Rear End Not Stable

[jamato998]

Hey Everyone I need some help as I cannot figure out my rear issue on my 2016 BMW M4. About two months ago I felt my car feeling unstable in the rear, shifting from side to side... kinda how a front wheel drive has torque steer when you accelerate, you are fighting the wheel. I have noticed this in my rear end where im fighting the car to stay straight. This happens on highway at cruising speed from 50 up and when I am full throttle it is even worse. I first started noticing it when I got my brakes painted but they did not take them apart just sand and sprayed (not sure if this is coincidence). So far I have replaced my stock suspension with hr springs to BC coilovers, new driveshaft, tried axles with 10k miles, upgraded diff pushings with powerflex, had the control arm bushings checked, all new subframe bolts which were torqued down to spec. Had the wheels road force balanced and two alignments. NOTHING HAS FIXED IT and it feels dangerous. Can anyone give me any input or experience this issue?

[michaelalex17]

Originally Posted by jamato998

Hey Everyone I need some help as I cannot figure out my rear issue on my 2016 BMW M4. About two months ago I felt my car feeling unstable in the rear, shifting from side to side... kinda how a front wheel drive has torque steer when you accelerate, you are fighting the wheel. I have noticed this in my rear end where im fighting the car to stay straight. This happens on highway at cruising speed from 50 up and when I am full throttle it is even worse. I first started noticing it when I got my brakes painted but they did not take them apart just sand and sprayed (not sure if this is coincidence). So far I have replaced my stock suspension with hr springs to BC coilovers, new driveshaft, tried axles with 10k miles, upgraded diff pushings with powerflex, had the control arm bushings checked, all new subframe bolts which were torqued down to spec. Had the wheels road force balanced and two alignments. NOTHING HAS FIXED IT and it feels dangerous. Can anyone give me any input or experience this issue?

Is it possible that the car is tramlining and that is what you are experiencing?

[jamato998]

I dont think its tramlining... I have tried different psi in the tires didnt fix it, i swaped on some different wheels didnt fix it. I changed the suspension to coilovers and it didnt fix it. And it does it on every road not just one certain road.

[hC1001]

Do you have matched brand/model/size tires in the rear?

How are your diff bushings?

Maybe the diff is not functioning correctly. When was the last time you replaced the fluid?

[jamato998]

Yeah they are newish michelin PS4 tires... I have brand new power flex diff bushings. the fluid was just replaced when I had my driveshaft done.

[Ropes]

Have you tried jacking up the car and applying force on the rear wheels in various directions to check for play in the suspension connection points?

[jamato998]

How would we do that? Jack the front up and just have pressure on the rear?

[maddmatth]

Do you have any BMW dealers or BMW specialists, or good performance/motorsport shops in your area? You really need someone who understands this suspension well to diagnose it.

Sounds like your process so far has been pissing away money at random things in the hopes of fixing it. That's the completely wrong approach, you need someone who can actually diagnose this and find the problem.

People can potentially have some good ideas on the forum but nothing beats having a good specialist taking a proper look at it.

[Crazy Penguin]

1 psi off between the rear tires can do this

[clutchdj]

Power flex are really for the track as they don’t yield or deflect like stock; even their street version of stuff. Not saying that is related to your issue... but need more info.

Do you have the readout on the front and rear toe settings?

[jamato998]

Car has been only to BMW dealers and they are the ones changing stuff, its them not me and they cant figure it out. I have alos brought it to a suspension shop and there answer was traction control (which is ridiculous).

[clutchdj]

You should really get a F&R toe set print out. Anything close to zero toe in the rear would could cause this assuming there are no mechanical issues with the car. Hit a bump the wrong way and you can knock the toe out especially in the rear. Your best case scenario is the toe is set to zero in the rear and it just needs to be brought closer to factory spec.

[deli333]

have you checked rear trailing arms? rear lower camber arms? btw have it checked by an experienced person as it is difficult to see/notice the anomaly.

also give more information like behavior under 10, between 10-20, bumps etc.

[G35POPPEDMYCHERRY]

Originally Posted by jamato998

Car has been only to BMW dealers and they are the ones changing stuff, its them not me and they cant figure it out. I have alos brought it to a suspension shop and there answer was traction control (which is ridiculous).

highly the worst place to go for a suspension problem is a stealership.

[knightarmor]

I had this happen once. Felt like the rear was riding on ice (it was a nice cool afternoon) under acceleration going up a mountain. It's the DSC, turned that off and haven't had anymore problems.

[jamato998]

How do you turn the dsc like traction control?

[deli333]

rear end of this car have different stability issues so you have to give more information or I wish you good luck moving from one shop to another. I have had a similar problem, it drove me crazy. just trying to help

[jamato998]

Not where you are getting im going from one shop to another. Literally have gone to the dealer. I think i have given a ton of information... The car is very bumpy and unstable in the rear at especially around 40-50mph.

[deli333]

https://f80.bimmerpost.com/forums/sh....php?t=1703457

check this. mine was rear lower camber arm. Read the topic can you see anything similar. Only one person said that straight away it is bend. Other than that no one could see a difference even side by one with a new piece. It is really difficult to notice.

also make a basic shock control, push the car from the rear right and left shock sides, does it settle after one jump or more? But probably they have checked it

[jamato998]

deli333

Did you get any new specific lower camber arms or just went with the stock ones?

[deli333]

it took 45 days to find out the problem,it made me paranoid so I am just changing the whole rear arms.

I went with stock, however I wanted to try this aluminium piece but needs different spring/coilover setup which people dont recommend it for various reasons. However like I said I hated it. Probably your only option is stock one.

https://www.irp.lt/individual-racing...f80-m3-f83-m4/

also your problem might be any of the rear arms, so it has to be inspected by an experienced person.(some track guys/garages have eagle eyes) I doubt that dealership guys can notice it.

[jamato998]

Did you buy the stock ones yourself? I cant seem to find lower camber arms, i just see lower control arms?