[Mrspeed636]

So.... On track at VIR over the weekend. Was very excited to try out the carbotech brakes as well as my Forgestar wheels fitted with Good Year Super Car S3Rs.

First session, very cold, ran the stock PS4s. Wasn't pushing it, ran a 2:09, things were slick, ran off on my exit thru Oak Tree one lap but no big deal. The brakes were fantastic! Great initial bite and modulation. I cant say that I was pushing them too hard due to the cold but initial impressions were that they are an improvement in feel and stopping power over the stock pads.

Then, I switched over to my 'track wheels'. I went UP in size to 285/305s and was really looking forward to my first experience on R comps. First turn might as well have been driving on ice. Thankfully I kept it on track and then proceeded very cautiously until I got some heat in them. Started noticing a bad vibration at 60+ MPH and decided that something must be out of balance. I drove straight to Quantum just outside of VIR and had them balanced. One of them required 2 full ounces!!!!!!! WTF!!!! All 4 tires required some balancing which I am not happy about since everything was supposed to be balanced and ready to go.

Now, back on track, temps were up into the 50s and I was excited to get things going. First straight away, BAD vibration and 'pull' from what felt like the right front. Almost like the car was 'binding up' then it would 'release'. The car flashed something about a problem with the AWD system. I slowed down pulled off track and put the car into 2WD mode. Same exact problem but the car never said anything was wrong. BimmerLink does not show any codes and I am struggling to understand what happened.

By now my confidence was completely shot and I was DONE. Loaded my shit and headed home. fl1by was there when I came off track which was super cool and he made a comment that the front right brake seem to be grabbing extra based on the coloring of the rotor.

Talking with some of my friends, the best theory we could come up with was the car 'knew' the wheel(s) were out of balance and was attempting to compensate by pulsing the brakes. Perhaps a bad wheel speed sensor or the sensor was somehow messed up due to the horrible balance of the wheel?

Now the real kicker, I spent Sunday putting the car back to stock so I can take to the dealer and say WTF, please fix... My friend was in the car during a test drive. pulled out of the neighborhood, at 45mph, BAD vibration coming from the right front. I made it 1/2 mile and turned around. When I pulled back out from the u-turn the vibration was completely GONE. I then spent the next 20 min or so driving the car increasingly aggressive in order to re-create some kind of vibration or issue, nothing.... I even changed to 2wd and back to 4wd, tried everything I could think of.

I realize this post is already insanely long but... does anyone here have any clue as to what could be going on here?

[mattburks]

Man… all I can say is I’m sorry you’re going through this and I FEEL your frustration, brother.

Thank you though for your testimony and I imagine this will help someone out in the future if you continue to update until a solution is solved.

I am rwd, but haven’t seen anything like this on the boards since receiving my car last April.

Sure hope someone smarter & more helpful can chime in soon sir— best of the best luck.

[Ximian]

Which compound of the CarboTech pads? Also, was this a vibration under braking or just at speed? Was it only the r-compound tire/wheel package or the factory wheels as well? It's not quite clear in the original post.

There's already a thread about the insane number of wheel weights on the wheels out of the factory, but that doesn't quite apply to the aftermarket wheels - https://g80.bimmerpost.com/forums/sh...1900058&page=2

If it's a vibration all the time -
Not sure if you have a before and after picture to see if any wheel weights are missing, but that would definitely cause issues vibrations. If none are missing, it's also possible that the tires themselves have a manufacturing defect. They can look perfect to the untrained eye but would be difficult to balance. Typically, these tires are spotted by being visually out of round, or would need crazy amount of wheel weight to balance.

If it's a vibration under braking only -
CarboTech pads are amazing but can tend to leave pad deposits or pad material transfer causing vibration. I had it happen after a cold, wet track day last year in my track car. Really bad cases like I had require grinding the brake rotor* to smooth it out. Otherwise cold track pads or other really abrasive pads should rub off the pad transfer.

*(because most places aren't comfortable with slotted/drilled rotors on a lathe)

[Mrspeed636]

Appreciate the support and the questions to help me sort thru this.

Carbotech XP20 up front and XP10 out back.

Vibration was mostly under acceleration, braking seemed quite smooth and predictable. After the initial 'big vibration' it was there all the time. The wheel guy asked me if I saw any weights had fallen off. I really dont think they did, I went thru the packaging myself and I feel like I would have seen weights. Nothing on the rims indicated one had fallen off after my initial session as well, the shop I went to said the same thing. All 4 tires took some amount of weight to balance them. Biggest offender was right front as indicated.

It will probably be this weekend before I can have the wheel balance checked again.

Another theory that was brought up to me was the difference in size of the stock compared to the new tires.

26.9'' New vs 26.6'' Stock

26.2'' New vs 26.7'' Stock

Could this have 'confused' the computer into thinking there was a speed matching problem between the front and the back?

Maybe I am grasping at straws here?

[MannyM5]

Originally Posted by Mrspeed636

Appreciate the support and the questions to help me sort thru this.

Carbotech XP20 up front and XP10 out back.

Vibration was mostly under acceleration, braking seemed quite smooth and predictable. After the initial 'big vibration' it was there all the time. The wheel guy asked me if I saw any weights had fallen off. I really dont think they did, I went thru the packaging myself and I feel like I would have seen weights. Nothing on the rims indicated one had fallen off after my initial session as well, the shop I went to said the same thing. All 4 tires took some amount of weight to balance them. Biggest offender was right front as indicated.

It will probably be this weekend before I can have the wheel balance checked again.

Another theory that was brought up to me was the difference in size of the stock compared to the new tires.

26.9'' New vs 26.6'' Stock

26.2'' New vs 26.7'' Stock

Could this have 'confused' the computer into thinking there was a speed matching problem between the front and the back?

Maybe I am grasping at straws here?

Your final theory sounds probable..

Overall diameter should be either slightly larger in rear or equal. You'll want to keep close to the stock ratio, especially with x-drive.

[Mrspeed636]

Agreed but the more I think about this, BMW had to assume that people would be changing wheels and tire sizes. I would hope the car would be able to figure this out. but maybe that is wishful thinking.

[Ximian]

Originally Posted by Mrspeed636

Another theory that was brought up to me was the difference in size of the stock compared to the new tires.

26.9'' New vs 26.6'' Stock

26.2'' New vs 26.7'' Stock

Could this have 'confused' the computer into thinking there was a speed matching problem between the front and the back?

That's almost a 3% difference in diameter to each other (1-(26.2/26.9)) which can cause some issues. Generally for xDrive, what I've read is to keep the diameter difference less than 1%. Personally, I wouldn't use these tires again.

At what point did you switch back to your street wheels to check for vibration?

[beachBmmr]

First, this sounds like a really hard track trip, and was probably demoralizing. It happens, were blazing the trail here, so you need to embrace failure, analyze, learn and adapt...


Couple of theories.

1. Did you have DSC on when you swapped your wheels? Were you in AWD or 2WD?

Changing tire diameter would mess up the AWD speed sensing (and traction management) as it would interpret either the front , or the rear slipping and apply brakes to try and solve for it.

2. Were there a lot of marbles on the track, and did you notice a lot of pickup on your tires (looks like gobs of melted rubber)? I have experienced this a lot on track and it feels like an out of balance issue, but its vibration that goes away as you turn and scrub off the pickup.

3. If. the problem came back with your stock wheels and tires, and went away, I suspect you are screwing with the cars speed sensing for traction control and its correcting itself.

These cars are fucking sensitive, and dont like shit out of spec...also Brake by wire sucks....and your experience tells me not to change from stock tire sizes until someone proves otherwise. You should experiment just doing street driving in awd, 2wd, DSC on, DSC off before trying on track.

[beachBmmr]

Originally Posted by Mrspeed636

Another theory that was brought up to me was the difference in size of the stock compared to the new tires.

26.9'' New vs 26.6'' Stock

26.2'' New vs 26.7'' Stock

Could this have 'confused' the computer into thinking there was a speed matching problem between the front and the back?

so I did some math here...
Stock ratio is .375% smaller front very low...

Did you do 305/35/19 in the rear?
If so then your changes were 1.1% larger front, and 1.8% smaller in the rear. With 285/35/19s in front and 305/30/19s in rear your new ratio is 2.38% smaller REAR. This is no bueno for the speed sensing and gearing. You need to stop running these tires ASAP.

To maintain the right ratio with those front tires, you need to have either 295/35/19. or 295/30/20 in the back...Neither of which are SC3R sizes....

you MIGHT get away with 305/30/20 in the rear which is a OD of 27.2 and a 1.1% smaller front and closer to stock...but that might also be enough deviation to cause issue.

Edit: vorsteiner runs this kind of deviation on their shop car with 20" wheels so it might work
https://www.instagram.com/reel/CaIIR...dium=copy_link

I think you have just convinced me to only run the MSC2* from now on on the 19x9.5 and 20x10.5 wheels.

[Ximian]

Originally Posted by beachBmmr

I think you have just convinced me to only run the MSC2* from now on on the 19x9.5 and 20x10.5 wheels.

Not really an issue for those of us with RWD. xDrive is much more sensitive.

[Erebor]

As other already mentioned, the transfer box of the xdrive system is extremely sensitiv and you might have already killed it with your chosen tire combo. Those vibrations while accelerating sound all too familiar to me.

First off, the car can't figure anything out. You have to get the right tire dimensions for it, or things go haywire. There are basically just two rules for xdrive cars:

1. Front to rear overall tire diameter must be withing 1% difference.
2. The rear tire must not be smaller in diameter than front.

So for tracking a car with xdrive, you either go with stock dimensions, or, as many other do, change to a square setup. With that, there won't be any issue. There's to hope your box isn't gone yet. Good luck.

[Mrspeed636]

Appreciate all the feedback!

I called BMW Genius yesterday and had a really good conversation. They are escalating this internally and even mentioned possibly getting BMW engineering involved. In the opinion of the guy I talked with (track enthusiast himself) the tire combo should not have been a problem and the car should be able to sort out the math. Here's hoping....


Also, to beachBmmr's questions:
1. DSC off - AWD mode. I did switch to RWD mode to see if that changed anything and it did not.


2. Track was quite clean of marbles, heavy rain Thursday night and we were first on track since the rain. I do know what you mean about picking up rubber and how it feels, I have experienced this before.


And yes, it was a difficult day but I agree this is uncharted territory for these cars and I get it. I hope the car isnt fucked.... I have been driving it all week to work with no apparent issues.

Is there anyone else that we know of that is tracking an AWD G8X platform? I'm sure someone besides me is?

[playswcars]

You sir, may have just wrecked your transfer case.

[Mrspeed636]

No, I dont believe the car is wrecked. I've put 250+ miles on it this week and there are zero issues or vibrations. Hopefully I will talk with BMW today and can shed some more light on this.

[BulkBen]

Originally Posted by Mrspeed636

No, I dont believe the car is wrecked. I've put 250+ miles on it this week and there are zero issues or vibrations. Hopefully I will talk with BMW today and can shed some more light on this.

It's been covered already but the tyre sizes being 1.8% apart seem the obvious issue here.

Your description of the feeling (car binding up) and the vibrations perfectly align with the expected symptoms of running too large a variation here.
No the first time those exact things have been reported when people made the same mistake.

Would be interesting for you to slot 285/30's on the back (square setup) and see how that goes.
I did wonder how 305's would go all round, but figure you'll need a bit of camber to ensure tyres clear the guards there.

[Mrspeed636]

I agree with you, wishing I had figured this out before hand but that's clearly on me. While I haven't read the manual fully, I wonder if this information is stated anywhere from BMW.

[Allerum]

I think a square setup is the safest. Since you have Xdrive you don't need as fat on the back as it will just send power up front if you get slip.

[Mrspeed636]

So... Finally a little bit of an update. After waiting on BMW 'Corporate' to get back to me I got a call asking if my issue had been resolved. After I stopped laughing I said no, and asked who I need to talk to. A few phone calls later I was directed BACK to the dealer to figure this out. I was told who to talk to (service manager) and was assured that they have a direct link with BMW NA Engineering. Well, after a few weeks of getting him to call me back we had a great conversation and he genuinely tried his best to help me.

Here is the last email I got from him after my last follow-up, I believe this tells the whole story. 'He' mentioned in the email below is the NA-Engineering contact.

"He was VERY non committal on everything that I presented to him. He kept iterating that you would have no issue if you just stay with the stock size. He didn't seem comfortable deviating from that BMW NA corporate stance. He did comment one time that you can go as wide as you want, as long as you can still turn the car within the wheel wells, but you had to stay at the same height as stock. I think that I may have already passed that comment along to you verbally. He also made the statement that there is an acceptable variance that has to be written into the control unit's parameters to account for tire wear and tire pressures, but he said that it would be next to impossible for him to obtain that information....which I also think that I shared with you over our previous phone conversation."

[Hendo741]

It's been mentioned numerous times, but since you are still chasing this rabbit it bears repeating, you cannot have that much of a deviation is tire diameter.

Your situation is quite resolved. The car is behaving as expected with an incorrect set up. Welcome to AWD!

My advice, as well as others above, find a square set up. I have been driving and racing AWD cars for almost 2 decades and find square easiest solution to ensure you never have issues. Also makes it easier when rotating tires to ensure even wear (which is also a good idea with AWD). My RS3 AWD system is so sensitive it threw a fit when I had to replace a tire after getting a flat on a road trip. I replaced both front tires with tires of an identical size, but different manufacturers, and the TCS was very unhappy because it thought it was sensing constant wheel slip. Existing tires were even very new with less than 1/32 wear.

Sorry that you wasted cash on tires that will absolutely not work. Could always buy 2 of the 285s and throw them on the rear. Pretty certain 305s won't fit on the front.

Also, not sure, but I'd be careful how high you want to try and take this. They (BMW) may have grounds to deny any future warranty claims against your transmission and AWD system and cite this ticket as out of tolerance use. I hope not, but you can never be to safe these days.

[LuvMyE92]

I'm going to throw a vote out there along with the other(s) who suggested pad buildup on the rotors. Some brands are really bad about that, and it can make for a most unpleasant driving experience.

It should be fairly easy to see on the rotors, and you can get some 60 grit sandpaper to clean it off.

Many people (myself included) do not like to change pad materials on the same rotors. In my track days, I used dedicated rotors that I only used with the track pads.

[Mrspeed636]

Hendo741 - I agree with you and am not chasing this up the ladder any more. This was them finally getting back to me with some kind of answer.

On your suggestion on a square setup, that is/was my thinking, however there is still a .4% difference in the stock setup Front to Back so... is a true square setup even safe on this car? That was my last question to BMW in hopes to get a thumbs up but they wouldn't even confirm that.

[BMWRacer523]

Originally Posted by Mrspeed636

Is there anyone else that we know of that is tracking an AWD G8X platform? I'm sure someone besides me is?

I am but running stock wheels/tires (PS4 and Cup2) in the OEM sizes. No issues. Definitely sounds like the AWD system did not like the wheel/tire change.