SyedUmarAhmad

Just had a few questions concerning my new tire/wheel setup on my recent e34. The car is a 91 525i auto.

The car came with e39 style 5s with its usual square setup --- I think 225/35/17 all around if im not mistaken. Didn't have any issues whatsoever.

Now just this past Friday I swapped those out with some M Parallel reps that I picked up running 225/40/18s in the front and 235/50/18 out back. First 10 miles of driving the ABS light popped on but I didn't think much of it, just assumed the computer needed to get acclimated to the new setup, brakes were still working fine. I had already read online that the ideal tire size for M-pars on an E34 were 225/40 in the front and 235/40 in the rear. It just happened to be the previous owner had 235/50s in the rear so I did a mild roll in the back and just popped them on--- still minor rubbing but drive-able. No other issues, drove 100% straight.

More recently, my neighbor (also a car enthusiast) was checking out my new setup and mentioned that I may have issues concerning the transmission or differential as the tire/wheel setup is severely different than what the computer is used to and will end up destroying the trans/diff in the long run if I don't correct the tire sizes. He faced a similar issue in his GS300 going from 17s to 19s with varying tire sizes.

After he mentioned this, I did begin to pay attention at the driving experience and the only thing I noticed different after switching the wheels was that upon hard braking at a high speed, I can almost feel the transmission down shifting and in random cases, at low speeds also. I only mention that because I could never really "feel" the downshifts before. Besides that, transmission shifts completely fine and it feels and drives exactly as it should.

So I guess my question is how severe is the current setup im running and what would be ideal so as to not fck up anything concerning the drivetrain? Rear sidewall clearly needs to go down a size to get rid of the rubbing but besides that I'm lost... I used this link for some assistance but it looked more like hieroglyphs to me
http://www.1010tires.com/Tools/Tire-...0R18/235-50R18

any help is much appreciated.

brandini

That's an 8% difference in wheel speed front to rear, get as close as possible:

www.willtheyfit.com shows the % difference between two sizes you input.

SyedUmarAhmad

I checked that out and I know im way off. But until I buy new tires is it safe to drive without damaging anything?

Fabrik8

The ABS light is because of the difference in tire diameter. The shift harshness is likely because the transmission controller is either confused or is in a reduced-function fault condition, for the same reasons of mismatched tire diameter. Whether this would damage anything is unknown, but I can't see how the diff would be in danger at all if the car is RWD and therefore not connected to another set of different-diameter tires....?

The overall tire diameter isn't really important (as long as the front/rear are closely matched) as the shift algorithms are based on wheel speed and engine RPM, etc., nothing that is actually based on real speed-over-ground (SOG). However, because the transmission controller is programmed for a certain SOG, RPM, etc., your shift points will behave differently because you've changed your tire diameter and therefore your SOG with respect to wheel speed.

So not only do you have the 8% front/rear difference with your current set of tires, that speed difference also changes when you're not traveling in a straight line....

So get some better matched tires.

Fabrik8

The rears are about 17.5% different from stock, that's pretty big.

SyedUmarAhmad

well that was very well put, thanks.

I've tracked down a set of 235/40s in my area so by switching those to the rear, would this eliminate the transmission controller issue and improve speedometer accuracy? (setup would be 225/40 front, 235/40 rear).

This guy dropped his rear sidewalls actually lower than his fronts (245/40 front, 275/35 rear) and apparently its working out for him
http://forums.bimmerforums.com/forum...tyle-37-thread

Fabrik8

I don't care what the sidewalls are, they're just a percentage of treadface width. So that random guy's front sidewalls are 40% of 245, and his rears are 35% of 275. So the sidewalls are within less than 2mm of each other (about 3.5mm total, because there are two sidewalls for the tire diameter).

The sidewalls have nothing to do with it by themselves, the only important thing is that the overall tire diameter is fairly closely matched. The sidewall number is just a percentage of the treadface width. So the treadface width, wheel diameter, and sidewall aspect are all important in what the overall tire diameter actually is, which is why they're the only 3 things that the tire size calculators need to know.

Anything that looks at wheelspeed (ABS, etc.) can only handle a certain amount of wheelspeed mismatch before it thinks bad things are happening (wheel lockup, skid, etc.) or thinks that the wheelspeed sensors are broken.


The setup you're looking at 225/40 and 235/40 is about 8mm difference in diameter, which is probably more than you want. I don't really know what the threshold for front/rear speed difference is, so it's hard to know. It's only about 1.24% difference for those diameters, which might be fine or might not.....

SyedUmarAhmad

Ok I understand what you are saying. So 225/40 and 235/40 may in fact not be ideal for this type of setup. I guess ill have to take different sizes into consideration while keeping the ratio as close as possible. no idea tire sizes went this far in depth.

Thanks for the breakdown, I was clearly missing crucial pieces of information

Fabrik8

There should be plenty of people around who are doing the same thing, so there should be a pretty good knowledge base on the BMW-specific forums. Sometimes the easiest thing to do is learn from someone elses experience with the same car or family of cars..

SyedUmarAhmad

agreed, thanks for the help sir