Have I made a bad choice in tyre sze
 

I finally got the mag pattern I liked, I redressed the bit of kerbing issues and set about giving them a new coat of VHF mag wheel paint. They are also the correct 19x9 and 19x10 for this model.


The book shows a plethora of sizes for the 19x10 with the 285/45R19 being the preferred size, I on the other hand had a pair of Kumho Atensa 275/45R19 that were still good for a year or two. The fronts were 255/50R19 and that was also the size fitted. These were new with only 2000km so they were in much better shape too. So I just had the shop fit this combination on to the car.


Here is the rub in this tale, on the drive down the car ran as it has since the fuel filter change, flawlessly. After the tyres were fitted I was driving home and initially thought it was road surface due to roudworks, but this irregular bump feeling at 60km/h-35mph once I got to the Expressway and smooth road surface I accelerated to 100km/h-60mph and in doing so I indeed did feel these bumps throughout the car and it felt like it was drivetrain snatching. However at reaching 100, and set cruise, there was nothing.
There were no warning on the tyre or 4x lights, not even a flash. All four were balanced, so I cannot see that as an issue. SoI am wondering if the rotational value of these tyre sizes is enough to cause problems in the transfer case?:dunno:


Any thoughts, Jeff

This is one of the main reasons I run squared tires and wheels.

In my view, you answered your own question. No issue before the tire change, issues after. I think that small diameter difference could be enough to cause transfer case issues. But only my opinion. And it also depends on tire type and brand. Some are more true to size and others not so much.

Also, the fronts having more tread if I understood you correctly, doesn't help your situation.

The difference is 1%, which shouldn't be a big issue. But the fact that the smaller tires are worn down more, combined with possible tire pressure differences, means that it may be more than 1%. If you are feeling like there is some shifting around in the tires then you may want to measure the circumference with a string and see what the difference is. If you truly have mismatched tires then you will soon find out what is going to break first in your drivetrain, something no E53 owner wants to ever have to do!

This post + thread might be helpful:
https://xoutpost.com/bmw-sav-forums/...ml#post1173532

In there, BMW says max of 2mm difference in tread depth (they appear to be assuming that the specified tire sizes are used, so nominally diameters are almost exactly equal).

With your difference of 275 vs 285, at an aspect ratio of 45, that is 4.5 mm smaller radius on the 275 tires. I have the stock 19" wheels, and once did the calculations to confirm that front, rear, and even the 17" spare all have almost identical diameters.

So you do seem to be out of spec, as you knew. About 9mm smaller diameter on a ~738mm diameter tire.

On my Cayenne, with 22" wheels, once when I had a flat and the collapsible spare tire on, I marked the tires and counted revolutions as my son drove it down the street in front of my house. Marked with chalk and masking tape. Very effective way to compare the actual rolling diameters vs. trying to measure with a tape measure or doing the math on tire specs.

Yes, I can understand what you are saying but if this is the case why would they manufacture and reccomend the vehicle with the tire fitments in the first place.
I had squared fit 9"x19" and it looked like cookie cutters on the rear!
I do realise I may have made a error on my rear size based on only the 1.00% variation, but did not allow for the difference of the new over used worn diameters. That may be the biggest issues.


I think this is a good way to see exactly how much variation I really do have, I am not doing any long driving... nor much of any driving at all at the moment.
I'm off to see how long the peice of string is...:rofl:

If 2mm tread depth is the limit according to BMW, then OP is slightly out of that spec. OP has about 3 mm difference if both tires are new, and has more difference if the small tires are worn out more. So it's possible that OP does indeed have a problem. I personally would think it would be OK, because other manufacturers allow for more % tread difference, and they are using the same kind of differentials. But I recognize I am not the expert.

If you have xdrive you can disable the front axle by pulling the appropriate fuse and drive RWD only to determine quickly if you are getting binding due to size differences.

When I did this with my client's 328xi, it made the windshield wipers run continually so if you want to go for an extended test maybe pull the fuse to the wipers.

The calculators for figuring out rolling circumference are only estimates so you may actually have a bigger difference than you think.

You could do an actual measurement by putting marks on front and back tires, drive down a straight road for a known distance say 1km and see how much difference in where the marks end up.

I think you may find they are more different than you thought.

Oldskewel, andrewwyn, Bdc101 and Autoparts Guy, Thank you for this valuable input.



It does indeed seem that I may have FUBAR the diameters and even if I only do a single revolution the front and the rear should end up in the same place or very close.

I will wait for tomorrow and head out to the side street and be doing some measures. Finding a KM of straight road without corners or no dicks racing on it would be impossible around here :confused:.

I do have the luxury of putting the other 9"x19" of the old mag wheels with the two new tyres on them also, if the rolling diameter is crappola. I'll just get two new correct size tyres for the 10"x19" if the problem goes away doing that and in the factory size.


Thanks all, you are all bloody Legends.:thumbup:

I rarely run exactly the same spec tires fr/rr. However, I always make an accurate measurement of the floor to center hub height. With the weight of the car on the tires and the tire pressures proposed you will get the effective radius front and rear. The number is obviously not relevant but they need to be a close match. Adjusting tire pressure may help but does not change the effective radius as much as you would expect. Also, 285/45-19s are unicorns and typically pricey.

FWIW, I have 19" x 9 & 10" wheels on the car at this time and on the 10" wide wheels I run Porsche spec (N0) Michelin latitude tour 255/50/19. They are about 20mm wider in the thread and have no trouble seating the heel on the rim when inflating. On the front I use M-B spec which is somewhere in between regular and N0. The hub height measurement is perfectly matched fr/rr and it runs without issues. (I have also used reg 255/50 on 10" wheels but they may need a bit of ingenuity to get them to seat, no not brake clean..... :-)

I finally managed to do some serious measuring. Side by side there is a noticeable height difference of maybe 1/2".

So that would be a 1/4" from the centre I figure. When I mark the start point and the tyre, then roll it for each, the front and the rear. The difference is a bit over 3/4". The front rolls further in one rotatation by this much than the rear.
Finally to set my mind at ease, I still had two of the old style rims, 255/50R19 on 9" same as the other tyres. Only the mags are different. Once set bckon the car, I took it for a drive and sure enough...No banging from the transfer case, no feeling like is was a fuel pump failure. back to running as smoothe as once more.


So, I have ordered a new set of correct size tyres and these should cost me about $360 AUD fitted. 285/45R19 111Y


Hopefully this will fix the issue this time.