Brake Drum out of Balance
I've been driving since 1948 and this is the first time that I've ran into this problem. I had a high speed vibration that would begin above 60 mph and then get progressively worse quickly with more speed. The vehicle is a '98 Jeep Cherokee Limited, 6 cy, 4 sp automatic with 4 wheel drive. The 4 speed has both part time and full time modes.
This vibration problem began after work was done in two shops:
1. Tire leak fixed at Firestone and tires rotated.
2. At Car-X: Front wheel U-joint replaced with new design CV joints, rear drive shaft U-joint at the differential replaced, and rear brake shoes installed.
A couple of days later was the first time that I had the occasion to get up to expressway speed and then the problem was sensed.
I noticed that Firestone after the tire patch, had moved the red dot balance mark on the tire. Firestone corrected this and rebalanced. This helped a tiny amount, but the problem persisted. Rebalancing all the tires with wheels helped a bit, but did not solve the problem.
The tire rotation moved the fronts to the back and to opposite sides. This reversed the rotation of these tires. Since I had bought the vehicle (3 yrs ago) at 70,000 miles and it now has 85,000 miles, I had no idea of past maintence except I knew that the tires had been changed at least once because the ones on the vehicle were not factory issue and had too much tread to have been on the road for 70,000 miles. It was possible that the tires had never been rotated until I did it. I switched the left and right rear tires to switch the rotation direction ot see if that made a difference. It helped a little, but as I ran the vechicle the next week, the problem persisted. It was smooth up to 50 or 60 mph and then the vibration would set in.
At this point, this vehicle was speed restricted. It was ok for grocery shoppng and most of my errands, but could not be used for a trip of any length. Incidentially, the Jeep is my winter driver; I use a more fuel efficient vehicle for the remainder of the year. My policy for the fair weather machine is if there is salt on the street, it does no go out. This practice has preserved its finish and structure.
Neither Car-X or Firestone could not pin point the problem or solve it. And then I noticed that one of the brake drums did not have a balance weight. It is possible that it never had one if it was one of the rare ones that did not need one, but I have seen many drums with balance weights. Since this vehicle had been driven in winter road salt and the drum's outer surface was rusty, it was had to determine if a weight had been lost. There was a faint indication that it may have had one. The problem is that no one in town is equipped to balance brake drums and several techs scoffed at the idea this could be a problem.
In desperation, I put on a pair new drums. I reasoned this was not a total loss since the old drums were about 1/2 used up. Both of the new drums had balance weights and one was quite large.
The problem went away. It is vibration free up to 70 mph.
Has anyone encountered an out-of-balance brake drum?