Diff failure,,,caused by tyres,,??

[Greydog]

[quote="plowy"]
So much opinion and discussion on this issue,,,the Master Mtech I am friendly with is utterly convinced tyres caused this failure.....in fact when I explained the symptoms to him his immediate response was what's tyre sizes and condition..??
[/quote]

Lots of debate and some strong opinions (especially from your Mtech friend) so for what its worth here is a view from an old engineer.
Transmission (the 3 diffs and gearbox) damage with road going 4 wheel drive system would need a pretty drastic difference in tyre sizes allied too locked diffs plus a fixed 50/50 split.
As I understand the X5 diffs are not locked and power split is not fixed, when released from the factory the difference in tyre wear (left right/ front rear)for recommended sizes would be anticipated in the design.
Subaru who have a fixed 50/50 split are one of the few who have known problems with tyre sizes though not tyre wear. I know from experience my wife loves her Subaru Outback 3.0RN it cam with 245hp as standard then while in the States on business I met someone running a Perrin tuned Subaru.
Short story I brought a complete engine from them and had it fitted to my wife's car her engine was shipped back exchange. The result is 415whp the transmission and diffs remain standard the only other upgrade is brakes and suspension, and no it doesn't have a noisy exhaust. So far 32k miles and it never misses a beat, it does get full fluid changes every 5k though but that is my/our choice. She doesn't hoon around unless from time to time she gets a boy racer who needs a shock (so look out for the sweet old girl in the sliver Outback 535boy).

So engineers hat on, has anyone investigated what condition the diffs are in? What condition the lubricant is in? With pressure heat etc all lubricants will thicken and deteriorate as they break down, this will accelerate wear.
So when if at all was the Diff and Transfer box lubricants changed, if there has been a leak is there even any Lubricant in the diff that failed?

It's all to easy for a Dealer (your MTech friend) to say "Tyres" then fit a new diff sell you a set of tyres and send you on your way with a melted credit card and kids living on a food free diet.

Even though this sadly doesn't change the outcome for you a blown diff is still going to need replacing as will the tyres or at least 2 of them by the sound of it.
The engineer in me would still want to know why? I would be checking the diffs even the blown one to understand why.

[plowy]

Personally I'm not 100% convinced it was solely tyres that caused it and as BMW Warranty is footing the bill it makes me less inclined to get to the bottom of it but on another note here is a word of warning,,,,,it took 4 days for the dealer to get authorisation from warranty underwriters because they had to download the cars mapping and forward it to them prior to authorisation,,,,and to think I very nearly had this car remapped last month..PHEW..!!