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      09-30-2020, 09:39 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, 08:12 AM   #2
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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, 03:47 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, 08:46 AM   #4
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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, 176 views)
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      10-12-2020, 09:34 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, 03:35 AM   #6
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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, 03:39 AM   #7
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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, 07:38 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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      08-20-2021, 02:06 PM   #9
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Latest Update

Good evening all,

I wanted to give you an update on my story and once again ask for your advice…

Since the last time I updated this thread, a lot has changed. Whilst the car behaved better with the “correctly” sized front tyres on it, I still was not satisfied with the handling. As such, I signed up to do a Masters in Race Engineering that also covered vehicle dynamics and car setup (it seemed like a good way to make the most of my free time during the initial COVID furlough). As such, I now have a much better understanding of what is going on and should be able to provide more helpful information…

So…the car was slightly unstable and as the months progressed, even started squirrelling about when crossing over the white line in the middle of the road. The TC light was also kicking in mid-corner as soon as I even looked at the throttle. As soon as I got the car in the air and had a look underneath the cause seemed obvious - the RL damper was completely shot.

I decided to replace the rear dampers with some Bilstein B8s (with F82 bump stops, after reading all of FaRKle! excellent write-ups) - the car has been on ACS springs for a while now and so the 20mm drop over the standard xDrive height meant that the B8 was the correct choice over the B6.

While I was there, I decided to also fit some Millway Street camber plates (again, thanks FaRKle) up front (managed to get a bit of a deal on eBay, barely used) and fit 4 new tyres.

My new setup was:
4x MPS4S 255/35 R19 XL
-2deg camber front and rear
+0.11deg rear toe (per side)
+0.06deg front toe (per side)

The car was transformed - fantastically stable and very competent on roundabouts (tons of grip, even if the steering still left me without real confidence in the car). However, on a spirited drive to enjoy my new rear dampers, I was mid overtake when one wheel hit a (small) bump and the car stepped about 300mm to the right! I thought it might be worth fitting the BMW F3X Steering Rack kit, especially as I has also begun to notice that there was also a very faint knock when stationary coming from the rack…

I had the B8s on the rear installed for all of 10 days when I was driving in the mountains when I came over a blind crest and found that the road ahead had basically all crumbled away. I hit the anchors and scrubbed off as much speed as possible but not enough - I managed to get some air before I came crashing down (the impact was absorbed by both front and rear axles). I was petrified I had ruined the B8s and was adamant the car felt slightly more nervous to drive after this.

In the meantime, I purchased the Steering Rack kit hoping to improve the steering. Whilst I didn’t follow the 70Nm specified by BMW (I tried but it was way way too stiff, at least with the particular thread in my rack and so I wound it back a quarter turn), the process was easy and the rack felt much better. The knocking was gone and the steering was more taut and precise than before, yet the weird feedback through the steering wheel continued.

I decided that it would be worth the investment of upgrading the front dampers to match the rears, thinking I could also fix the steering - I waited for Bilstein to find some stock of the B8s and got those installed (with an accompanying wheel alignment).

After the change, the front axle absorbed bumps nicely and felt more supple and more controlled. However, the steering was still not how I remember it when I bought the car; I tried to convince myself that it was all in my head…

5 days had passed before I bounced my FR through a hideous pothole on the motorway (hidden in the shadows under a bridge on the join between two patches of tarmac). Deafening - it sounded like the damper had compressed the bump stop to nothing and gone metal-metal. I had not had a chance to seriously test the B8s on the front axle before this happened and as such have no idea if there is a difference in the handling but again, after the impact felt the car was more nervous to drive.

3 months have passed and this is my description of the handling/some things of note:
1. The front axle has a LOT of grip (in part due to the 255 up front, in part due to the beautiful tyre (thanks Michelin), in part due to the 2deg of camber and in part due to the dampers)
2. The steering is dreadful - it does not load up as one would expect and is erratic…sometimes it goes light mid corner and other times it gets much heavier for no discernible reason…
3. The steering dead zone is large and seemingly variable
4. There is an instability under braking and even in a straight line (almost as if you feel a slight “shake” of the front axle (this happened to me on my previous car, an Alfa G - one of the driveshaft tripods had rusted causing a vibration)
5. You are never really sure how much steering input is right for a corner (even at normal speeds) - sometimes you are surprised that the front end turns to the extent it does and you need to get off the throttle/open the steering significantly/correct the other way (technically “oversteer’ but not in the conventional sense that the rear axle slides), other times it under rotates and more steering input is needed (this is especially scary on a motorway even with big, gentle curves)
6. The teering weight changes between drives (this week it is heavier than last)
7. TC sometimes kicks in when accelerating away from a T-junction (and changing road surface)
8. Even with everything turned off (DSC included) and the car in manual mode, I can feel that in some corners the car will not accelerate until I straighten the wheels (I am on circa 20% throttle, I purposefully keep my foot at a fixed position and on corner exit as I straighten the wheel, the power kicks back in)
9. The steering column has a “creak” that sounds like it could be the steering column position sensor scraping… https://1drv.ms/u/s!ApJ3UNdoA4Zhg5Yb...4jkXg?e=r9et7s I have put a couple of audio files here, I hope it is loud enough. The sound is from within the wheel, it is always present)
10. The front axle, particularly the FR is much “crashier” than it was when the B8s were first installed. They do not absorb changes in road surface (e.g. when a new bit of tarmac is laid down on top of old tarmac and there is a step between the two) nicely, the balance is upset and there is a metallic clunk
11. The car tramlines significantly; I would expect some tramlining due to the wider front tyre, however this problem seems exacerbated by the fact that bumps in the road result in instantaneously heavy steering that feels “locked”
12. There are a couple of (very shallow) gulleys in my driveway at home; when reversing the car into the garage I pass over one of these. If the front left runs along the length of the gulley and then lifts up out of it (I am pretty much full right hand down at this point), there is a small jolt as if the wheel loses traction. It feels “sloppy”

The most alarming thing however, is the fact that the car behaved in an identical fashion to last year when I asked you all for your help, in exactly the same corner on exactly the same road…:

I was down in the South of Italy, visiting my girlfriend’s family. I was driving down the same road where the car “sidestepped” 90deg last year – this year I put the car into Sport at the beginning of the road and noticed that in the first turn, the car was simply not composed; it understeered which then turned into snap oversteer (small amounts as I was going very slowly). For the next corner (the culprit from last year), I decided to see if the car would repeat the behaviour from the previous corner; I went into the corner at a very pedestrian speed; the car understeered for 5m or so, this then turned into snap oversteer that I corrected with an armful of opposite lock to get it pointing in the right direction…

It occurred to me that it could be diesel on the road; as the front tyres hit the diesel they understeered, once they came out the other side they found some grip, rotating the front end as the rear axle passed into the diesel patch - the difference in axle grip front-rear would then cause the snap OS.

However, I find it highly unlikely (read:impossible) that:
a) 2 years in a row that I do that corner, the car behaves identically and
b) there was diesel in 2 consecutive corners and
c) there was not a pile of cars on the outside of the corner where everybody had understeered straight on. It is worth noting that the day before passing by that road with my car, I drove the same bit of road, several times, with another car and not once did the car misbehave…

So, what could be the cause of these issues?

1. Last year I traced the “big moment” issue to the discrepancy in front and rear tyre diameters…I am now suspecting that it was a mix of tyre diameters and rear damper…I am wondering if the damper(s) have been damaged during use thanks to the dreadful road surfaces and potholes in Italy (I would expect FR and potentially complete rear axle) and/or were defective. I have performed a visual check, there if nothing immediately obvious (i.e. fluid leak/bent piston) – I think it is highly likely that the dampers are playing into this (especially given how the rear hops over bumps)
2. Bushes/bearings – the car has had a visual check underneath; some bushes flex slightly but nothing looks worn
3. Steering rack (rack & pinion) - I know there have been many issues with the racks and BMW changed the design 6 times…the car is servotronic (no VSS) and mid 2015 so I suspect is mid-rack updates. I don’t know if there is a SIB update that would fix it, however I doubt it as the sensation was not there when I bought the car and so the problem is mechanical, not software
4. EPS - likely culprit given that the level of assistance is what appears to be varying continuously. I tried pulling the fuse for the EPS and going for a drive. The steering was so damn heavy at anything up to 40mph (and quite frankly, awful in terms of feel) that I didn’t get any meaningful result from it
5. Camber plates - with the original BMW dampers there was a “wobble” noise at low speed. With the B8s, this is all but gone, however I do wonder whether the Millways are simply too rigid for Italian roads. Could the road surface be so dreadful that removing the rubber blocks in the top mounts causes every road imperfection to work its way through the controls creating this sense of imbalance? I think it’s unlikely…
6. Steering position sensor - I don’t know if and how this is used by the EPS. I would assume that the EPS inputs are vehicle speed, steering input and (maybe) Ay; a poor/inconsistent reading on the steering position would then explain why the steering weight changes from light to heavy at random. Without expert knowledge on EPS inputs, I can’t rule it out
7. Transfer case - it crossed my mind that I did indeed cause some damage to the transfer case when I was running around last year with the wrong size front tyres. Reading through the forum, people have had different experiences with the case going bang, usually there is a noise (in my case there is not). Maybe a transfer box oil change + xDrive review would be a good idea to rule it out(?) I didn’t think it was the root cause, however given that I had surpassed the recommended service limit for the gearbox, I took the car to XF Milan a few weeks ago - no filings on the magnet, oil dark but “clean”; gearbox much smoother now
8. Several of the above. I think that dampers would explain the instability (based on previous experience), however I am not sure it would explain the steering weight (which varies from week to week even when at low speed/stationary). Could an issue with the steering rack/EPS cause the instability? Quite likely. Occam‘s razor would suggest that only 1 thing is at fault here…

Ideally I would take the car to a local indy who lives and breathes BMW and who has seen all these problems before. Unfortunately, the concept of an indy simply does not exist in Italy (at least, I haven’t found one yet). My local mechanic is a lovely guy who is happy to work together with me on the car but who is not experienced in the problems I am having, particularly with BMW (Fiat maybe yes). I am contemplating booking an appointment with a decent indy in the UK for when I next go back to visit my parents, though it will be a while and means I have to drive the car through France as is. As such, I am trying to eliminate the various possibilities that have come to mind so far with various tests while driving.

I am considering:

1. Taking the car to a damper expert I know and having him check all 4 on the dyno to check for damage
2. Swapping the Millways for the original top mounts for less NVH and a more supple ride (I would be very sad to lose the front camber)
3. Checking the wheel alignment to see if for some reason something moved and I have a huge imbalance left-right

So what do you all think? Does anyone have any ideas on what to do next or where to bang my head up against a brick wall?

I know that some of you over in the US have good relationships with your BMW dealers, however I have sceptical that an Italian dealer would be able to diagnose without giving me hell for having non-standard components fitted. I am also convinced that I would be unable to speak to the mechanic who handles my car, instead having to rely on a middle-man in the front office who understand the square root of 0.

I am desperate to get this fixed and get back to enjoying the car. Recently I have been driving my girlriend’s company car, a G21 318d Business. Despite the lack of grunt, the light steering and the body roll, the chassis is a complete peach. Even with the long wheelbase, the way it rotates is sublime and the thing does make decent progress if you can keep the momentum up. I imagine that a 340d MSport on some serious rubber would be completely unstoppable.

Thank you in advance, sorry about the length of this post…

Yours,


Dom

Last edited by ildottorebardi; 08-20-2021 at 02:17 PM..
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      08-20-2021, 03:41 PM   #10
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I would start investigating in this order:

1st: get a obd interface to check for error codes since steering angle sensors, hight sensors, wheel speed sensors and abs sensors can all influence the steering and acceleration behavior.

2nd:allignement check

3rd : damper check

then go from there
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