06-23-2015, 10:10 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 |
06-23-2015, 10:27 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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06-23-2015, 11:12 PM | #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, 06:00 AM | #4 | |
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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, 06:01 AM | #5 | ||
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And they come from the factory staggered.
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06-24-2015, 06:51 AM | #6 | ||
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06-24-2015, 06:57 AM | #7 | |||
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06-24-2015, 07:12 AM | #9 |
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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, 08:16 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, 09:04 AM | #11 | |
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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, 09:07 AM | #12 |
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06-25-2015, 07:22 AM | #13 | |
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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, 08:20 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, 10:36 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, 02:10 PM | #16 | |
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01-27-2017, 12:30 AM | #18 | |
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01-27-2017, 06:49 AM | #19 | |||
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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, 07:14 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, 05:27 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, 10:41 PM | #22 | |
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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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