Quote:
Originally Posted by JCL
Tire size differences can certainly cause damage with the pre x-drive transfer case. I would strive for 3-4 revolution per mile difference, but I don't think that is a hard cutoff...
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Quote:
Originally Posted by JCL
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*The non x-drive (nxd) is an open diff. It can handle speed differences
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*BMW has a towing limit, with one axle raised, of 150 km at 50 km/hr for the nxd. Towing the xd with one axle raised is not permitted due to the risk of transfer case damage.
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I've got a 2001, non-xdrive, NV125 transfer case, 19" style 63 BMW wheels, 285/45R19 107V rear, 255/50R19 103V front. The spare is of course a 235/65R17 with zero wear.
My rear tires are worn. That is a fairly rare size, but I can get a pretty good deal on some lightly used, nice 275/45R19 tires, but of course I'm concerned with causing TC problems.
I know that buying a new set of 4 Michelins at the dealer, and replacing the never used, but 14 years old spare as well will work, and many here would do just that. Not me.
So first question on the TC tolerance on difference in rotations front to rear - JCL quotes 3-4 per mile should be good, and I think I saw that somewhere else too. But then the quote on towing (also confirmed in the owner's manual) - that seems EXTREMELY more stressful than the 3-4 revs/mile difference for driving. It would make me think that the real limit may be higher than 3-4. Comments?
Also, I ran some numbers for myself calculating the differences in tire diameter and revs per mile for different tires. Following is the output for my current situation.
1 - 285/45R19 on rear, with about 8/32" of wear gone (not remaining)
2 - 255/50R19 on front, 4/32" worn
3 - 235/65R17 spare, 0/32" worn
4 - 275/45R19 candidate replacement for rear, 2/32" worn
revs_per_mile =
705.2 700.5 694.8 704.7
diameter_mm =
726.4 731.3 737.3 726.9
Comments - the candidate tire fits better than the existing one due to lower wear. The spare is further out.
BTW, if I did not account for tire wear, here are the results (assuming all tires were brand new with no wear):
revs_per_mile =
693.1 694.5 694.8 701.6 (brand new, all specified tires are in a tight range)
diameter_mm =
739.1 737.6 737.3 730.1
I know this does not account for manufacturing tolerances, inexact tire design, inflation, load, etc.
Does this seem like a safe plan? BTW, the TC is pretty new, dealer installed by the PO a year before I bought it.