Vibration

Keith Fry

Member
Hi all
Just had the rear calipers and diff mountings replaced. The diff mountings were polyurethane rather than rubber. All the fitting was pretty straightforward.

When I drive the car now at about 60 mph a vibration starts which gets worse and worse - it does not drive through it, so to speak. It is characterised by a sound rather than a shake, but there is a tremor.

So, any ideas about the cause - I would like to get it right. Is it possible that the new bushes being less flexible than rubber, give less and show up a vibration which was there before but was absorbed by the rubber mounts?
Could it be the ZF5HP22 which has had to have a mount made up - could this be to blame.

I don't know how to tackle this other than having rubber mounts back and fitting them to replace the poly bushes.

The supplier says he has sold 30 plus sets of these with no bad reports.

All help appreciated.
 
Hi, Go back to basics first, try checking all six universal joints there could be one or more seized or starting to seize. Inspect all round for witness marks, like the propshaft bolts clipping the diff front crossmember.

Colin
 
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It may well be that the new bushes are firm enough that an underlying problem is now felt. Colin's plan make sense as a first try.
 
When it makes the sound, try pulling up a notch on the hand brake lever? I replaced my engine mounts. Things sit a little higher now, I need to adjust that little adjustable mount on the back of the gear box. I did mess with it, raising the back of the box up a bit. I probably should have left it alone. For my car when the hand brake lever is all the way down, part of the mechanism seems to be making contact with a uni joint in my car. This was rather noisy, and it was only doing it some of the time. Bring the lever up a notch or 2 and then it would stop.

Anyway you could record when running, and link us to it from youtube? With sounds, that helps a lot more than trying to describe the sound.

Check clearances around the new gear box and the base unit0. Also check how the exhaust pipe is hanging. Perhaps one of the rubber donuts has perished in the back and it is making contact with a brake disc or the half shafts.
 
The new bushing will certainly change the resonant frequencies, but the fact that it starts and keeps on getting worse suggests it's not a resonance issue. They had a lot of trouble with vibration on the P6 in development. What are you using for a prop shaft? A stock shaft will have the u-joints slightly out of phase, and the phase is different for V8 vs the 4 cyl cars.

Yours
Vern
 
very simple thing to check is the wheels.

A balance weight could have fallen off or moved whilst your work was being done. Try swapping the tyres front to back or have them balanced.

Sometimes it is the less obvious things that cause the issues.
 
What are you using for a prop shaft?Check the prop alignment. A stock shaft will have the u-joints slightly out of phase, and the phase is different for V8 vs the 4 cyl cars.

Early and late V8 have a different alignment as well. I did think of prop alignment, but my understanding is that the OP said the vibration came after the diff mountings were fitted, and the prop alignment shouldn't have been altered doing that.
 
There's another thread with some suggestions here, as well:
Vibration at 55mph or over

As I put in there, I'm having a pretty similar problem, but it feels more like a vertical oscillation in my case, rather than a shimmy, or a sound. My diff mounts have been checked and are all fine - I'm planning to have a good look around under there this weekend. Thinking about it, though, my vibration may have started after changing the gearbox (I can't remember when I first noticed it), so I'm starting to wonder if the mount bolts have been tightened a bit too much. That may also be the problem in this case.

It's not getting any worse, so I'm just living with it at the moment, but I want to sort it out as it gets a bit tiresome on a long-ish journey.
 
Keith where did you buy poly bushes for the diff?
As far as I knew they didn't exist, and I'd been planning to make some with aluminium housings actually.
You should post some photos of your zf mounting and diff bushes if possible, it might well help with our Internet diagnoses.

What gearbox do you have Martin, LT77?
How is it mounted?
Are the prop bolts all tight?
Diff oil level
Jim
 
Hi Jim, yes, it is an LT77. It's mounted using the standard LT77 rubber mounts onto an adapted SD1 crossmember, that bolts up where the auto crossmember would be. Haven't checked the prop bolts yet, am planning to get under there for a good look at the weekend, but I've no reason to believe that they wouldn't be tight.

Regarding the diff oil level, I don't know. When the car went in for it's MOT a few weeks ago I had the gearbox oil changed and was going to get the diff done at the same time, but the fill plug was stuck tight and the garage reckon they can't shift it without damaging it. That reminds me, I must get a spare one so they can do whatever they need to do to it to get it out.
 
Hi Keith,

Was the vibration evident before having the polyurethane diff mounts fitted? If it wasn't, then the mounts would almost certainly be the issue. If it was there prior to fitting the mounts, then it could be any number of potential items. Is the harmonic balancer located on the pinion extension case undisturbed? Was the tailshaft balanced following any changes that you may have made to it?

Ron.
 
Been under there this morning and I think that the culprit may be the extension tube flexible mounting. All of the prop bolts are tight, and there doesn't appear to be any play at all between the front and rear sections of the prop. All of the engine and gearbox mountings are tight. The only thing I couldn't check was the alignment marks on the propshaft - they weren't visible, but I've no reason to believe that it isn't aligned, as I'm sure the vibration wasn't there when I bought the car.

Should there be visible movement in the extension tube mount when the propshaft is pushed up? If I put a bit of pressure on the diff end of the prophaft, there is a small amount of deflection in the mount. Not a lot, but it is enough to be able to see the mount moving. I tightened the mounting bolt a bit, but it didn't make an appreciable difference to the amount of movement.
 
Fixed it! Took it out for a blast yesterday evening and the vibration is gone.

Unfortunately, I don't know exactly which of the many thing I did whilst under there actually stopped it. My guess is either the tightening of the extension tube mounting bolt, or slackening off the transmission mount snubber bolt a bit. The only other things I did, other than checking things, was slackening off the gearbox mounts and re-torquing them, and tightening slightly the cross member to body bolts, which I don't think would have that much of an effect.
 
Thanks everyone for your thoughtful help and replies. I will go through all the points and let you know as soon as possible the outcome.
 
Hi again

Well according to my mechanic, the prop shaft uj was worn and there was some gearbox flange which was loose. He replaced the prop shaft uj and adjusted/tightened the flange and now all is well. Went for a bit of a blast down the M27 and she performed well.
 
When I had the ZF fitted, the mechanic changed both propshaft UJs not because they were worn but because they were corroded. Given the very low prices of these, it makes sense always to do this when the prop is out and you are in any way uncertain of them.
 
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