GMC 1976 ZEO 6083 Maintenance Manual page 603

10- 8
WHEELS AND TIRES
inside half of the tire at the excessive rate . This type
of tire shows much the same appearance of tread
wear as tire wear caused by negative camber .
Second, the transfer of weight may also overload
the outside tires so much that they are laterally dis-
torted resulting in excessive wear on the outside half
of the tire, producing a type of wear like that caused
by excessive positive camber .
Cornering wear can be most easily distinguished
from abnormal camber wear by the rounding of the
outside shoulder or edge of the tire and by the rough
ening of the tread surface which denotes abrasion .
Cornering wear often produces a fin or raised
portion along the inside edge of each row in the tread
pattern. In some cases this fin is almost as pro
nounced as a toe-in fin, and in others, it tapers into
a row of tread blocks to such an extent that the tire
has a definite "step wear" appearance .
The only remedy for cornering wear is proper
instruction of operations . Driving more slowly on
curves and turns will avoid grinding rubber off tires.
To offset normal cornering wear as much as possible,
tires should be rotated at regular intervals.
Uneven Wear - Uneven or spotty wear is due to
such irregularities as unequal caster or camber, bent
front suspension parts, out-of-balance wheels, brake
drums out-of-round, brakes out of adjustment or
other mechanical conditions . The remedy in each
case consists of locating the mechanical defect and
correcting it .
Power and Speed - Excessive speed has always
been harmful to tires. Speed creates heat - heat
softens tires.
Stops and Starts - Quick stops and starts grind
off tread in a hurry, may cause flat spots which
continue to grow for the life of the tire .
Temperature - Considerably less mileage can be
expected from a tire used in all warm weather driving
as compared to all cool weather driving, or from a
tire first put into service in warm weather.
MECHANICAL IRREGULARITIES
Following are some wheel or vehicle irregulari-
ties which may cause rapid or uneven tread wear :
Toe-In - The wheels on the same axle are closer
together in the front than they are in the rear . When
toe-in is excessive the tire wear shows feathered
edges on inside edge of the skid design .
Toe-Out - The wheels on the same axle are
closer together in the rear than they are in the front .
Tire wear shows feathered edges on outside edge of
the skid design .
Camber - This designates the tilt of the wheel.
Positive camber is when wheels are closer together at
point of road contact. Negative camber is when
wheels are closer together at top. Too much camber
results in excessive wear on one side of tire .
Caster - This is the backward tilt of the axle or
inclination of the kingpin at the top. Too little caster
causes wheel to wander or weave - result, spotty
wear . Excessive caster may cause wheel "flight" or
shimmy wear . Unequal caster causes wheel to pull to
one side, resulting in excessive and uneven wear .
Sprung or Twisted Frame - Will cause rapid or
uneven tread wear .
Grabbing Brakes - Brakes out of adjustment
and out-of-round brake drums or discs cause tire
treads to wear rapidly in spots. Out-of-round brake
drums usually wear out tires in a single spot . Im-
properly adjusted brakes produce several worn
places .
Worn wheel bearings, unbalanced wheels, or
wobbly wheels all result in uneven and irregular
tread wear .
At first sign of uneven tire tread wear, check and
correct all mechanical irregularities .
WHEEL INSPECTION
Do not use wheels with bent rims . The continued
use of such wheels will result in excessive tire wear
and, if wheel is mounted on front of vehicle, dif
ficulty in steering will be experienced. Wheels that
are thought to be distorted may be checked as fol-
lows, referring to Figure 12 for checking points :
1 . Raise wheel at side to be checked and safely
support underneath .
2. Tool for checking run-out may be readily im-
provised as follows :
Secure block of wood approximately 6" x 6" x
14" or material with suitable base so it will remain
positioned . Secure thin piece of wood or suitable
material 10 inches long, such as ruler or yardstick,
and fasten to wood block to a height in relation to
rim surfaces as shown in Figure 12 . Tighten screw
sufficiently so pointer will hold its position when
adjusted .
3 . Position pointer at crown of rim (A, figure 12).
Slowly revolve wheel and move pointer toward wheel
until it contacts wheel at nearest point.