[HighlandPete]

Originally Posted by xenon

The x drive system must be pretty shite if it can be damaged by tyres outside of a 1% tolerance.

For xDrive torque bias sensitivities, we want a very close tolerance. At the same time tyre tolerance logic means 'backing off' the locking torque. Bit of a conflict.

Any differences in axle rotation speeds, has to be eliminated in one of two ways. It is either through tyre scuff, or transfer box slip, (possibly a bit of both), there is no other way. The wider the tyre tolerance, the more the issue.

The result of different tyre rolling dimensions, is extra heat and possible wear. Can show up as transmission bunching and judders, even if the transfer box isn't showing wear.

This is not a new thing, just with BMW. AWD systems have always been subject to tyre issues. Some of the manufacturers have been even more specific about tyre wear differences across axles and between axles. Hence the tyre shaving often used on AWD vehicles, to save fitting a pair or a full set of tyres. Users fitting one tyre, would have it shaved, to a similar tread depth to the existing tyres.

BTW, Porsche have tight limits as well. I understand with Porsche approved tyres, the difference in tread depth between axles is a maximum of 30%.

[xenon]

But why don’t they have front to rear differential? That’s another way to cope with differing axle speeds, surely? Never ever an issue with Quattro. I've had two quattro cars, my last A6 with the pukka permanent AWD quattro system and my wife had an A3 which was badged "quattro" but actually had the Haldex shite. However, tyre issues were not a concern on either - I don't recall the handbook making any reference to tyre wear and the forums are not awash with dire warnings of irreparable damage to the system if your tyres differ by slight amounts. You say some manufacturers are even more specific; which ones are those as a matter of interest? Maybe they are all similar in that case, other than quattro. What about "traditional" AWD such as Range Rovers with all-terrain tyres? Is it an issue there?

(incidentally, I'm not provoking an argument - I've known your posts for many years and I fully respect your opinion and knowledge )

[HighlandPete]

Here's a 'broad brush' article on the subject.

https://www.tirerack.com/tires/tiret...e001630a18050d

Audi and Subaru have a history of making recommendations. I also notice Porsche is mentioned, the figure I had posted earlier. Not sure on the current LRJ product, but Land Rover was very susceptible to transmission issues in the past. I'll have to see how LRJ are doing.

[JNW1]

Originally Posted by HighlandPete

Here's a 'broad brush' article on the subject.

https://www.tirerack.com/tires/tiret...e001630a18050d

Audi and Subaru have a history of making recommendations. I also notice Porsche is mentioned, the figure I had posted earlier. Not sure on the current LRJ product, but Land Rover was very susceptible to transmission issues in the past. I'll have to see how LRJ are doing.

So the article linked is suggesting that, to avoid problems with a 4WD system, all four tyres should be the same brand, tread design and have the equivalent tread depth. This I understand but what I still can't get my head around is why they should also need to be star rated?

To me the only way that would make a difference is if the manufacturing tolerances for star rated tyres are much tighter than for non-star rated but is that the case? I thought star rated tyres just had a slightly different combination of compounds which are - in theory - better matched to the characteristics of some BMW models but has it ever been claimed they're also manufactured to closer tolerances and as a result are less likely to cause problems with the X-Drive system?

[Dave_3]

Originally Posted by JNW1

So the article linked is suggesting that, to avoid problems with a 4WD system, all four tyres should be the same brand, tread design and have the equivalent tread depth. This I understand but what I still can't get my head around is why they should also need to be star rated?

I would expect a single tyre size from a respected manufacturer to have a good variance or consistency in rolling diameter between individual tyres of the same marked size.

What would give me greater confidence buying star rated tyres is when you run staggered setups. In that case I also want the same tight variance in all the rolling diameters between the front and rear axles.

For example my 435D runs 225/45 x 18s on the front and 255/40 x18s on the rear.

If they are manufactured with the knowledge that they will be specifically used in combination in an xdrive setup and get as close as possible to a consistent overall rolling circumference for all my tyres, that's better than just as stand alone marked tyre size.

D.

[HighlandPete]

Manufacturing tolerances for the RC for the typical generic range of passenger and 4x4 tyres from the nominal size (ETRTO standard), is +1.5/-2.5%, dimensions to DIN 70020 at 37 mph (60 km/h). Easy to see why an approved tyre (particularly for xDrive use) may need much tighter tolerances, or move the tolerances in manufacture to be more like +x.x/-0% or +0/-x.x%, when two different tyre sizes are used together.

One of the personal examples I've come across, (and know other users have had transmission issues with), was with the E83 X3.

The E83 X3 was particularly susceptible to tyre selection. My son had a situation where the * marked tyres were out of stock nationwide. The only * marked front tyres in the country were available from BMW Direct. Our local National Tyres depot had a pair of non marked tyres of the same dimension. The manager wouldn't fit them, even if my son wanted him to, he said he had witnessed too many issues with the transfer box due to tyres, policy was "star marked only". He was willing to fit approved tyres, if my son could find them anywhere.

I looked into it at the time, to see what could be adrift with the generic sizing. I recall to keep to BMW tolerances for the staggered setup, no way could the star marked tyre be to generic dimensions, BMW must have had a unique tyre sizing to suit the xDrive.

[Dave_3]

Originally Posted by HighlandPete

I looked into it at the time, to see what could be adrift with the generic sizing. I recall to keep to BMW tolerances for the staggered setup, no way could the star marked tyre be to generic dimensions, BMW must have had a unique tyre sizing to suit the xDrive.

Exactly what I was thinking. Star marked staggered tyre sets, because of their specific tailored application, err to overall group rolling circumference consistency rather than matching true marked (expected) tyre size.

D.

[HighlandPete]

Originally Posted by Dave_3

Exactly what I was thinking. Star marked staggered tyre sets, because of their specific tailored application, err to overall group rolling circumference consistency rather than matching true marked (expected) tyre size.

That's where my knowledge takes me.

A quote from an E83 X3 Owner's Manual. BMW are very careful what they put in print these days, the 1% allowance for tyres and wear, used to be quoted in Technical Data.

[xenon]

Taking in to account all of the above: why the f*ck you want X Drive in the first place? If it’s that crucial then what a waste of everybody’s time.

[Dave_3]

Originally Posted by xenon

Taking in to account all of the above: why the f*ck you want X Drive in the first place? If it’s that crucial then what a waste of everybody’s time.

If I could get an S-Drive I would be after that, but marketing and customers want the perceived virtues of X-Drive. Alpina's (RWD) versions just a bit pricey for me.

Personally, I am just lugging about heavy transfer case, clutches, rotating inertia mass, half-shafts, bearings, UJs for 99.99% of the time. I wouldn't miss them.

D.

[xenon]

What car are you driving, Dave?

[Dave_3]

Originally Posted by xenon

What car are you driving, Dave?

At the moment one of the last run out "platinum edition" F32 435Ds (apart from lack of sunroof, all the toys). Managed to find it just before Christmas, for literally half the price of a new 4 series. Old model, 1900 kms, just before Christmas - good time to buy.

Still very much keeping an eye on the upcoming MHT 430D and M440Ds. But guess they will arrive very slowly what with the present electrical component shortages affecting the availability of options?

D.

[JNW1]

Originally Posted by xenon

Taking in to account all of the above: why the f*ck you want X Drive in the first place? If it’s that crucial then what a waste of everybody’s time.

Agreed! If BMW offered the M340i as RWD for the UK market there's no way I'd opt for an X-Drive version although that's mainly because I don't think the traction advantages in certain conditions outweigh the permanent disadvantages of extra weight, inferior fuel consumption, etc. The apparent sensitivity to tyres is IMO just another reason to stick with RWD where possible!

[verysideways]

I'd happily have had a rwd m340i too.

Back when I owned an E83 X3 3.0sd I had a conversation with BMW UK about tyre shortages in the correct fitment for my staggered 18" wheels and the person I spoke to said that it was both rolling radius and weight that was part of the quality control process for star marked tyres over the equivalent sized non star marked. Weight!!

But then I also remember the early 1990's, when a puncture in a cavalier 4x4 or calibra 4x4 meant 4 new tyres!

[xenon]

When I bought my 330i I wasn’t looking that far up the food chain and hadn’t realised the 340i was X-Drive only.

[Stevie_B]

Originally Posted by Stevie_B

Trying to get hold of a full set of Star rated PS4Ss at the moment and like others have said it's proving difficult - I can find fronts but getting quoted 16+ weeks for rears.
Not sure if there would be any issues with having Star rated fronts and Aftermarkets in rear...? I highly doubt it but I'm no tyre expert.

Also makes me question if a dealer would just end up putting aftermarkets on too given availability of Star rated ones.

Well I'm picking my car up tomorrow and BMW dealer themselves have confirmed today they can't get hold of Star marked PS4S and can't confirm when they could.

So very much looking like non-Star marked all round (or star front, non star back as Azure has done…).

Bit of a ballache but also doubt I'll notice.

[Charlie (Wessex)]

I've got Goodyear F1* on.

Any * all season tyres out there, as find that they're good enough for most winters in southern England

[AndyTo]

I've learnt something from this vid