On-car Wheel Balancing
Introduction:
On-car wheel balancers contain a drum driven by an electric motor. This drum is positioned against the tire on the vehicle, which allows the electric motor to rotate the wheel. Many on-car balancers have a strobe light with a meter and an electronic vibration sensor.
Off-car and on-car wheel balancing: Off-car wheel balancing and on-car wheel balancing are a complementary combination for fine-tuning wheel balance. For example, a wheel vibration problem may still exist after an off-car balance procedure. If this problem occurs, the on-car balancer may be used to correct the problem. The on-car balance procedure corrects imbalance problems in all rotating components, including brake drums or rotors.
When a front wheel on a rear-wheel-drive car is balanced, use this procedure:
1. Perform the preliminary checks listed previously in this chapter.
2. Raise the wheel being balanced 5 in. (12 cm) off the floor and be sure the chassis is supported on safety stands so the wheel drops downward.
3. Install the electronic vibration sensor between the lower control arm and the floor.
4. Chalk mark a reference mark on the outer sidewall of the tire.
5. Spin the wheel just fast enough to produce vibration on the front bumper.
6. When vibration causes strobe light flashing, move the balancer drum away from the tire and allow the tire to spin freely.
7. Shine the strobe light around the tire sidewall and note the chalk-mark position.
8. Note the pointer position on the meter.
9. Use the balancer’s brake plate to slow and stop the wheel.
10. Rotate the wheel until the chalk mark is in the exact position where it appeared under the strobe light. The heavy spot is now at the bottom of the wheel and the balancing weight should be attached 180° from the heavy spot. Install the amount of weight indicated on the meter.
11. Spin the wheel again. If the wheel balance is satisfactory, the meter pointer will read in the balanced position. When the pointer does not indicate a balanced wheel, shine the strobe light on the tire sidewall. If the installed wheel weight is at the 12 o’clock position, additional weight is required, whereas the 6 o’clock weight position indicates excessive weight. A 3 o’clock or 9 o’clock weight position may be corrected by moving the weight 1 in. (2.5 cm) toward the 12 o’clock position.
Additional on-car wheel balancer precautions include:
1. Do not spin the wheel at excessive speeds.
2. Do not spin the front wheels on a front-wheel-drive vehicle with the floor jack under the chassis and the suspension dropped downward. Under this condition, severe angles exist in the front drive axle joints, and these joints may be damaged if the wheels are rotated with the balancer. Place the floor jack under the lower control arm to raise the wheel.
Rear Wheel Balancing
If an on-car balancer is used on the rear wheels of a rear-wheel-drive car, the technician must determine if the vehicle has a conventional or a limited slip differential. With the transmission in park, or in gear with a manual transmission, rotate one rear wheel by hand. If the vehicle has a limited slip differential, the rear wheels will not rotate, whereas a free-turning wheel indicates a conventional differential.
CAUTION: On a rear-wheel-drive vehicle when a rear wheel is rotated with an on-car balancer, do not allow the speed indicated on the speedometer to exceed 35 mph (56 km/h), because the wheel speed is much faster than the speed indicated on the speedometer.
When the vehicle has a conventional differential, use the same front wheel balance procedure on the rear wheels. Raise only the wheel to be balanced from the floor, and do not allow the speed indicated on the speedometer to exceed 35 mph (56 km/h). When the other rear tire is resting on the floor, the speed of the wheel being balanced is 70 mph (112 km/h).