[boost3dx]

I noticed some months back my car making a strange clunking sound sometimes when in reverse. It only happened after I left off the throttle, but didn't happen 100% of the time.

I have my car in for service right now and I brought it up to them, and kudos to Henderson BMW in here Vegas, they actually took the issue seriously and got their shop foreman involved, and had the car for a couple of days to diagnose the issue. They had me go back down there to do a test drive to confirm the sound I heard with the foreman.

After I replicated it and he heard it which was about 24 hours ago, I just got a message saying they've confirmed failure of the front diff.

Just wanted to share this out there in case anyone else has a similar sound, this may be your issue. Seems crazy to me that the front diff would fail so soon, only about 15K on the car. It's never been abused, just a couple of track events and fun here and there.

[shawnhayes]

I had concerns about the front diff fluid not being changed in this car as part of the break in and have planned to do mine at 10,000 which I have not gotten to. Can’t say that will mean anything concerning your case, but that diff works hard at the track.

Shawn

[Littierally]

My dealership changed the diff fluid on my CS at the break-in service. I understand not all did.

After I installed my springs, I had a similar sound for the first day. Haven’t heard it since.

Do you have any suspension mods?

[Holset]

Sorry to hear that.
The drivetrain are generally stout on our cars and rarely breaks.
The transfer case seems the most vulnerable but that mostly to user error running wrong tires front to back or dyno on 2wd dynos.

[boost3dx]

Originally Posted by Littierally

My dealership changed the diff fluid on my CS at the break-in service. I understand not all did.

After I installed my springs, I had a similar sound for the first day. Haven’t heard it since.

Do you have any suspension mods?

No sir, car is mostly stock. Just intake, secondaries, and MPE.

[shawnhayes]

Originally Posted by Holset

Sorry to hear that.
The drivetrain are generally stout on our cars and rarely breaks.
The transfer case seems the most vulnerable but that mostly to user error running wrong tires front to back or dyno on 2wd dynos.

What about 2wd mode?

My GT-R was terribly sensitive to wrong tires/wheels. I learned from that experience. The rear tire is EXACTLY the size of the GT-R's rear tire.

Shawn

[Holset]

Originally Posted by shawnhayes

What about 2wd mode?

My GT-R was terribly sensitive to wrong tires/wheels. I learned from that experience. The rear tire is EXACTLY the size of the GT-R's rear tire.

Shawn

2wd mode is safe to use on the road but some people mentioned that if you dyno in 2wd with locked front wheels the car can switch to 4wd after some time and burn out the transfer case but there is apparently also a dyno mode that i don’t know what it does you can use but not sure if they used that.
It’s recommended to use a front to back linked 4wd dyno.

The F90 M5 has 1,1% difference in circumference front to back on stock 20 inch wheels.
I asked the genius at my Bmw dealer on how much difference that is okay and he said 2% and tried to look up some documents on it but didn’t find any then but said that he had got educated on it before.
Funny thing is that the winter wheels from Bmw for the M5 have 2,3 or 2,6% difference depending on which online calculator you use so i don’t think the cars are super sensitive about it but to be on the safe side i would stay within 1,1% atleast for summer tires.