PopRivet · Feb 14, 2018

I previously read a Thread that suggested the problem of why the rear-diff tended to knock when the gears were changed. I recall it was something to do with the rear-diff's mountings. However, I cannot locate what I read so wonder if someone could remind me of the cause of this thumping-sound from my (car's) rear.
And rather than buy each and every single item involved with the rear, I would appreciate any advice.
Thanks.

GrumpyGel · Feb 14, 2018

The diff has 3 mounts. Its usually the one at the front that has gone, but possible (but far less likely) the 2 back ones have as well.

Could also be the old "cracked chassis" chestnut. (details come up on a search).

Nodge68 · Feb 14, 2018

The front bush is the most likely to fail as it takes all the drive reaction torque. So as the wheels try to rotate forward under drive, the diff tries to rotate backwards. The constant load, unload of torque eventually causes the front bush to fail.

The rear pair of bushes take the weight of the diff, and propshaft torque. The propshaft torque is less than 1 3rd of the drive shaft torque, so they last considerably longer in normal use. However if the VCU has begun to stiffen or the vehicle has mismatched tyres or both, then propshaft torque can be very high which kills the rear bushes faster.

Arctic2 · Feb 15, 2018

As above change them all for poly bush ones and do your self a favour, from being in the same position further down the line,

PopRivet · Feb 15, 2018

Thanks, guys. I've looked at the bushes and right enough, the front/forward one seems soft and subject to large amounts of movement.
And as for your information, Arctic2, I'll do what you suggest and replace the lot of 'em with poly-bushed units.
Thanks to all for your help.
NB. I've just bought a set of three poly pieces, from Ebay, posted to a UK address. Getting them posted to Spain can be as much as the original cost - or even more. I'll pick them up soon.

kernowsvenski · Feb 15, 2018

I was chatting to a fellow FL1 owner about this very subject this week as I'm going to be sorting mine this weekend. He said that when he poly bushed all three of his diff mounts, the road noise increased dramatically. Anyone had the same?

Nodge68 · Feb 15, 2018

kernowsvenski said:
I was chatting to a fellow FL1 owner about this very subject this week as I'm going to be sorting mine this weekend. He said that when he poly bushed all three of his diff mounts, the road noise increased dramatically. Anyone had the same?

I'm reluctant to use poly on a road car, simply because it doesn't absorb (noise) as well as rubber. Manufacturers don't use poly bushes on production vehicles for this reason.
While the poly rear diff wouldn't necessarily increase tyre noise. It would undoubtedly increase transmission noise. This would travel to the body through the same path as tyre noise. This would make the noise from the diff sound similar to tyre noise. So in this case, less absorption in the bush would make the vehicle more noisy.

kernowsvenski · Feb 15, 2018

I thought that might be the case Nodge. "Road" noise probably wasn't the right term in the description.

However, I wondered whether it was worth a poly bush on the diff front mount for improved longevity and standard mounts on the rears.

Nodge68 · Feb 15, 2018

kernowsvenski said:
I thought that might be the case Nodge. "Road" noise probably wasn't the right term in the description.

However, I wondered whether it was worth a poly bush on the diff front mount for improved longevity and standard mounts on the rears.

I've thought about that too. But the best plan is to locate a stronger rubber bush.

PopRivet · Feb 16, 2018

Oh bugger! I bought the poly-flaming-stuff. I'm persuaded by the comments of you learned few; and although I've ordered a threesome-set of poly bushes, I'll do the same for rubber.
And then I guess it's going to be a case of trial-&-error. This could be very interesting.

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