[aquazi]

I’ve been driving quattro audi’s for years.... never had an issue replacing tyres as long they were done on the same axle.....

I used to get around 15-20k on the front tyres and 40k+ on the rears so at some point the difference would be massive.... but also had the option to rotate tyres with them all being the same fitment.

I expect the xdrive to the same... except the rears to go first... and no way to rotate with the staggered offset.

I bet a tyre company wouldnt care though and would sell you a single - even if the tyres on the same axle were down to 1.6mm!

[doncarlyon]

Originally Posted by aquazi

I’ve been driving quattro audi’s for years.... never had an issue replacing tyres as long they were done on the same axle.....

I used to get around 15-20k on the front tyres and 40k+ on the rears so at some point the difference would be massive.... but also had the option to rotate tyres with them all being the same fitment.

I expect the xdrive to the same... except the rears to go first... and no way to rotate with the staggered offset.

I bet a tyre company wouldnt care though and would sell you a single - even if the tyres on the same axle were down to 1.6mm!

My mates A4 Quattro often has 4 different tyres on it with various tread depths and like you he has had no issues. Car is on about 55k-60k miles.

There are a couple of different "Quattro" systems, so dont know if they cope differently?

[JNW1]

Originally Posted by 1966-TR4

That is how BMW sold my AUC xDrive to me (well, 3.5 on the rear). Despite me querying with the dealer.

I've seen comments before to the effect it's not good practice to have new tyres on one axle and part-worn ones on the other with X-Drive (because it can result in problems with the transfer box).

However, if it's crucial that all four tyres have a similar tread depth wouldn't you have thought there'd be some reference or warning to that effect in the owner's handbook? Can't find any mention of it in mine.....

[doncarlyon]

My thoughts on this:

If the x-drive "setup" is so fragile that it cannot cope with a tread depth difference, how on earth does it cope with cornering when all 4 wheels will effectively be rotating at different speeds?

In old school 4x4 setup's you can run into issues with driving in 4x4 mode on grippy surfaces as you end up with "transmission wind-up", hence you are advised to only use 4x4 when really required. With x-drive you have no way of disengaging 4x4.

[1966-TR4]

Originally Posted by JNW1

I've seen comments before to the effect it's not good practice to have new tyres on one axle and part-worn ones on the other with X-Drive (because it can result in problems with the transfer box).

However, if it's crucial that all four tyres have a similar tread depth wouldn't you have thought there'd be some reference or warning to that effect in the owner's handbook? Can't find any mention of it in mine.....

Agreed, there is nothing in the handbook. There have also been very few posts on this forum regarding transfer box issues (compared to many of the other issues with these cars, or compared to well known issues on earlier iterations of xdrive).

I'm on 95k miles now, and the rears have always been replaced a 4 or 5 thousand miles before the fronts. When the new rears go on, they already have 2mm more rolling circumference than brand new fronts, so accounting for wear on the fronts by the time the new rears go on (let's say 2mm, or 4mm RC) then there is a 6mm RC differential. So far, no signs of issues which hopefully means the system is robust enough to deal with typical differences.

[HighlandPete]

Originally Posted by doncarlyon

My thoughts on this:

If the x-drive "setup" is so fragile that it cannot cope with a tread depth difference, how on earth does it cope with cornering when all 4 wheels will effectively be rotating at different speeds?

In old school 4x4 setup's you can run into issues with driving in 4x4 mode on grippy surfaces as you end up with "transmission wind-up", hence you are advised to only use 4x4 when really required. With x-drive you have no way of disengaging 4x4.

It is not that xDrive is fragile, it is the fact demands on the system increase with differences in tyre tread depths. Something has to give, as I explained in an earlier post. XDrive is designed to be working all the time, sensitive to any individual wheel slip. It's not really comparable to an "off-road" 4x4 system.

Cornering and occasional slip is the least of the issues, as it is intermittent and variable. But take the example of the car that has to accommodate about 15 metres a kilometre in rotational difference, front to rear. Put that car on the motorway (or autobahn) and spend 4+ hours at speed. Where's the extra heat going due to forced clutch slip in the transfer box? Tyres, transfer box parts and oil! It is only clutch slip that prevents transmission windup, there's no magic way we can 'relieve' the rotational differences with the design of a system like xDrive.

What happens over time? Extra wear and oil degradation = shorter service life.

[HighlandPete]

Originally Posted by 1966-TR4

Agreed, there is nothing in the handbook. There have also been very few posts on this forum regarding transfer box issues (compared to many of the other issues with these cars, or compared to well known issues on earlier iterations of xdrive).

Most of the current generation xDrive models will still be on lower mileage and likely running OE or at least decent tyres. As cars get into the used market we typically see more issues, (in many areas) as users see more of the weak links in any model.

There is allowance for tyre variations.

BMW xDrive tyre tolerance logic

The tire circumference can fluctuate up to 1% or more as a result of mixed tires or wear. The tire tolerance logic decides depending on the driver's command and driving situation whether the slip is to occur in the transfer case clutch or at the contact area between tire and road.

If the slip is permitted in the transfer case clutch, the locking pressure set by the pre-control is reduced in order to keep the work loss low. In the driving dynamic control situation, the clutch is locked slightly more than normal, the four wheel drive is always guaranteed when required.

For maximum xDrive performance, tires (and wheels) of the same diameter should be installed on the vehicle."

Some of BMW documents (like TIS) use the 1% figure, usually translates to 2mm variation allowance for xDrive.

Additionally for all-wheel drive vehicles:

The tyre size, manufacturer and tyre tread must be identical on all wheels; if mixed tyres are fitted, different tyre sizes are permissible.

The tyre tread difference between tyres in all wheel positions must not exceed 2 mm (normal quality of the wheel control systems and wheel alignment requirement)

[Hooded]

Originally Posted by 1966-TR4

There have also been very few posts on this forum regarding transfer box issues (compared to many of the other issues with these cars, or compared to well known issues on earlier iterations of xdrive).
.

Seems a lot more common on the US part of the forum

https://docs.google.com/spreadsheets...BNlq8/htmlview

Taken from here: https://f30.bimmerpost.com/forums/sh....php?t=1407191