Rear susupension vibration at 50 kph

Recently had my diff rebuilt and now have a strange vibration when accelerating through 50 kph. The vibration is worse when turning left and not there when turning right.
Any idea’s?

Prop shaft mis-aligned?

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Hello mate …

If I understand your troubleshooting correctly the vibration is not so bad going straight, disappears as you turn to the right but gets worse turning to the left.

Is there any type of noise associated with this?

I don’t believe an unbalanced/misaligned prop shaft would care which way the car is turning. This leads me back to your rebuilt differential. Did this start happening as soon as you reinstalled the diff ?

I would get the car up to 60 KPH put it in neutral and try it turning left then do it again and turn right.
If you don’t have the vibration you have narrowed it down to the drivetrain. The only thing in the drivetrain that cares which way you are turning is the diff. Especially if your car has limited slip. I would expect however that a problem with the diff would result more in noise and not vibration, but you never know !

Check that the tires have been mounted correctly. AFAIK, if the rear A arms have been removed and separated from the wheel assy the eccentric securing bolt is the adjustment that affects the rear alignment.

Only applicable to the cross over xj40/x300

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Good to know Robin, thanks

First thing I’d try would be swapping the rear wheels from side to side to see if the vibration swaps over too, it could be a damaged tyre even if they look ok. While the back end is raised carefully check the 4 universal joints on the drive shafts for any sign of play.

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Thanks to all respondents. There is little or no noise, it is a transmitted vibration only when the car is accelerating moderately or higher, it is possible to drive the car with an easy right foot and not induce the vibration. Worst condition is an inclined pull away on a left turn with some throttle. The LH UJ’s were replaced recently, why is the propshaft centre bearing biased? If the centre bearing is is not biased - what happens? regards, Derek.

I’d follow casso’s advice and try swapping the wheels. I put up with a left rear vibration for a while, evident when I gave it gas pulling on to the freeway.
Disappeared when I mounted new tires.

Your differential differentiates only when one wheel spins with different speed in relation to the other. With current world design - this happens when you turn. Turning left makes your left wheels turn slower than your right wheels so yes, there may be an assembly/rebuild problem. Few things to consider/eliminate:

  1. Check differ’s temperature with hand after longer ride (motorway). If Jack the “heavy spanner” made some mistakes on rebuild or forgot to leave some gear backlash - it will run hot, not warm as it shoud.
  2. Lift rear wheel from one side, secure the front with wheel chocks and run it on drive and reverse, repeat for the other wheel
  3. Lift the rear axle entirely and let it run as per pt 2.
  4. Take off one wheel off and run as per pt 2.
  5. Take remaining wheel off and run as per pt 2.

Doing so, you can diagnose if the problem lays in your wheels (wheels on/off in drive) prop shaft (vibrations on rear end with wheels off) or problem with differ itself (hot, noisy in drive and reverse, vibrations while in drive with one of the wheels up). Naturally you can have compound errors on what’s above. Usually, workshops don’t care about correct prop shaft balancing / not even marking bolt holes locations for re-assembly…

Not sure what you understand by vibration… Would thst be something with higher frequency or random pulling which may suggest alignment issue…?

Another definition of vibration could be judder, if I was to guess a frequency it would be around 5 cycles/sec, it starts at around 45 kph and disappears at 55 kph the more the acceleration the worse it is. It is not what you would expect from an out of balance wheel. It is vaguely reminiscent of live axle leaf spring judder under hard acceleration. Not sure if this helps or confuses things. I put the post up with the hope that somebody has had the same experience. Once again thanks to contributors for their time and thoughts, much appreciated.