Front and rear diff swap, 3.91 to 3.73

[Horizon]

[quote="X5Sport"]
I'm probably out on a limb here, but my guess is tyres.  Either wide tread difference now or in the past, or use of non-approved tyres on staggered set ups.

I suggested that perhaps as the value of these cars falls justifying £800 on a full set of rubber becomes much harder.  The risk is that a poor choice could destroy a diff.  Some non-approved sets have a rolling difference of >30mm and that cannot be good.

I might be wrong and am happy to be educated if there is another reason.
[/quote]

I totally agree with X5Sport on the tyres, when I picked up my X3 it was showing signs of stress on the drive train, as I had noticed on the test drive viewing that it had non star marked tyres I had pre ordered a set of tyres to replace the fitted set. As my rims were only 13 mm wider on the rear  ( staggered set up )I fitted 4 identical tyres all round so guaranteed to be the same rolling circumference.
All the faults went away, as did the Lurch.
On the X3 the transfer box is the weakest link in the drive train, on the E53 the diffs are probably the weakest link.
On the E70 LCI the transfer box is the weakest link.
When I measured the circumference of the tyres I removed there was a 35mm difference , between the different makes ! All were marked up as being the correct size.

[henrym3]

So, the E53 transfer case with epicyclic gear and chain are able to absorb these tyre size differences (although 35mm is a mega difference) but the facelift, with active 4x4 isn't able, it winds up and breaks?

[X5Sport]

Basically, yes.  Same with the E70.

[steve1986]

Yes tyres !

I had two new rear tyres fitted to my X5 but some little oik punctured the n/s, I had another n/s new tyre fitted made by Accelera, it often brought on the traction control light, the car felt as if rumbling when accelerating hard and it also whined slightly, this was a few months/ 2k miles ago, the other day I checked the new tire as was going to order a set of replacements and found the new accelera to be bald, the day after huge bang and diff exploded.

So it looks like my problem was inflicted by me not sorting the issue straight away, also the accelera looked physically narrower than the other tyre even though it it's still a 315/35/20

Thanks
Steve

[Greydog]

When I got my 4.6 it was fitted with a set of Michelin * rated Diamaris which were almost new, the car had covered 76k and had a full service history. When I got the car it had a full service at the BMW specialist I use as a precaution At 82k the rear diff disintegrated no prior noises no prior warning I was travelling at 40mph on a level road and suddenly no drive.
While I accept that poor tyre choice will cause drive train damage (on any four wheel drive) in my case I don't believe it was. The Warranty Company said new diff that was fun waiting for delivery from BMW !!! I was able to examine the old diff, the crown wheel was very brittle in paces and not others and the bearing carrier on the drivers side showed the same signs. I believe the bearing seat gave way first causing a chain reaction the result was spread over the workshop floor to see. The BMW specialist I use had sourced a Diff repair kit but on inspection the old diff was beyond repair so the Warranty company was right in saying don't waste your time new diff.