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      06-23-2015, 11:10 PM   #1
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xDrive 1% Difference in Rolling Diameter - Myth?

its almost a given now that the rule of thumb to follow for xdrive vehicles is to keep the rolling diameters within 1% front and rear. However, from an online manual (which the link is expired lol) it states:

"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-con- trol 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."


The last part mentions yes, for maximum performance the same diameter should be installed. but how does it account for a set of front tires that are balding and a set of rear tires with close to 100% tread. this will more than likely cause the rolling diameters to be >1% no?

Is there any concrete evidence from a BMW rep or anywhere online that states specifically, the xdrive system will explode and cease to function if the wheels are indeed >1%? Or is this just a myth that has become fact by way of word of mouth? any personal horror stories? any members running wheels clearly >1% diameter on xdrive with no issues
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      06-23-2015, 11:27 PM   #2
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I always wear my rear tires out first. After a week I'm sure I'm off by 20%. Ran my 2009 xdrive hard to 120k miles. Never a problem. (They were staggered too)

Just don't run different tires.
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      06-24-2015, 12:12 AM   #3
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There was a thread in the wheel section about some guy who installed staggered wheels and as soon as he started driving, drivetrain ML issue came up and the dashboard lit up like a christmas tree. Basically, its not a myth, I'm assuming the active power distribution relies the wheel turning radius as an input for wheel slippage, basically monitoring all the 4 wheels and as soon as some show inconsistency, the system freaks out. And wearing down your tires will most definitely keep you within that 1% tolerance.
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      06-24-2015, 07:00 AM   #4
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Quote:
Originally Posted by Blubaron79 View Post
I always wear my rear tires out first. After a week I'm sure I'm off by 20%. Ran my 2009 xdrive hard to 120k miles. Never a problem. (They were staggered too)

Just don't run different tires.
There's no possible way you could be off by 20%. That's like having a 30" circumference tire and a 24" circumference tire.

The stock 19" wheels using a 225/40/19 and 255/35/19 have rolling circumference of roughly 26" of which a one percent difference would put you at 25.74". Considering the tread depth of the Potenza S001 RFT is only 10/32 of a an inch or .31", you'd have to have a new tire and an almost bald one to have them more then .26" in circumference difference.
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      06-24-2015, 07:01 AM   #5
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Quote:
Originally Posted by ND40oz
Quote:
Originally Posted by Blubaron79 View Post
I always wear my rear tires out first. After a week I'm sure I'm off by 20%. Ran my 2009 xdrive hard to 120k miles. Never a problem. (They were staggered too)

Just don't run different tires.
There's no possible way you could be off by 20%. That's like having a 30" circumference tire and a 24" circumference tire.

The stock 19" wheels using a 225/40/19 and 255/35/19 have rolling circumference of roughly 26" of which a one percent difference would put you at 25.74". Considering the tread depth of the Potenza S001 RFT is only 10/32 of a an inch or .31", you'd have to have a new tire and an almost bald one to have them more then .26" in circumference difference.
I was being sarcastic. I just burn up my rear tires a lot faster than my front.

And they come from the factory staggered.
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      06-24-2015, 07:51 AM   #6
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Quote:
Originally Posted by Blubaron79 View Post
I was being sarcastic. I just burn up my rear tires a lot faster than my front.
If you do then it's from literally burning them up, and that wouldn't happen with AWD, only RWD. Front tires tend to wear faster, due to the scrubbing action when they're turning. Front to rear rotation every 5k miles or so evens the wear.
Quote:
you'd have to have a new tire and an almost bald one to have them more then .26" in circumference difference.
+1.
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      06-24-2015, 07:57 AM   #7
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Quote:
Originally Posted by Billfitz
Quote:
Originally Posted by Blubaron79 View Post
I was being sarcastic. I just burn up my rear tires a lot faster than my front.
If you do then it's from literally burning them up, and that wouldn't happen with AWD, only RWD. Front tires tend to wear faster, due to the scrubbing action when they're turning. Front to rear rotation every 5k miles or so evens the wear.
Quote:
you'd have to have a new tire and an almost bald one to have them more then .26" in circumference difference.
+1.
Wrong. Drive an xdrive car. The rears always wear quicker. This is my 3rd BMW with xdrive. These cars are rear drive biased.
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      06-24-2015, 08:11 AM   #8
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Quote:
Originally Posted by Blubaron79 View Post
I was being sarcastic. I just burn up my rear tires a lot faster than my front.

And they come from the factory staggered.
The only thing that's staggered is the widths though, the rolling circumferences are roughly identical.
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      06-24-2015, 08:12 AM   #9
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Quote:
Originally Posted by Blubaron79 View Post
Wrong. Drive an xdrive car. The rears always wear quicker. This is my 3rd BMW with xdrive. These cars are rear drive biased.
Rear drive biased, yes, but if you accelerate hard enough to burn the rear tires more power is directed to the front to prevent rear wheel slip. I do have xDrive.
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      06-24-2015, 09:16 AM   #10
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I recall there being multiple incidents of people having issues on the E90 side when running incorrect staggered wheel sizes.

I ran a set that had a 2.0% difference in diameters (forgot the exact sizes) on my F30. I had some cases of what I felt like the transfer case locking up, some jerkiness in acceleration, and cases of the TC light coming on. After switching to a 235/35 and 265/30, I had less issues. Now with a square set, I have zero jerkiness when accelerating.
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      06-24-2015, 10:04 AM   #11
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Quote:
Originally Posted by bahasad View Post
I recall there being multiple incidents of people having issues on the E90 side when running incorrect staggered wheel sizes.

I ran a set that had a 2.0% difference in diameters (forgot the exact sizes) on my F30. I had some cases of what I felt like the transfer case locking up, some jerkiness in acceleration, and cases of the TC light coming on. After switching to a 235/35 and 265/30, I had less issues. Now with a square set, I have zero jerkiness when accelerating.
my brotha bahasad, i can always count on you for wheel input lol.

2% is pushing it, i was gonna run 235/35 265/30 but decided to go with 245/35/19 and 275/30/19 on my 19x9 and 19x10, which is right at the threshold of 1%.

But its funny cause i have a little bit of jerkiness on my acceleration right now slightly even on my stock wheels (which are square).

what did you do about the TC light coming on and killing your acceleration...should i run into this issue.
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      06-24-2015, 10:07 AM   #12
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Quote:
Originally Posted by Blubaron79 View Post
Wrong. Drive an xdrive car. The rears always wear quicker. This is my 3rd BMW with xdrive. These cars are rear drive biased.
interesting, my f30 is about 2 years old now, tread seems decently even front and rear.
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      06-25-2015, 08:22 AM   #13
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Quote:
Originally Posted by kzflags View Post
my brotha bahasad, i can always count on you for wheel input lol.

2% is pushing it, i was gonna run 235/35 265/30 but decided to go with 245/35/19 and 275/30/19 on my 19x9 and 19x10, which is right at the threshold of 1%.

But its funny cause i have a little bit of jerkiness on my acceleration right now slightly even on my stock wheels (which are square).

what did you do about the TC light coming on and killing your acceleration...should i run into this issue.
The jerkiness could be just the transmission programming, and gear engagements especially 1st to 2nd seem a little rough.

After I swapped to the 235/35 and 265/30, I rarely if ever saw the TC light come on, so I never had to get it dealt with.

You could always pull the fuse for the AWD system and see what happens
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      06-25-2015, 09:20 AM   #14
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Here's a nifty little tool to make sure your wheel diameter is within tolerance, as well as other wheel metrics.

http://www.rimsntires.com/specspro.jsp
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      06-25-2015, 11:36 AM   #15
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Did you actually read what you posted? It doesn't warn that the discrepancy will cause it to "explode". It simply says that for errors greater than 1% it will allow the transfer case to slip and make up the difference. Logic would suggest that if you continuously drive around with the transfer case constantly slipping due to mismatched tires that it will eventually wear the clutches. It's not rocket surgery.
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      06-25-2015, 03:10 PM   #16
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Quote:
Originally Posted by BavarianFanatic View Post
Did you actually read what you posted? It doesn't warn that the discrepancy will cause it to "explode". It simply says that for errors greater than 1% it will allow the transfer case to slip and make up the difference. Logic would suggest that if you continuously drive around with the transfer case constantly slipping due to mismatched tires that it will eventually wear the clutches. It's not rocket surgery.
I hate to be a pickler, but, rocket surgery?.
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      06-25-2015, 03:32 PM   #17
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Quote:
Originally Posted by Piggieback View Post
I hate to be a pickler, but, rocket surgery?.
Rocket science/brain surgery mash up...
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      01-27-2017, 01:30 AM   #18
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Quote:
Originally Posted by bahasad View Post
The jerkiness could be just the transmission programming, and gear engagements especially 1st to 2nd seem a little rough.

After I swapped to the 235/35 and 265/30, I rarely if ever saw the TC light come on, so I never had to get it dealt with.

You could always pull the fuse for the AWD system and see what happens
Was the jerkiness really bad on the 235/35 and 265/30? I was planning to go with 245/35 and 275/30 but am now not sure now.
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      01-27-2017, 07:49 AM   #19
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Quote:
Originally Posted by Billfitz View Post
If you do then it's from literally burning them up, and that wouldn't happen with AWD, only RWD. Front tires tend to wear faster, due to the scrubbing action when they're turning. Front to rear rotation every 5k miles or so evens the wear.
+1.
Quote:
Originally Posted by Blubaron79 View Post
Wrong. Drive an xdrive car. The rears always wear quicker. This is my 3rd BMW with xdrive. These cars are rear drive biased.
Quote:
Originally Posted by Billfitz View Post
Rear drive biased, yes, but if you accelerate hard enough to burn the rear tires more power is directed to the front to prevent rear wheel slip. I do have xDrive.
The fronts wear faster than the rears even on RWD.

This is based on daily driving as well as HPDEs.

This is why I went from the OEM 400M staggered setup to a square setup.
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      01-27-2017, 08:14 AM   #20
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I run staggered on all of my bmw's 17 I have in production right now will be number 5 and my rears always wear first. x-drive or rear wheel drive. Continental DW I just pulled off my 19's were 8/32 front 4/32 rear. That is with my wife driving it not me. My X3 when I switched to my winters was just at 7/32 front 5/32 rear running Pirelli non run flat 19's staggered. I have never had issue with x-drive but I use calculator on e90 forum and try to stay under 1%. I will be running 235-35, 275-30 or maybe 245-35 275-30 20 on my new one. Waiting on new PS4S in March and HRE whenever custom power coating is done.
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      02-03-2017, 06:27 AM   #21
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So what happens if the tyres do damage the transfer box, does the car break down and need to be recovered, or is it lesser damage that leaves the car driveable?
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      02-23-2019, 11:41 PM   #22
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Quote:
Originally Posted by teaston View Post
So what happens if the tyres do damage the transfer box, does the car break down and need to be recovered, or is it lesser damage that leaves the car driveable?
This is a damn good question -- it's a shame no one has spoken up over the past couple years. If I had to guess (and I do, 'cause I'm a noob), I'd say that once the clutch gets worn to the point of failure (the clutch that links the laterally-offset front-axle driveshaft to the main, centrally-positioned splined shaft in the transfer case), the car, in all likelihood, would simply lose its ability to effectively transfer power to the front axle. Some sensor would detect the failure and light up an annoying light -- possibly multiple annoying lights -- in the gauge cluster.

Meanwhile, the rear-axle driveshaft, seeing as how it's hard-coupled to the output shaft of the tranny (yes, I just said "shaft of the tranny"), would likely keep on a-truckin', allowing you to motor onward safely and uninterruptedly. I can't imagine BMW would program in some kind of instant limp-mode (let alone engine-kill mode) in response to a clutch finally reaching the end of its useful lifespan, as this would be unnecessarily risky and dangerous were the clutch failure to occur under inopportune driving circumstances. Nonetheless, it bears repeating; I'm a noob, and the above speculation is just that: speculation.

By the way (writing now to the thread at large), I checked out the diameter difference -- on the stock 19" staggered set-up -- between the fronts (225/40R19) and the rears (255/35R19), and it turns out the rears are 0.4% smaller! How perverse is that?!

This led me to come up with a potentially max-pimpin', max-practical solution to those who would wish to upgrade the meats on their xDrive F3Xs and really accentuate the staggered "bigger haunches" look. My solution, because it's a set-up where the rears are 0.4% larger, would also help balance out the issue of uneven tread wear for those who tend to wear out the rears faster than the fronts; as the rears rapidly wear down, the front/rear diameter disparity would lessen, as opposed to growing worse!

Here's my proposed set-up: 235/40R19 front, 275/30R20 rear

Oh yeah, you read that right: 19s and 20s... Muahahaahaaa!! Bold, pimpin', meaty, and badass! I would totally do this, even if it required a bit of fender rolling...
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