Quote:
Originally Posted by HighlandPete
Something to keep in mind. Are you calculating the diameter from generic sized tire charts/calculators? That is not 'definitive' sizing for 'star' marked approved tires.
Also remember those are nominal sizes, not allowing for manufacturing tolerances, (even within the appropriate standards), can be quite wide. BMW could well be setting tolerances which may make front and rear RC (rolling circumference) for the OE tires different than it appears.
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Quote:
Originally Posted by shawnhayes
We had to learn this the hard way in the GT-R world. Many manufacturers and tire size calculators that were "just right" still blew center differentials over and over.
A guy named Shep from ShepTrans (famous in the EVO/DSM/GT-R world for drivetrain) finally gave us the reality. Only way to know for sure is to install the damn tires, and put them on the car at running pressure, and mark them and do a slow rollout.
Unless they were REAL close, you'd eventually blow the center diff.
BMW's system is probably a little more tolerant. Who knows? I don't be the one to find out. I install either BMW "listed" tires and setups, or I install "square" (same tire all the way round) setups.
OP, reality is you can change the OD, but lots of unpredictables can occur as the result. Just giving you a suggested pathway to ensure safety.
Shawn
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Great points, guys. It seems there would be some favorable tolerances built into the BMW 4WD systems, because there are thousands of folks here on the forums changing their rides from OEM tires and wheels to aftermarket products. (Granted, 2WD much less critical, so G8x M3s and M4s are newer to the game). I only seem to read about people with rubbing problems and strut clearances, which they fix, but never any differential problems (unless they just keep quiet
)
I can only base my most experience off my 2 M5s. The OEM 20" summer wheels have a front to back diameter difference of 0.28", or 1.02%.
The OEM 19" summer set has a front to back diameter difference of 0.32", or 1.16%.
The BMW approved 19" snow set has a front to back diameter difference of 0.64", or 2.34%.
(The above based on nominal sizing, because I have no other data to go on)
"They" say to always have your front to back diameters within 1% on an AWD car. The OEM summer 20s and 19s are close to 1% (still over), but the winters are are considerably over a 2% difference in diameter. I saw no ill effects during 3 winter seasons, so I'm thinking we may have a little leeway here.
And
shawnhayes, what did you mean by "REAL" close when you do the roll out? How close is REAL close in Shep's world to not blow out the center diff?
Thanks for the input. This is what I've been looking for.