[AWD Addict]

This one stings, and it's embarrassing enough I'm going to post it.

I put some new wheels on the car this spring, and was hoping to get my old tire setup on the new wheels. I'm now running the square M3 CSL wheels (style 359) upgraded from my old VMR CSL reps.

https://www.e90post.com/forums/showthread.php?t=1995024

Wheels are a bit wider up front than my prior setup (9" vs prior at 8.5"), and during mounting, my tech was concerned they were stretched too much. So the tech got a set of new fronts with 245 width, and we kept the rears.

It didn't click for me at the time to check, (and maybe I thought he'd think of this), but I started having a bit of a surging feeling as I came into gear soon after the swap. It feels like the car is slipping as it catches, even though the clutch at that point is fully engaged for a while. I can feel something after the clutch is disengaged too, while I'm slowing down. As I tried to diagnose, it struck me maybe it's a transfer case issue. I ran the math on total diameter with the new fronts, and I'm double the 1% variance threshold.

Old setup was about 0.8% different front to rear:
F 235 30 R19
R 265 35 R19

New setup is almost 2% different front to rear:
F 245 35 R19
R 265 35 R19

So I noodled around a bit trying to figure out how I'd bridge the gap, while preferring to keep the wider tires on the rear. Long story short, it doesn't look like there are too many options:

1) If I keep the 245 35s up front, the widest I can go is 255/35 (0.8% diff.).
2) Going wider to 275/30s, I' be 1.2% different, and they'd probably rub with my drop anyways.
3) Keeping the 265 rears, I'd have to drop the fronts to 225s, which is a bit too narrow for me for a summer tire.
4) The 235/40s would have been perfect up front on the math (0.4% diff.), but as noted, they were really stretched over the 9" wheel.

I think given I was running a .8% difference for so long (15 years), that's an okay variance to live with. Otherwise, I'm going to have to run a square 245. So I'm going with 255/35 rear, 245/35 fronts so I can keep a bit of a stagger.

When I swap the rears, I'm planning on doing a fluid flush on the TC. Any thoughts on this? Think I killed it? Anything else I should do with the transfer case to check it out?

[Brian86]

My best guess, grains of salt of course...

But the BMW T-case under-drives the front axle relative to the rear. Somewhere in the 2% range (Never opened up a T-case to count teeth, but I've counted rotations on the output shafts, and it seems like ~~ 49/50 F/R)

The idea is that when the car is rolling, the front driveshaft is rotating more slowly than the rear, so when the wet clutch is engaged the slip provides forward torque on the front axle.

With the smaller tires up front, the input/output of the T-case is spinning at almost exactly the same speed, so there is a tendency for the clutch to 'lock' to 'fully engaged', which causes binding and surging until it slips again.

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The car will probably work fine with larger diameter tires on the front, at the expense of additional T-case clutch wear, but that's a silly looking way to run a car.

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I don't think you've damaged anything. The T-cases are very robust.

[AWD Addict]

New rear rubber goes on tomorrow, so we'll see if that's the culprit for my surging/slipping issue.

Along with the fluid flush, we're going to add some friction modifier treatment as belts and suspenders.

[AWD Addict]

Originally Posted by Brian86

The idea is that when the car is rolling, the front driveshaft is rotating more slowly than the rear, so when the wet clutch is engaged the slip provides forward torque on the front axle.

With the smaller tires up front, the input/output of the T-case is spinning at almost exactly the same speed, so there is a tendency for the clutch to 'lock' to 'fully engaged', which causes binding and surging until it slips again.

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The car will probably work fine with larger diameter tires on the front, at the expense of additional T-case clutch wear, but that's a silly looking way to run a car.

This is interesting. Do you think Where my fronts were -0.8% different before, and now I'll be +0.8% that there'll be a meaningful effect, based on your note above? I'll be 1.6% difference in the other direction. Is that what you meant on your comment about the larger tires up front?

[AWD Addict]

Tires swapped. Slipping / surging condition is fixed.

Hopefully the transfer case holds up!

[Cyril Sneer]

You have square wheels on a lowered AWD car. Why would you want anything other than a square tire setup? 245 or 255 square should be perfect. - better cornering balance - ability to rotate tires - no concern for xfer case - no concerns for traction.

No a single person ever will be able to tell that your tires are 10-20mm wider or narrower, so why the added aggravation?

[AWD Addict]

It's a fair point. I figured with the rear-bias of xdrive, a bit more meat back there would be preferred.

I typically only get a couple years out of my summers, so I'll likely end up going square on the next set.

[Cyril Sneer]

Good plan.

I found xdrive to be far too front-biased until I used an xdelete custom setting to limit some of the torque to the front. Much better this way. I think you'd need mega HP to worry about traction on an xdrive.

If you upsize your fronts to match your rears, you can't argue you're losing rear traction by going square 🧐.

(Minor) downsides to wider fronts are more unsprung weight on the front, and slightly more turning resistance at low (parking lot) speeds.

4x 359 fronts were a great choice, btw.

[AWD Addict]

I'm super happy with them. The differences to the replicas are subtle, but definitely noticeable.