[dakine]

You will have problems if the rolling diameter is not the same or off by around 1% or more (the 1% is a recommended tolerance reported by others on this forum).
- you will trip your DTC while driving normal
- you may blow up your transfer case
- you will not be happy

Can an admin make this a sticky at the top?
I saw more threads/questions pop up and want to make sure people who are new to the XI, and wouldn't even think to search for DTC + new wheels/tires unless they encounter a problem (after it's too late), read this prior to purchasing new wheels.

Related posts:
http://www.e90post.com/forums/showthread.php?t=190455
http://www.e90post.com/forums/showthread.php?t=186089

Thanks

[Ph0zenstone]

Originally Posted by dakine

You will have problems if the rolling diameter is not the same or off by around 1% or more (the 1% is a recommended tolerance reported by others on this forum).
- you will trip your DTC while driving normal
- you may blow up your transfer case
- you will not be happy

seems to be a little exagerated. I've seen many people run wheels past the 1% threshold with no problems. Just like people argue that non staggered can't be used on Xi. Of course I don't recommend it but its not detrimental.

[who's ur daddy?]

Not that I disagree with the advice of matching front and rear diameters, but where does this magical 1% threshold come from other than some members mentioning it? Is it from a BMW engineer or something? If its on the Internet it must be true...

Again, it makes inherent sense to keep the diameters the same to preserve the clutch pack, but I just question what the appropriate threshold is.

[scheherazade]

I think "1% tops" is another way of saying "as close to 0% as you can".

Granted that if you run staggered wheels, you'll have a hard time getting to 0%... but something "as good as zero (aka some low % like 1%)" will suffice.

-scheherazade

[Nikolas]

Oh dear god don't forget the POLAR BEARS!!!!!!!!!!!!!!!!!!!!!!!!!! This is huge!!!!!!

[JunkStory]

well that sucks. So what if the diameter shrunk a little bit due to normal tire wear?

[Lassaxi]

Originally Posted by who's ur daddy?

Not that I disagree with the advice of matching front and rear diameters, but where does this magical 1% threshold come from other than some members mentioning it? Is it from a BMW engineer or something? If its on the Internet it must be true...

Short answer: Nowhere. Someone pulled it out of his ass one day, posted it on this forum, and it became a fact.

Long answer: I've been on this forum for more than 2 years. As far as I can tell, this 1% figure came from a group of former Subaru owners who knew that their Subies had a 1% overall rolling diameter tolerance. This is true for Subarus that have viscous center couplings as their AWD system (which would be most manual transmission models, except for the STi). The reason for this is that a viscous coupling is not a particularly heat or stress tolerant form of differential, and the fluid in the coupling can overheat and break down over time if the axles that feed it are constantly spinning at different rates.

Of course, xDrive doesn't use a viscous center coupling. It uses an electromechnical clutch pack, with a heat-resistant clutch. I've never seen anything that provides a specific tolerance for an acceptable difference in rolling diameter between front and rear wheels.

During a trip to my local independent service shop about a year or so ago, I was flipping through the SA's xDrive dynamics manual, and saw a secton on wheels. I remember that the manual said that the xDrive system can tolerate differences in wheel diameter, but I don't remember there being anything specific about how big of a difference was acceptable. I was just sort of flipping through the book though, and I wasn't really looking for an answer to this question, so it may be in there and I may have just missed it. If any SAs out there have their xDrive dynamics training manual, and can lay this issue to rest once and for all, I would welcome it.

EDIT: To clarify, my post should not be read as suggesting that you will have no problems if you put monster truck wheels on the rears and heelys on the fronts. My point is simply that, so far, I have never seen any real basis for the 1% figure.

[Lassaxi]

Originally Posted by Nikolas

Oh dear god don't forget the POLAR BEARS!!!!!!!!!!!!!!!!!!!!!!!!!! This is huge!!!!!!

Finally! Someone else understands the true issue!

[Kev]

Seriously, there are a lot of people running staggered on the XI platform and so far I've only see one person reporting DTC light on but it was because he put on wrong-sized tires. I've been running staggered on both the old and new XI for 1.5 years and neither car has detonated, nor have I been attacked by a Polar Bear yet.

[BMWFanboy]

Originally Posted by phozenstone

seems to be a little exagerated. I've seen many people run wheels past the 1% threshold with no problems. Just like people argue that non staggered can't be used on Xi. Of course I don't recommend it but its not detrimental.

I'm a little confused. Most of the posts seem to indicate people think a staggered setup is bad with the xi, but you're saying some people argue the opposite?

[who's ur daddy?]

Originally Posted by Lassaxi

Short answer: Nowhere. Someone pulled it out of his ass one day, posted it on this forum, and it became a fact.

Long answer: I've been on this forum for more than 2 years. As far as I can tell, this 1% figure came from a group of former Subaru owners who knew that their Subies had a 1% overall rolling diameter tolerance. This is true for Subarus that have viscous center couplings as their AWD system (which would be most manual transmission models, except for the STi). The reason for this is that a viscous coupling is not a particularly heat or stress tolerant form of differential, and the fluid in the coupling can overheat and break down over time if the axles that feed it are constantly spinning at different rates.

Of course, xDrive doesn't use a viscous center coupling. It uses an electromechnical clutch pack, with a heat-resistant clutch. I've never seen anything that provides a specific tolerance for an acceptable difference in rolling diameter between front and rear wheels.

During a trip to my local independent service shop about a year or so ago, I was flipping through the SA's xDrive dynamics manual, and saw a secton on wheels. I remember that the manual said that the xDrive system can tolerate differences in wheel diameter, but I don't remember there being anything specific about how big of a difference was acceptable. I was just sort of flipping through the book though, and I wasn't really looking for an answer to this question, so it may be in there and I may have just missed it. If any SAs out there have their xDrive dynamics training manual, and can lay this issue to rest once and for all, I would welcome it.

This type of response I can appreciate! Thank you.

[dakine]

The point of starting this thread was to let current or future XI owners know if you plan to run a staggered setup, make sure the rolling diameter of your wheels are as close to 0% as possible before going out and buying the wrong sized tires/wheels like I did, so for a fact, 245/40/18 rear with 225/40/18 front (on stock staggered rims) is out of tolerance. By the way, what do polar bears have to do with this thread? I may be too old or young to understand the reference... Someone please enlighten me....

[dakine]

nevermind about the polar bears reference... I did a search for polar bears and staggered setup and was viciously chased by a polar bear... good thing I'm running a staggered winter setup or I would've been done for.

[nicknaz]

Originally Posted by Lassaxi

Long answer: I've been on this forum for more than 2 years. As far as I can tell, this 1% figure came from a group of former Subaru owners who knew that their Subies had a 1% overall rolling diameter tolerance. This is true for Subarus that have viscous center couplings as their AWD system (which would be most manual transmission models, except for the STi).

If that is a true story, then fail by those Subaru owners because the actual Subaru spec is "Within 1/4-inch of tire circumference or about 2/32-inch of each other in remaining tread depth." (using the OEM diameter for an '02 WRX as reference, the acceptable difference in tire diameter is 0.05%)

Also, this spec is true for both viscous center diff WRX and multiplate clutch pack STi

I'm biased because I am a 'former Subaru owner' but if I had a xi I would use that Subaru spec in the absence of an equivalent BMW spec because all AWD systems have an ideal rolling diameter difference of 0% and an overly tight spec is much better than $$$$ differential damage

[undecided]

What if one tire is under or over inflated by let's say 5-10 psi? would that cause a problem?

[nicknaz]

Originally Posted by undecided

What if one tire is under or over inflated by let's say 5-10 psi? would that cause a problem?

It would cause a problem with tire wear for sure over the long term.

Would it cause a problem with the differentials? I'm not sure but I suspect it would not because I don't think that difference in pressure will change the tire's diameter very much (driving an AWD with one flat tire (conventional tire with no air) would be a big problem though)

[Lassaxi]

Originally Posted by Nick Nazareno

If that is a true story, then fail by those Subaru owners because the actual Subaru spec is "Within 1/4-inch of tire circumference or about 2/32-inch of each other in remaining tread depth." (using the OEM diameter for an '02 WRX as reference, the acceptable difference in tire diameter is 0.05%)

I don't know whether it's a true story or not, but it's the best I've been able to discern over the years. If anyone has any more reliable info, I'd love to see it.

Also, this spec is true for both viscous center diff WRX and multiplate clutch pack STi

I did not know that. My '04 Outback, which has Active AWD (which includes the multiplate clutch pack) apparently does not have that same tolerance. I know this because I got a flat at about 17,000 miles, and needed to have a tire replaced. I asked the dealer about needing to replace all four tires, and was told that the tolerance on the Active AWD system is not as tight as on the viscous center coupling systems. I checked my manual, and sure enough, it only says that the tires need to be of the same make, model, and size, and there is nothing about uneven wear. My friend's WRX manual, in contrast, had a sentence or two about uneven tire wear.

[nicknaz]

Originally Posted by Lassaxi

I did not know that. My '04 Outback, which has Active AWD (which includes the multiplate clutch pack) apparently does not have that same tolerance. I know this because I got a flat at about 17,000 miles, and needed to have a tire replaced. I asked the dealer about needing to replace all four tires, and was told that the tolerance on the Active AWD system is not as tight as on the viscous center coupling systems. I checked my manual, and sure enough, it only says that the tires need to be of the same make, model, and size, and there is nothing about uneven wear. My friend's WRX manual, in contrast, had a sentence or two about uneven tire wear.

I'm getting info from Sube tolerances from the factory service manual for WRX/STi. I don't have the CD with me so I can't post the pdf. I don't think anyone here will care anyway

[Ph0zenstone]

Originally Posted by BMWFanboy

I'm a little confused. Most of the posts seem to indicate people think a staggered setup is bad with the xi, but you're saying some people argue the opposite?

if you got the ZSP and 18" wheels, then you got staggered wheels on an Xi from BMW.

[Lassaxi]

Originally Posted by Nick Nazareno

Did you have the non turbo 2.5L outback? the AWD in that car is part time. the car is FWD and then based on wheel spin in the front wheels a transfer clutch locks up the center diff and powers the rear wheels. that's why the tolerance isn't as tight (especially for the rear wheels) the center diff isn't routing power all the time.

If you had the 3.0 or 2.5 turbo automatic I'm surprised the tolerance isn't tight because the center diff uses a planetary gearset to provide fulltime AWD with rearward torque bias and a transfer clutch to control the front rear torque split. that system (like the viscous coupling type) would care about tolerance because the center diff is routing power all the time

anyyyway.... i am surprised this differential topic comes up so often on this forum and that no one with a xi factory service manual has posted with some tech.

Actually, I have an H6-3.0. But, in '04, the the Variable Torque Distribution system (the full-time electromechanical clutch system you've described) was only available on the "VDC" model Outback. I have an L.L. Bean model, which didn't get the VTD AWD system until '05. That might explain it.

[nicknaz]

Originally Posted by Lassaxi

Actually, I have an H6-3.0. But, in '04, the the Variable Torque Distribution system (the full-time electromechanical clutch system you've described) was only available on the "VDC" model Outback. I have an L.L. Bean model, which didn't get the VTD AWD system until '05. That might explain it.

I deleted my post above since it is incorrect speculation on my part. I looked through some of my old Impreza docs and all of the awd systems (viscous (manual trans), clutch (auto) , planetary and clutch (auto w/ VTD)) are full time, but with different default torque splits.

I frankly have no idea why the tread depth specs appear to be more relaxed in your outback. Impreza service has the same spec for all cars

[Lassaxi]

Originally Posted by Nick Nazareno

I deleted my post above since it is incorrect speculation on my part. I looked through some of my old Impreza docs and all of the awd systems (viscous (manual trans), clutch (auto) , planetary and clutch (auto w/ VTD)) are full time, but with different default torque splits.

I frankly have no idea why the tread depth specs appear to be more relaxed in your outback. Impreza service has the same spec for all cars

Neither do I. EDIT: I was told he story I recited above- that the fluid in the viscous coupling is prone to overheating, which is why it's more sensitive to differences in tread depth. But if you're saying that the technical manual lists the same spec for all Imprezas, I've got nothing.

I suppose the possibility exists that the dealer simply lied to me and got away with it, but I've had one tire with 17,000 fewer miles on my OB wagon for the last 20,000 miles or so, and haven't had any problems with the AWD system. That reminds me- it's nearly time to start looking at new tires!

I also didn't realize that VTD was available on any of the Imprezas. I've also never owned an Impreza, so that probably exlpains that.