Section 00-04: Noise, Vibration and Harshness
1996 F-150, F-250, F-350, Bronco and F-Super Duty Workshop Manual
Rear brake drum imbalance can cause a vibration condition that sometimes cannot be compensated for by wheel balancing.
If rear brake drum imbalance is suspected, raise the rear end of the vehicle, taking proper precautions. Remove the rear wheel and tire assembly.
Before testing, reverse conical-type lug nuts and install all to retain the drums to axle flanges. If drums do not have balance weights, run the vehicle up to the speed where vibration occurred during road test.
If the drums exhibit out-of-balance symptoms, they should be removed from the vehicle and balanced as follows:
Position the brake drum on a bubble-type balancer and, noting the heavy side of the drum, place a wheel balance weight on the drum directly opposite the heavy point. Select the appropriate number of weights until the bubble balance indicates a balanced condition. Mark the location of the light side of the drum, and note the total amount of the balance weight needed to balance the drum.
A drum that already has a balance weight attached, or requires less than 56.6 grams (2 oz.) of weight to balance, is not the prime cause of the vibration.
Fabricate a piece of mild steel to fit the curvature of the drum face and equal to the balance weights required to balance the brake drum (an old flywheel ring gear can be cut and used as a balance weight).
A small postal scale may be used for determining the weight of both the balance weights used and the weight of the fabricated drum balance weight needed.
CAUTION: Under no condition should any other welding method be used other than arc welding. To prevent drum distortion, do not use a current setting of more than 100 amps.
Position the drum balance weight on the drum face in the location on the light side of the drum indicated, as shown. Arc weld (tackweld 6.35mm [1/4 inch] long) the balance weight to the drum face.
Install the balanced drum on the vehicle.
Static balance both rear wheels and tires and install on vehicle.
Road test to verify condition.