Most drums have little flat iron weights that are welded on the outer
surface by the vendor to balance them. My dad once had a 1957 Ford that he
balanced and rebalanced the tires to no avail. The car still vibrated
badly at certain speeds. He then balanced the drums by welding weights
onto the drums. The vibration problem was solved. I wonder if, in that
instance, the drums came from the vendor unbalanced.
Vibration can also be caused by out of round tires. No amount of balancing
will solve this vibration problem. Bad tires will need replacement. It
should be easy to see by jacking the car up and spinning the wheels fast
enough to see if the tire is out of round (doesn't rotate smoothly), has any
lumps or bulges, etc. Early nylon tires in the late 1950s and early 1960s
were notorious for developing out of round spots by just being parked
overnight, etc. They then had to be driven quite a ways to come back into
round and stop thumping, which also took longer in cold weather!
Usually, tire vibration becomes more pronounced at higher speeds such as
above 50 MPH. I would think that vibration in the 35-45 MPH range would
be a driveshaft problem. Driveshafts also usually have weights welded onto
them at the factory and don't usually need rebalancing. I would inspect it
to make sure that it hasn't been slightly bent, which is a frequent problem
in boneyards where cars are lifted with fork lifts. Otherwise, the most
likely culprit is a bad universal joint. Even if they seem tight, they
frequently begin going bad by binding at certain positions of the
driveshaft. I would pull them apart and make sure that they have not gone
dry of lubricant. Check too to make sure that the universal joints are
perfectly centered. If a universal joint was previously replaced and not
done right, such as being slightly off-center with respect to the
driveshaft, it can cause significant vibration.
Also check for play where the driveshaft attaches to the back end of the
transmission. I once had a Dodge van that had a dipstick that incorrectly
identified the full level as too low. As a result, the bearing sleeve in
the rear of the tranny wasn't getting lubricated and would prematurely wear
out, which gave enough play in the front of the driveshaft to permit it to
jump around and cause a horrible vibration. Replacement of the sleeve and
"over-filling" the tranny solved the problem.
Good luck, Jim
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