Posted By : John Davies, 26-Jul-2007,
10:31am
Paul,
This question is a little like going to the doctor and asking, "Why do I have a headache?" You will get asked a lot more questions before you get an
answer!
Such a vibration means there is an unbalanced rotating component somewhere. As it only happens over 60mph, then it is unlikely to be in or in front of
the gearbox, and anyway, the largest and heaviest rotating components are the wheels. Have you had them balanced recently? That would be my first step.
Then inspect the wheels while the car is on stands and they spin - any wobble due to distortion, perhaps from kerbing?
Then have you done anything to the transmission recently? Taking the propshaft off and replacing it with the front or rear flanges bolted up in a
different orientation to the gearbox and diff flanges is alleged to cause an imbalance. Only two positions are possible at each end, so that there are
four permuations of position. One will be right, two will be so-so, one will be awful. The propshaft itself is balanced in manufacture by a small steel
plate welded onto the shaft. If this is lost by corrosion or removed, then the shaft itself becomes unbalanced and can vibrate. A propshaft can be
re-balanced if you can find the right workshop. Seems to be a truck thing rather than for domestic vehicles, but don't let that stop you asking. And
the propshaft has one end with sliding splines to accomodate small changes in length - this should be to the rear, but is very easy to re-install at
the front. If this can cause vibration, I don't know, but it is WRONG!
Moving back, are the halfshaft flange nuts properly tightened? Is a half shaft bent, even slightly? Run the car while up on stands to see if the shafts
are straight or not - this is DANGEROUS and is best done from an inspection pit, rather than lying under the car. Perhaps a mirror?
I'm sure others can suggest other questions you need to answer on the way to a solution.
Good luck!
John