BMW xdrive and tyres - surely not that limited tolerance?

[DickP]

Hi,

I understand it as widely mentioned the BMW xdrive has limited tolerance to differences in rolling diameter between front and rear axles.

I also understand that the front axle is not engaged by the transfer box until the car detects slip. I think it uses a clutch mechanism in the transfer box? Have I got both of these right?

If it is a clutch in the transfer box, then how can differences in diameter damage the xdrive if it only engages the clutch and front axle when there is slippage, at which point the wheels may be turning at different speeds anyway?

Is this widespread "worry" all just based off what BMW said, which might have actually been to support the marketing of their star rated tyre range rather than being an engineering limitation of the xdrive?

Thanks,

[Jakg]

DickP said:

I also understand that the front axle is not engaged by the transfer box until the car detects slip. I think it uses a clutch mechanism in the transfer box? Have I got both of these right?

This is not correct.

For a 2011 X5 specifically:

TIS said:

During normal straight-ahead driving, the front and rear axles are powered with the normal 40 to 60 force distribution.
{...}
When turning and in stable cornering, the transfer box transfers up to 80 percent of the force with electronically-controlled multidisc clutch to the rear axle.

Obviously there have been several generations and the number may differ.

[DickP]

Thanks for the quick reply. More specifically what s the set up then on the F and G series range 3, 4 and 5 series?

[Jakg]

DickP said:

Thanks for the quick reply. More specifically what s the set up then on the F and G series range 3, 4 and 5 series?

For a G31 530d xDrive, it's 50:50 by default, with up to 80% to the rear axle during cornering.

[samoht]

DickP said:

Hi,

I understand it as widely mentioned the BMW xdrive has limited tolerance to differences in rolling diameter between front and rear axles.

I also understand that the front axle is not engaged by the transfer box until the car detects slip. I think it uses a clutch mechanism in the transfer box? Have I got both of these right?

I think so, yes.

DickP said:

If it is a clutch in the transfer box, then how can differences in diameter damage the xdrive if it only engages the clutch and front axle when there is slippage, at which point the wheels may be turning at different speeds anyway?

My understanding:
- Normally, with all four wheels having the same rolling radius, front and rear driveshafts are turning at the same speed and so the clutch will be disconnected when driving in a straight line. Only when one axle loses grip or the car is cornering, the clutch will engage, increasing traction and stability. So the clutch is disconnected (open) ~99% of the time.
- If the rolling radius differs between the axles, the front and back are rotating at different speeds even when driving steadily straight ahead. So now the clutch is engaged (closed) ~99% of the time.

The effect of having the 4wd engaged all the time, rather than as-needed, leads to premature wear of the centre coupling clutch etc.

DickP said:

Is this widespread "worry" all just based off what BMW said, which might have actually been to support the marketing of their star rated tyre range rather than being an engineering limitation of the xdrive?

Thanks,

It's not specifically related to BMW-marked tyres, you could fit eg four bog-standard new PS4Ss and they'd all have the same rolling radius. The main issue is that if you end up with different wear depths across your four tyres, whether by differing wear rates over time or replacing fewer than all four tyres, it could cause the above.

I don't know how much mismatch in rolling radius BMW XDrive can accept without damage and how much of a problem it is, however I do recall AWD Volvos being notorious for this issue in the past. Someone gets a puncture and the Volvo dealer hands them a bill for £1k for four brand new tyres, cue letter to Autocar.

[babo456]

I hadn't really thought about this too much initially myself. I've had several xdrive BMW's M340i, M4, M3 all had star marked tyres but didn't think too much of it other than it's probably a marketing gimmick.

I've now got an X3 m40i and it's a royal pain finding 'star marked' tyres for it/limited selection (21" rims). I wanted to get some cross climate tyres but there aren't any that are star marked. I've ended up buying a second set of 19" wheels and star marked winter tyres.

There are already a few threads of this and many varying opinions but my conclusion was that I wasn't going to risk running non star marked tyres on the car mainly due to any warranty issues BUT I had read anecdotal posts of people using non star marked tyres fitted and transfer box whine (I think there was a post on here with someone in an X5?) and they swapped back to star marked and the issue went away.

The star marked tyres tread pattern and compound is also specific to xDrive cars in what I've come across (someone even had a picture of non star Vs star and you can see the difference) so for whatever reason xDrive seems to be more sensitive than other all wheel drive systems hence the need for specific tyres.

Star marked tyres are readily available summer wise but a bit of a pain for all seasons or winter - as limited or NO choice.

I'm sure there will be folk out there who have used non star marked tyres and done 8 million miles with no issues but it's not a risk I'd be willing to take for the sakes of not much more money for the star marked tyres - winter tyre/all season tyre selection put to the side.

Also interestingly if you put your BMW xDrive details into the Michelin website they will say their all season non star marked are fine for your car (they specifically say they don't have to be manufacturer approved or did when I checked months ago) which is directly contradictory to what BMW suggest/the research I've done.

Edited by babo456 on Wednesday 1st April 00:10

Edited by babo456 on Wednesday 1st April 00:13

[GeniusOfLove]

Do you only use BMW branded screenwash too

Joking aside tyre reviews did a bit on the differences in OE approved/specific tyres and there were some differences. I don't think using non OE marked tyres will have any material impact on your AWD transmission though.

[Smint]

It makes perfect sense not to have diffs of any description working permanently to account for excess rolling radius differences.

As such i've never replaced less than 4 at a time on any 4WD vehicles i've owned, the only exception for that would be selectable 4WD (typically old school pick ups) where the centre diff is manually engaged only for off road or really poor road conditions such as snow/mud/ice, these vehicles sometimes go for years operating in RWD only the center diff never getting engaged.
If a semi worn tyre was damaged beyond repair on a modern 4WD but the other 3 still worth running i'd try and find a suitable part worn.

[DickP]

Hi,

So from the responses above I am best to assume all XDrive cars will be sending power to both axles at all times, and the proportion is only what varies?

The tyre size difference tolerance is about 2mm then too?

[lemonslap]

babo456 said:

I hadn't really thought about this too much initially myself. I've had several xdrive BMW's M340i, M4, M3 all had star marked tyres but didn't think too much of it other than it's probably a marketing gimmick.

I've now got an X3 m40i and it's a royal pain finding 'star marked' tyres for it/limited selection (21" rims). I wanted to get some cross climate tyres but there aren't any that are star marked. I've ended up buying a second set of 19" wheels and star marked winter tyres.

There are already a few threads of this and many varying opinions but my conclusion was that I wasn't going to risk running non star marked tyres on the car mainly due to any warranty issues BUT I had read anecdotal posts of people using non star marked tyres fitted and transfer box whine (I think there was a post on here with someone in an X5?) and they swapped back to star marked and the issue went away.

The star marked tyres tread pattern and compound is also specific to xDrive cars in what I've come across (someone even had a picture of non star Vs star and you can see the difference) so for whatever reason xDrive seems to be more sensitive than other all wheel drive systems hence the need for specific tyres.

Star marked tyres are readily available summer wise but a bit of a pain for all seasons or winter - as limited or NO choice.

I'm sure there will be folk out there who have used non star marked tyres and done 8 million miles with no issues but it's not a risk I'd be willing to take for the sakes of not much more money for the star marked tyres - winter tyre/all season tyre selection put to the side.

Also interestingly if you put your BMW xDrive details into the Michelin website they will say their all season non star marked are fine for your car (they specifically say they don't have to be manufacturer approved or did when I checked months ago) which is directly contradictory to what BMW suggest/the research I've done.

Edited by babo456 on Wednesday 1st April 00:10

Edited by babo456 on Wednesday 1st April 00:13

That was my F85 X5M nearly 4 years ago, good memory! No noise, but weird surging feeling at speed in a straight line that gradually got worse. Been fine ever since running star marked tyres.

[The Gauge]

I was pleased when I found out the 730d I was buying didn't have XDrive, though I would enjoy the benefits in winter with all season tyres. Instead I have a spare set of smaller wheels with all season tyres, and swap then over twice a year.

However I fancy buying an X5 next year and I will have to decide whether to go the XDrive route, and risk having to swap all 4 tyres if I get a non repairable puncture.

Strange how far cars have evolved, yet an extra mm difference in tyre tread can cause expensive problems.

[Dashnine]

While OEM vs standard tyres probably doesn’t affect the transmission, there is quite a difference between standard and star marked PS4S tyres - construction, compounds, tread pattern which pretty much makes it a different tyre:

[Panamax]

Smint said:

It makes perfect sense not to have diffs of any description working permanently to account for excess rolling radius differences.

I don't think it matters if an "open" diff is rotating a bit. It's limited slip diffs that can suffer increased wear (and presumably heat) if they're "working" all the time.

As regards 4x4 systems, different tyre diameters could cause the front and rear diffs to be "working" even if the centre diff is idle. Similarly the centre diff could be "working" even if the front and rear diffs were both idle. It all depends which tyre is where.

With modern electronically controlled brake systems it's debateable whether 4x4s need LSDs at all. Each manufacturer makes their own choices.

[Smint]

Panamax said:

I don't think it matters if an "open" diff is rotating a bit. It's limited slip diffs that can suffer increased wear (and presumably heat) if they're "working" all the time.

As regards 4x4 systems, different tyre diameters could cause the front and rear diffs to be "working" even if the centre diff is idle. Similarly the centre diff could be "working" even if the front and rear diffs were both idle. It all depends which tyre is where.

With modern electronically controlled brake systems it's debateable whether 4x4s need LSDs at all. Each manufacturer makes their own choices.

Our Hilux being selectable 4WD had a rear LSD instead of a lockable rear diff, so fierce was the thing it wasn't far off 'locked' in effect on a wet road, i had to sell the OE Pirelli tyres within 1000 miles of purchase and fit far better wet grip rubber because the back end was skittish as hell, decent tyres sorted that issue.

Our Prado as you rightly say doesn't have lockable front or rear diffs instead relying on electronic control, which to be fair works very well.
More basic Prado models don't have the electronics but come instead with a locking rear diff.

[The Gauge]

Similar thread here from 2022..

https://www.pistonheads.com/gassing/topic.asp?h=0&...

[DickP]

Hi,

So the consensus is the XDrive really should be avoided? That’s the vibe I’m getting?

[lemonslap]

DickP said:

Hi,

So the consensus is the XDrive really should be avoided? That s the vibe I m getting?

i think the challenge is that BMW decides to install staggered wheel set up on xdrive vehicles, my X5M runs 285/35/21 at the front and 315/30/21 at the back. If it was a square setup then there wouldn’t be such an issue with tyres or system.

What I will say regarding my experience of the xdrive and electronic rear diff on our ours is that we have never managed to get stuck, even trying to get out of a very muddy lydden hill racetrack once. We were next to a Volvo XC90 that refused to move in those conditions, I was deeply impressed!

[lemonslap]

Dashnine said:

While OEM vs standard tyres probably doesn t affect the transmission, there is quite a difference between standard and star marked PS4S tyres - construction, compounds, tread pattern which pretty much makes it a different tyre:

When it comes to staggered set up tyres, star marked will be exactly the same rolling diameter. Others won’t be, that’s the crucial difference for xdrive vehicles. I did have a photo of the different diameter of the Pirelli P-Zeros that I had to remove after we purchased the vehicle. There was a good 1cm+ difference between front and back despite being the same size as the staggered star marked set.

[The Gauge]

My main concern with using non star tyres would be BMW warranty refusing to cover any transfer box issues. Anyone know of they use this tactic?

[DickP]

A square set up will be okay then? Which I assume is what has to be done for using all seasons too?