aftermarket aids.
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