[IanH]

Xdrive isn't full time awd, so why would it have an issue with 2 different diameters? It could just slip the clutch to compensate, and likely does this regularly for all sorts of driving conditions.

The only thing that can really be an unsolvable issue would be the computational elements based on wheel speed, which shouldnt cause xdrive to fail but it may not work to its fullest capabilities.

[mjr24]

Originally Posted by Hiddenknife

I know it might be stupid sounding question, but does your BMW have xDrive, do you run non star marked non runflat tires? For how long and if any issue so far?

BMW star marked tires are mostly "star marked" because they are approved in rolling circumference (same RC all four)but of course there are some other star marked tires that use different rubber compound like Michelin Pilot Sport 4S

I’ve put hundreds of thousands of miles on high end BMWs with zero issue…..ever. Maybe ran OEM tires for 10 percent of that…..yes lots of that were on XDrive models….X5M, X7, M5, M8, M550i.

It’s like how BMW recommends Shell gasoline. Sure, maybe the car runs “optimally” on it….but the gas from other places works just as good.

That being said, I stay within the 1 percent rule. That’s something I wouldn’t want to risk going outside of.

[Hiddenknife]

Originally Posted by mjr24

I’ve put hundreds of thousands of miles on high end BMWs with zero issue…..ever. Maybe ran OEM tires for 10 percent of that…..yes lots of that were on XDrive models….X5M, X7, M5, M8, M550i.

It’s like how BMW recommends Shell gasoline. Sure, maybe the car runs “optimally” on it….but the gas from other places works just as good.

That being said, I stay within the 1 percent rule. That’s something I wouldn’t want to risk going outside of.

If i strictly follow the tiresize, load rating, speed rating written on the B pillar, does that mean im going to be within the 1%?

[pbonsalb]

Yes. Same size tires as stock are what BMW specifies. When you alter sizes, stay very very close to the original front to rear circumference variance, especially on AWD cars. If that variance is 1%, don’t exceed 2%. The traction control, ABS and AWD transfer case are programmed for that stock variance.

[RockCrusher]

Originally Posted by smrtypants44

Yeah if BMW had such tight tolerances on xdrive then units would be failing all over the place just from manufacturing and wear variances.

The Porsche sanctioned tires identified with the 'N' on the sidewall have a bit less tread depth. I believe this arises because the tire maker removes some tread to ensure the tires sizes are more precise/matched that what the result from the tires being used directly from the mold.

The BMW sanctioned tires I looked into likewise have less tread depth.

[RockCrusher]

Originally Posted by IanH

Xdrive isn't full time awd, so why would it have an issue with 2 different diameters? It could just slip the clutch to compensate, and likely does this regularly for all sorts of driving conditions.

The only thing that can really be an unsolvable issue would be the computational elements based on wheel speed, which shouldnt cause xdrive to fail but it may not work to its fullest capabilities.

xDrive is a full time AWD system.

BMW xDRIVE technology is an advanced, permanent all-wheel drive system that ensures your BMW has the best possible traction at all times. This helps to keep you safe in inclement weather and in imperfect driving conditions, and helps provide smooth, sporty performance.

https://www.bmwofsudbury.com/new-bmw...ippery%20roads.

The system can adapt to some differences in tire diameter. But slipping the clutches can (will) lead to premature wear. Which is one reason why the proper tires should be used from the get go.

[IanH]

Originally Posted by RockCrusher

xDrive is a full time AWD system.

BMW xDRIVE technology is an advanced, permanent all-wheel drive system that ensures your BMW has the best possible traction at all times. This helps to keep you safe in inclement weather and in imperfect driving conditions, and helps provide smooth, sporty performance.

https://www.bmwofsudbury.com/new-bmw...ippery%20roads.

The system can adapt to some differences in tire diameter. But slipping the clutches can (will) lead to premature wear. Which is one reason why the proper tires should be used from the get go.

That description is wrong, the front wheels can be completely decoupled to 0 torque, aka clutch release. The rear wheels are driven at all times.

here is a good writeup

https://f30.bimmerpost.com/forums/sh...38&postcount=2

[visionaut]

Tread depth 2mm between tires, is 4mm diameter difference..

I run the Michelin PS4S 245-255-35/19 combo on my F36 435iX. Michelin’s exact difference (all brands/styles aren’t same-same) is only 8 revs/mile. So even tho’ the diameter is cited as .2” (25.8 “ vs 26”) that’s probably rounded up.

My OE staggered 19s (225/40 & 255/35) were 1.5mm diameter difference.

It’s my understanding that BMW’s tire inflation specs for specific tire sizes are how they make up the small differences to try to get equal F & R. They’ll slightly over or under inflate the front or rear axle to match up… (They raised rear pressure, the 255 being smaller than the 225. I run my 245 fronts slightly higher since they’re the shorter.)

Regarding BMW ‘Star’ tires. They’re different than the non-marked versions. They’re developed by the car & tire manufacturer in combination, tweaking the ‘standard tire’ to the car makers priorities. Good write-up here:

https://www.michelinman.com/auto/aut...d-marked-tires

[wilbur_the_goose]

For what it's worth, I haven't found any non-runflats/all season tires with the BMW star logo.

Has anybody found such a tire?

[omasou]

Originally Posted by Hiddenknife

Hi folks!

I ve been searching alot on this topic weather you should pick OE tires for your BMW model or regular non OE tires. Interstingly i ve not yet found any conclusion on this matter if you should run OEM tires to avoid any potential xDrive issues or run normal non OEM tires because of the better price & comfort being non runflat. Nobody gives an clear answer why you should pick one over the other technically. Theoretically BMW approved tires have the same rolling circumference, being created specifically to meet the stringent low RC tolerances of xDrive.

Yesterday i found something very interesting. I compared two brand new tires one Pirelli Sottozero 3 (star marked) runflat and a regular Michelin Pilot Sport 4s non runflat of the same size, load rating and speed rating. 245/45/20 103V

In the photos attatched you can see the OE tire is slightly taller than the non OE one even if it has 0.5 mm less thread than the non OE tires. (8 for MPS4S and 7.5 for Pirelli)

I ve seen many people post around that if you mount tires based on the readings on the B pillar specs you are totally fine as long as it matches the exact tire size, load & speed ratings. But the question is, are they acutally the same? Obviously it seems they are not. Are we exposing ourselves to transfer case/xdrive issues? i dont know. Maybe we dont see an immediate effect on the drivetrain but maybe it stresses it s componets running non approved tires.

Making this post is not because i am an advocate of OEM tires but because i wanna see what othets think about this matter.

Potential X drive damage can occur when the tire are significantly different in diameter side to side. This could happen if the tires a near replacement, you get a flat and only replace one tire. Other than that you're over thinking this.

I always swap out my tires for Michelins non-run flats regardless of what came w/the vehicle.

[wilbur_the_goose]

Run-flats are horrendous. I can’t determine why the Star essentially means OEM which means run-flat. Yuck!

[DrVenture]

As to the notion that BMW raises tire pressure to accommodate slight size variations, on the 2022 M550 with 20" wheels the fronts are 245/35-20 and rears are 275/30-20. Continental (DSW06+) says that the fronts are 26.8" in diameter and the rears are 26.5" in diameter. The DSW06+ are commonly used, but not oem.

The 3 psi higher front pressure would seem to worsen the issue, rather than alleviate it. The rears are ~1% smaller than the fronts when new. Would the fronts wear faster from the added weight? Or the rears wear faster from being RWD biased? My experience has been the rears wear faster.

So, we start out with the rears shorter by 1%, pump up the fronts more than the rears, and wear the rears faster. LOL - wrap you heads around that!

Are the "star" tires different. Or the stock runflats? Are they better matched in terms of sizing?

[BMWbiker]

Just to confuse things a bit more, the G26 M440i x-drive GC comes with:

19x8.5, 245/40 R19 front
19x9.0, 255/40 R19 rear

This is more than a 1% difference in diameter.

[snowbimmer]

The F90 came with 275/35 and 285/35 tires - a little over 1% larger in back. I bought their OEM winter tire and wheel set that brought the rim down to 19" from 20". The tires were 265/40 and 285/40 - a little over 2.3% larger in back. Ran them for several seasons. Where's BMW's 1% thinking on that set of wheels? Right, there wasn't any.

Hundreds, if not thousands, of people on these forums are changing to aftermarket wheels and tires all the time. Larger diameter rims, much wider tires. I can't say I've read about anyone with problems created by this - there probably are, but it seems rare. And these were people that probably went just a little bit cray cray without checking too closely what they were actually getting in to.

I know, because when I eventually went to sell the F90 set locally, I had no inquiries from F90 owners. These people didn't understand bolt patterns, offsets, wheel widths or tire profiles. They just wanted the wheels - because they looked cool (Blacked out 705M). I patiently went through all of them and each time lost the sale when I said they wouldn't work. Finally sold them to a young guy who's father was sure they would fit his car. I had my doubts, but unloaded them. They can always resell them if they have to.

The actual real world tolerances for front to back diameters are probably far greater than we are led to believe - they just need to scare everyone and keep them within some guardrails. I can't imagine a car manufacturer, like BMW, that would build an AWD system that would be so prone to failure just by looking sideways at it. Still, I always try to stay matched up within the 1% - not that hard to do.

[KevinC]

Originally Posted by mjr24

There is zero difference between star tires and normal non-star tires…..at least not one that you or I will ever notice.

Please stop spreading misinformation.

https://youtu.be/COA630Juf_U?si=NSfOjLasRPhsLSer

[DrVenture]

To be fair, he concluded, while power sliding around a track with tight turns, that the dry was better (for the stars) and the wet was worse. And he only tested one brand of max performance summer tires.

No way I'd be able to tell the difference. But, if looking for best times on a track, his findings seem relevant.

[mjr24]

Originally Posted by KevinC

Please stop spreading misinformation.

https://youtu.be/COA630Juf_U?si=NSfOjLasRPhsLSer

[Neusser]

Originally Posted by mjr24

I’ve put hundreds of thousands of miles on high end BMWs with zero issue…..ever. Maybe ran OEM tires for 10 percent of that…..yes lots of that were on XDrive models….X5M, X7, M5, M8, M550i.

It’s like how BMW recommends Shell gasoline. Sure, maybe the car runs “optimally” on it….but the gas from other places works just as good.

That being said, I stay within the 1 percent rule. That’s something I wouldn’t want to risk going outside of.

Exactly. These are just corporate partnerships that bring in additonal short-term revenues for both companies.

My M2 has a big sticker on it saying BMW recomends Shell gas; meanwhile current independent testing shows that Aral Ultimate 102 actually bumps hp and contains superior cleaning agents.

It's largely all smoke and mirrors in the pursuit of cash.

[ShocknAwe]

Gotta watch out for the Xdrive inspection gnome that climbs out of the transfer case and looks at your tires in secret at night to make sure they're Star Spec. If they're not, beware! Angry inspection gnomes sabotage Xdrive systems.





Obviously it's hogwash. Run similar circumferences F/R from the SAME TIRE (meaning tire model dimensions and manufacturer) and you'll be fine.

[DomMuc]

Originally Posted by Hiddenknife

Hi folks!

I ve been searching alot on this topic weather you should pick OE tires for your BMW model or regular non OE tires. Interstingly i ve not yet found any conclusion on this matter if you should run OEM tires to avoid any potential xDrive issues or run normal non OEM tires because of the better price & comfort being non runflat. Nobody gives an clear answer why you should pick one over the other technically. Theoretically BMW approved tires have the same rolling circumference, being created specifically to meet the stringent low RC tolerances of xDrive.

Yesterday i found something very interesting. I compared two brand new tires one Pirelli Sottozero 3 (star marked) runflat and a regular Michelin Pilot Sport 4s non runflat of the same size, load rating and speed rating. 245/45/20 103V

In the photos attatched you can see the OE tire is slightly taller than the non OE one even if it has 0.5 mm less thread than the non OE tires. (8 for MPS4S and 7.5 for Pirelli)

I ve seen many people post around that if you mount tires based on the readings on the B pillar specs you are totally fine as long as it matches the exact tire size, load & speed ratings. But the question is, are they acutally the same? Obviously it seems they are not. Are we exposing ourselves to transfer case/xdrive issues? i dont know. Maybe we dont see an immediate effect on the drivetrain but maybe it stresses it s componets running non approved tires.

Making this post is not because i am an advocate of OEM tires but because i wanna see what othets think about this matter.

it´s the same size but you compare a summer with a winter tire... on the other hand bmw says you should not mix different tire manufacturer

[pbonsalb]

I read somewhere that one of the star spec Michelins has 0.5mm less tread than the non star spec. I forget which size/application. I don’t believe that was for transfer case longevity; I think it was more likely for ever slightly better handling.

Somewhere there is a post that says BMW pulls the Michelins and installs Continentals on M5’s at their 2 USA driving schools. That was from a guy who did one of the schools.

For the M5, BMW allows square snows — all 265/40/19. So 0% difference is OK instead of 1%. BMW also allows 265/40/19 and 285/40/19 snows, so 2% is OK instead of the stock 1% difference. Seems like the 0-2% range is OK for the transfer case, ABS and traction control. I personally would be hesitant to go further in either direction.

My take on the above is that non star spec vs star spec tires in the same sizes won’t make any difference. I may put Continentals on my M5 next year. Not sure yet — it will depend more on size availability in the wider sizes I want than on star or no star.

I am also not scared of mixing similar level tires front to rear. I would not mix side to side. And I would not mix different level tires front to rear (i.e., cheap all seasons in front and PS 4 on the rear). But who would?

[pmsteinm]

I can confirm the F90s run a square 285 Continental setup at Thermal. Although I also drove one with Michelin 275 fronts for a 1/2 day. Continental is a sponsor and provides tires.

The technical guides on the ATC13-1 VTG in the G12 docs are somewhat vague:

"Since a rigid connection of the rear axle with the front axle is not possible due to possible differences in the wheel speeds, there is a multidisc clutch inside the transfer case. The multidisc clutch performs the task of variable torque distribution between the two drive axles."

"The multidisc clutch in the all-wheel drive transfer box allows the torque to be distributed to both axles within certain limits. Seen statistically, the torque distribution between the front and rear axles on the current BMW all-wheel drive vehicles is 40:60. In the G12, the torque distribution to the two drive axles was split equally in the direction of 50:50. In terms of dynamics, however, other important parameters such as different wheel slip values play a part. It is no longer possible to speak of a 50:50 torque distribution with different wheel slip values at the two drive axles. In this case, the drive torques are distributed variably in the range between theoretically 0:100 and 100:0 corresponding to the driving situation."

"Efficiency Mode is always used when there is no all-wheel drive request from the DSC control unit and the multidisc clutch is therefore open."

But then:

"The tire tread depth, tire rolling circumference as well as the tire manufacturer should be the same on the front and rear axles if possible, in order to ensure proper functioning of the xDrive. It is also recommended to use only tires that have been approved or recommended by BMW."

I know the M5 only uses the front wheels when the rear is slipping since numerous documents talk about that. But for the G12/G05 I'm not clear what is going on. The 40/60 50/50 text above seems to contradict the efficiency mode statement .

But no matter what, if the front and rear wheels don't match perfectly (which I think is impossible in the real world) the VTG clutch will always slip some, as it is designed to do. Looking at the picture the chain loops between the 2 sprockets. There is no "gearing". So if the clutch is at 100% both axles turn at the same speed.