05-03-2016, 06:04 PM | #1 |
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Xdrive tire sizes seem wrong
Hi guys,
A friend of mine just bought a 2 series xdrive with factory staggered wheels. Only thing is that the tires seem wrong for an all wheel drive car. 225/40/18 front and 245/35/18 rear. This works out to over 2% difference in wheel diameter. The golden rule for other xdrives has been less than 1% variance, or a square set up. Even other factory staggered setups adhere to this rule. My question is, why is the 2 series different?? |
05-03-2016, 10:24 PM | #2 | |
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http://www.2addicts.com/forums/showthread.php?t=1198464 |
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05-03-2016, 11:09 PM | #3 | |
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05-04-2016, 06:21 AM | #4 |
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Calculations
How did I get over 2%? Weird.
On a second look though the size difference is 1.35 according to the calculators I used. Still not under the 1% golden rule but still less concerning than over 2%. My concerns remain amount undue stress on the xdrive system right out of the factory. |
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05-04-2016, 09:33 AM | #5 | |
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Last edited by x233; 05-04-2016 at 09:42 AM.. |
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05-05-2016, 11:11 PM | #6 | |
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I theorize that the stock smaller diameter rear tire is read by xDrive as slipping sooner than it should and power is shuffled to the front. With the equal front/rear diameter and a wider rear, the car feels more rear wheel planted and less likely to shuffle power to the front. I actually feel like there is less understeer with the more staggered but equal diameter setup, which I was concerned about because in theory there should be more understeer. |
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09-12-2019, 09:50 PM | #8 | |
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I went from the factory 18x7.5 in the rear to 18x9 in the rear. It looks awesome but the car feels slower- like it is harder to get rolling? Could always be a boost leak but since I just changed my rollers I have been asking around. Thank you in advance!
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09-13-2019, 08:48 AM | #9 |
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Feeling slower could be related to the fact that your 9x18 is heavier than the 7.5x18 stock. And a higher rotating mass is creating more resistance (needing more power to spin) and if you started wit 7.5 I assume you were on 225er and now on 245-265 ... wider tire more rolling resistance and these are possibly heavier than the stock tire too. And with rotating mass the further away from the axel it is the more if has an effect (and the wider rim and wider tire are far away from the axel).
BUT you are in a 235 with tune and must have a really good calibrated butt dyno to notice these changes. When I drove a Fiat Cinquecento with 55hp the change from 13 inch to 15 inch was very noticeable but that was an underpowered POS To the original discussion 225/40 and 255/35 would be as close in diameter as possible for a staggered set |
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09-18-2019, 12:29 PM | #10 |
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It is widely rumored that this requirement of <1% might be a bit conservative, and serves to juice up tire sales more than it protects transfer cases (even moreso for the recc's about side-to-side matching, where unequal rotations are a given during anything but pure straight line driving). I figure that if the factory was OK with it, it won't be a major concern. Those handling effects noted above (less understeer with larger rears) seem counterintuitive for this, more likely reflecting the change from summer to winter tires; marcthedark seems right on that the slower sensation is likely the weight. You'll really notice changes in unsprung weight, especially rotating weight.
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