Quote:
Originally Posted by gtaccord
So I have a theory based on my experience.
Stock tire size:
F - 225/40-18 (637.2 mm dia)
R - 255/35-18 (635.7 mm dia)
Notice that the fronts are slightly larger than the rears
Common 19" size:
F - 235/35-19 (647.1 mm dia)
R - 265/30-19 (641.6 mm dia)
Notice once again that the fronts are larger than the rears. This is the sizing that I run with no problems so far. (20 k miles)
My winter setup:
225/40-18 (637.2 mm dia)
265/35-18 (642.7 mm dia)
Rears are larger than fronts. This is similar to the 225/265 19" setup (640.1 mm dia front vs 641.6 mm dia rear) that the OP has. With this setup I have a lot of surging at constant interstate speeds (60 + mph with cruise on) when under load (going up hills etC). The car still performs well and I have not had any issues with the TC so far (10k miles this winter on this setup). I even had the car on the dyno with these tires. But as soon as I went back to my summer setup the surging went away.
I theorize that the computer and awd system as a whole is setup with the consideration that the front rolling diameter is >= the rear (within reason on the > of course). This being a rear biased awd system (transfer case instead of center diff) the computer wants to see the front driveline turning at the same speed or less when compared to the rear. If you think about it, since the rear wheels can never be disengaged like in a car with a center diff, the TC clutch can only be used to slow down or speed up the front driveline using clutch slip in relation to the rear driveline. The front driveline can never be driven faster than 100% of the speed of the rear driveline. In normal driving (non slip conditions) the transfer case can easily attempt to turn the front driveline faster when using larger front tires(using rear driveline speed as a target) but it cannot make the front driveline constantly turn slower when using smaller front tires (tire speed vs driveline speed). So basically the smaller front tires cause the car to fight itself and will probably cause excessive tc wear.
This is why the 235/35 - 275/30 19" setup still works because the rolling diameters are basically exactly the same.
I would modify the typical advice and say that as long as the front tire diameter is >= the rear tire dia within 1% you will be good. I would avoid going with a setup where the rear tire dia is larger than the front. I am actually considering changing my winter setup because of the obvious drivability difference I am seeing.
Hope this helps.
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Thanks for taking the time to share your experiences and thoughts. This is the kind of technical discussion i'm looking for since I've exhausted my abilities. I did want to question a portion of your theory. You mentioned that the front axle can never be driven faster than the rear, but my understanding of xdrive is that it can transfer 100% of the torque to the front. This is needed when a car has the rear wheels on ice and the front wheels have grip.
http://en.wikipedia.org/wiki/BMW_xDrive The description section on that link also says that, but to give you a better source i think i can find one of the e9x technical documents (
http://www.bmw.com/com/en/insights/t...mm_xdrive.html) that goes into more detail. Also the difference that you calculated for my setup is within a few 32nds of uneven treadwear so I still can't understand why it didn't work fine.