[ildottorebardi]

Dear all,

I have a 2015 F32 430xd MSport - it was utterly sublime in every single condition that I ever threw at it, until I potentially ruined it...

I bought it last Summer; for what I paid for it, I was more than happy to accept the 18" 400M wheels that came with it as I was buying with the view to upgrade them to 19s. A couple of months ago, I upgraded to a set of as-new 437Ms from an M2.

I was running 225 45 R18 front and 255 40 R18 with Goodyear Eagle F1 Asymmetric 3s.
I am now running 245 35 R19 front and 265 35 R19 with Michelin Pilot Supersports (the M2 tyres that came with the rims).

I have read multiple threads on this forum regarding the difference in wheel rolling diameter and how for an xDrive it is important to maintain a difference of maximum 1% front-rear...

Original %D in wheel diameter = 0.23% (1.5mm)
Fronts have changed -5.6mm = -0.85%
Rear have changed +6.9mm = +1.04%
New %D in wheel diameter F-R = 2.12% (14mm) - I know this is outside the limit, I thought I would try them anyway given the tyres were included with the rims)

When I put the new rims on, I had an alignment done (although admittedly to the factory BMW values that don't take into account the increased track widths, larger wheel widths, and ACS springs that I had fitted late last year). There were/are no warning lights on the dashboard.

I have driven the car for the last couple of months; the handling of the car is "interesting":

At the beginning, I had 2x episodes in which the car has sidestepped significantly at low speed when going round a 90degree corner. One of these occurred in Sport mode, the other in Sport+. The traction control *used to* gather it up rather nicely if I gave it a bootful in a low-speed corner; I was barely breathing on the throttle in these 2 instances and it was anything but smooth.
When pushing it, the front is very very slow to respond (looking at the alignment data I would ideally go more toe out at the front but would expect the car to be much more nervous than it already is as a result), but then the car rotates quickly, almost as if the traction control is getting confused with the wheels speeds it is seeing, as a result under rotating the inside and over rotating the outside. This happens in a very unsettling manner. To be clear, if I drive the car in a manner that is pleasing to the local constabulary, this unsettling behaviour is not present.
In general, there is no cornering sensibility; the front end is incredibly light and hence I have no confidence in chucking it into a corner and expecting the front end grip. The front end does still have a lot of grip, but it doesn't inspire confidence.
The car used to be incredibly precise and a complete joy in weight transfer left-rightÂ…now it is anything but. That precision has been eradicated.
As I mentioned in the intro, the car used to be very competent in all conditions - it just inspired confidence every single time I drove it. I was over in the UK this month for a few weeks (I live in Italy) and drove it back to Italy a few days ago; on a sopping wet French motorway (with very poor drainage), I could feel the car nervously changing direction within the lane as it encountered puddles of water here and there.
The car feels slightly jerky on gearshifts (particularly when pulling down a cog on the motorway from 8th - 7th unless I give it a slight touch of throttle to smoothen it out).
There is a slight vibration (“purr”) when reversing without throttle on a flat surface - I am sure the vibration is present always, it is just so subtle that I cannot feel it unless I have the above conditions.


Here are my questions for you:

1. Has anyone exceeded the maximum 1% limit for wheel diameter with an xDrive and if so, what happens?
2. Given that the 2x "episodes" that I mentioned have not repeated themselves (and I HAVE tried to replicate them in a safe environment), can the traction control "learn" new relative wheel speeds?
3. The fronts are 2.12% smaller than the rears; I suspect a lot of the handling issues I have described are due to the fronts being forced to rotate slower than they would otherwise want to (the xDrive takes up 1% of the slack but the remaining 1.12% causes the front end to absorb that extra energy that isn't being used to propel the car). Does anyone have any useful knowledge on this and what effect this would actually have?
4. Anyone with 437Ms on an xDrive, did you notice any improvements in handling after the switch?
5. Anyone with 437Ms on an xDrive, please could you share your wheel alignment data? How did your wheel alignment place play with the values to account for different track widths/tyre widths to the BMW guidelines?

I believe that I need to change my front tyres to something with a similar diameter to the rear in order to destress the xDrive and sort out the majority of the handling issues.

These are my 2 options:

If I went to 235 40 R19 on the frontÂ…there would be a 0.37 %D difference in diameter (2.5mm, fronts would be slightly larger in diameter than rears)
If I went to 225 40 R19 on the frontÂ…there would be a 0.83 %D in diameter (5.5mm, fronts would be slightly smaller than rears as before changing to new rims)

The 225 40 R19 would be a slight stretch on the 9J, however I know that some people on the forum have done it and that they seem happy with it.

This leads me to my final question - can the fronts be slightly larger than the rears (provided the supposed 1% limit is respected) or is this 1% slack only allowed in one direction (i.e. the rear must be up to 1% bigger than the front but cannot be smaller)?

Thank you all for any help you can offer.

Yours,


Dom