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      09-30-2020, 10:39 AM   #1
ildottorebardi
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Poor Handling & Confused TC - 437Ms on xDrive F32

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
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      10-09-2020, 09:12 AM   #2
marcthedark
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Hi, my M240ix has 225/40-18 and 245/35-18 from the factory and that is around 1.2% difference (with the front wheel being slightly larger).
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      10-09-2020, 04:47 PM   #3
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With a 2.12% difference your transferbox is constanly fighting the different speeds between front and rear tires. Eventually it will give up with a high price tag. This is what you mainly experiance around corners and lower speeds.
A wider tire always reacts more nervous at higher speeds especially when staggered. Not to mention the effect a broken in tire can make coming from a differenz car/suspension geometry.
Get at least some of the tires changed
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      10-12-2020, 09:46 AM   #4
ildottorebardi
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Hello all,

thanks for your responses.

Last week, I bought some taller tyres to put put on the front end (235 40 rather than a 245 35). As mentioned in my previous post, this brought me to a 0.37% difference in diameter with respect to the rears.

I wanted to make sure that the "right" size tyres did fix the handling issues and did not just ensure the xDrive was no longer under stress (my thought process being that if the car still handled poorly in comparison to the 400M 18s, I would sell the 437Mss and go back to 18s). I therefore did not fancy buying some new 235 40 R19s for the front, only to find out the handling was not back to normal and that I needed to shift 2x brand new tyres as well as the rims. The tyres that I bought were a pair of Continental ContiSport 5s, second hand on eBay with 5mm of tread left (conveniently, my rears are at 6.25mm of tread and hence my diameters are now identical!). These tyres are not the same as the rear (Michelin PSS), but given that BMW fit both the Conti and the Michelin to the M4, I figured they would be good enough for my test (even though the forum has many posts saying that different tyres front to rear is never a good idea).

I went for a 5 min drive to see “what happened”...what happened was that the new fronts were in need of some serious scrubbing in! The traction difference front-rear was so great that the traction control kicked in every single input I gave it (even when going in a straight line and breathing lightly on the throttle). I suspect the tyres had been sitting in a warehouse for a year and were in need of a decent clean.

I took the car back out for a 2hr drive in which I got the tyres warm and gave them a proper scrubbing with some heavy braking and slow speed corner, aggressive throttle stuff (with everything turned off) to get them spinning and cleaned up. After I got through that initial part of the drive, I tried all modes (Comfort, Sport, Sport+, DSC off) and can report the following:

The car is pretty much back to how it was before the new rims went on...it is not identical, in that it is pointier and the initial rotation when entering a corner is much quicker than it used to be, however the weight transfer is back to being a truly joyous occasion, and the mid-corner composure and grip is just sublime. I can feel the TC working in all modes (bar DSC off) when I give it huge bootful at the apex, but I think that is down to the masses of torque (the engine has been chipped) and the difference in rubber front-rear. The next set of tyres that go on will be all matching.

I still need to test it in the rain, and on a motorway, but the signs are good...

Now that the car is behaving as I had hoped, I was wondering whether anyone would like to help me make a definitive F3X xDrive 437M alignment guide? I have scoured the forums; unfortunately several people have asked for help with few replies.

I have attached my current alignment data, I am thinking a touch more toe on both front and rear (rear with circa 0.1deg toe than rear total) + a little bit more rear camber might make the car a little more stable on corner entry, even if the initial rotation is slightly slower.

Sorry the alignment document is in Italian, FYI:

Campanatura = Camber
Convergenza = Caster

One final thing...with the 235 40s on the front, when manoeuvring around a carpark, the increased diameter means that they rub slightly on the arch lining at the front. It is not excessive, but it is slightly annoying.

Thanks again.

Yours,


Dom
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File Type: pdf 200731 Convergenza.pdf (46.5 KB, 10 views)
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      10-12-2020, 10:34 AM   #5
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Good we got your transferbox saved and and you back on the road in a more stable way.
I can't help on alignement due to the fakt I only got 2WD which behaves differently. You could have a look in the M2 section of this forum, these guys seem to play around much more with alignement settings since they use the car much more often on the track.
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      10-13-2020, 04:35 AM   #6
ildottorebardi
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Thanks for the advice re: M2 forum. I am going to let the car settle down for a month with these new front tyres and see how the driving experience evolves (if the DSC does indeed have the capability of "learning", I would hope that it can become slightly less intrusive than before, or rather, less noticeable).

In the meantime, I am reaching out to some colleagues of mine that specialise in vehicle setup and who might be able to give me a few pointers. If anyone else would like to chime in with their alignment values and comments on handling, please do!
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      10-13-2020, 04:39 AM   #7
ildottorebardi
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Quote:
Originally Posted by marcthedark View Post
Hi, my M240ix has 225/40-18 and 245/35-18 from the factory and that is around 1.2% difference (with the front wheel being slightly larger).
Thanks for this info; that is weird! I would not have expected BMW to run those values if the 1% is a hard and fast rule. Maybe 1.2% is the true limit
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      11-01-2020, 08:38 PM   #8
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For 19's it should be 225/40/19 & 255/35/19 or 245/40/19 & 275/35/19 to stay within the 1%. Had a friend that had to change the transfer case which most likely resulted from running outside the 1% difference. If you look at other threads, others have had refusal of TC on warranty due to the wheels not matching the specs on the door tag. I've had staggered 19's and 20's and just switched to 245/40/18 square and the ride is much better than staggered in every way obviously that's only my opinion.
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