The technical definition of balance is the uniform
distribution of mass about an axis of rotation, where the center of gravity
is in the same location as the center of rotation. A balanced tire is one
where mass of the tire—when mounted on its wheel & the car's axle—is
uniformly distributed around the axle (its center of rotation). Balanced
tires can spell the difference between a positive & negative driving
experience. Drivers of high performance vehicles will be more sensitive to
imbalance problems, but no driver is happy with an annoying vibration.
An out-of-balance tire & wheel assembly:
- Degrades ride quality & driver comfort.
- Shortens the life of tires, bearings, shock absorbers
& other suspension components. Vibration is the most noticeable effect
of imbalance.
- It is dependent on vehicle speed.
- It often first becomes apparent between 40 & 45 mph
& increases in magnitude with greater speeds.
Sources of Imbalance
Two sources of imbalance occur in tires: heavy or light spots in the tire
& radial or lateral run-out. Imbalance also can be caused by:
- Variations within the wheel, such as thickness &
welds.
- Rotor & axle imbalances.
Heavy or Light Spot Imbalance
There are two types of imbalance caused by heavy or light spots: static &
dynamic.
- Static imbalance:
Occurs when there is a heavy or light spot in the tire so that the tire
won't roll evenly & the tire/wheel assembly undergoes an up-and-down
movement.
- Dynamic imbalance:
Occurs when there is unequal weight on both sides of the tire/wheel
assembly's circumferential centerline. The tire/wheel assembly has a
side-to-side movement.
Heavy or Light Spot Balancing
Achieved either statically or dynamically, depending on the type of
imbalance that has occurred.
- Static balance:
Achieved with a bubble balancer but does not correct for dynamic
imbalance.
- Dynamic balance:
Achieved with a spin balancer where the tire/wheel assembly is balanced
both statically & dynamically.
Radial or Lateral Run-out Imbalance
This results from poor bead seating on the rim or the placement of
components. Poor bead seating is usually the result of improper mounting or
the use of improperly made wheels. A small degree of this imbalance is
acceptable, but too great a run-out causes vibration & excessive tire
wear.
- Radial Run-out:
An "out-of-round" situation where vibrations are produced as the wheel
spindle moves up & down.
- Lateral Run-out:
A side-to-side or wobbling movement of the tire & wheel. It is less
common than radial run-out. Sensitivity of a vehicle to vibration from
radial run-out is four to eight times that of wobble from lateral run-out.
Run-out Balancing
Depends on whether it is radial or lateral run-out.
- Radial run-out balancing: Achieved by rotating the
wheel & tire assembly two stud positions on the hub, or by rotating the
tire 180° on the wheel. If run-out is still over specification, check
wheel run-out & mark the low point. Rotate to match the high point of
the assembly run-out with the wheel low point. If the assembly run-out is
still too high & the wheel is within specification, replace the tire.
- Lateral run-out: Achieved by using a run-out gauge to
check both the tire & wheel. Chalk-mark the highest point of run-out on
both the wheel & tire. Replace whichever (wheel, tire, or both) is out
of specification.
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