Unstable over 80 mph

[Leafsrule531]

My car seems to drive perfectly below 75 mph. The steering is tight and it feels very planted and resposive. As soon as I go over 80 mph, it comes super unstable. I get a slight vibration in the steering. Steering starts having play, and car feels wobbly/ unstable while even turning on the highway. Even when I brake from that speed, it almost feels like car becomes unstable like it’s going left and right and I have to hold the steering very tight to make sure it doesn’t swerve. Any thoughts on why this could happen ? I replaced the steeeing coupler around 15k miles ago. Has new tires on it. Kw v2 coilovers. Just had an alignment done too.

Getting pretty frustrated because that’s not how an m3 coupe should drive. Any advice will be appreciated!

[Halftim3]

Some ideas, Miles on front control arm bushings and tie rods?

[mr pete]

I know you mentioned new tires, but are they properly balanced? Check lug nuts also.

[Slideways]

Have the shop double check the alignment and wheel/tire balance.

[Tbonem3]

Happens a lot when the hub and hublips are corroded which makes it easier to mount the wheel crooked. Make sure it's clean and smooth and make sure you tighten wheels in the star pattern.

[Nate047]

Start by looking to see if there's a clean spot where a wheel weight or two could have recently fallen off.

Also take off the front wheels and torque the lug bolts/nuts/studs correctly, and like someone above said make sure there isn't corrosion or grime preventing them from sitting flat.

[bigjae46]

Happens a lot when the hub and hublips are corroded which makes it easier to mount the wheel crooked. Make sure it's clean and smooth and make sure you tighten wheels in the star pattern.

Gotta keep the wheel mating surfaces clean AND rust free.

Check the front suspension, steering and the subframe. Make sure everything is torqued up. I'd consider that you might have 2 issues. Sounds like you have a bad RTAB as well.

[x X Cloud X x]

I had this recently happen to me after hitting a dip on the highway same symptoms you described, turned out one of the wheel weights came off on one of the rear wheels... After getting a new one put back on the car felt back to normal

[DJM]

Here's a thought out of left field. Quite a few years back I was plagued by the sudden onset of instability at speed. Curves previously taken at 80 mph were setting off the DSC at 70. Driving on a straight at 50-60 just felt kind of 'squirmy." Got alignment, had several mechanics look at it without solution. One day I was lying on the ground behind gazing at the underside and noticed that my recently installed Michelin PS2s with the asymmetric thread pattern were pointing in the same direction. One tire had been mounted inside out. Got that tire remounted properly and problem went away!

[9kracing]

Post your alignment sheet if you can

Improper toe can exhibit darting at high speed

[bigjae46]

Here's a thought out of left field. Quite a few years back I was plagued by the sudden onset of instability at speed. Curves previously taken at 80 mph were setting off the DSC at 70. Driving on a straight at 50-60 just felt kind of 'squirmy." Got alignment, had several mechanics look at it without solution. One day I was lying on the ground behind gazing at the underside and noticed that my recently installed Michelin PS2s with the asymmetric thread pattern were pointing in the same direction. One tire had been mounted inside out. Got that tire remounted properly and problem went away!

That's a good idea.

A lot of brands tires with asymetric tread patterns that you can mount either side for dry conditions. Higher end brands like Michelin, Pirelli and Continental have asymetric tire compounds that MUST be mounted correctly.

[Leafsrule531]

I did double check the tires. They all say “outside” on them which means I’m assuming they are installed properly ?

attached is my alignment spec. I don’t run -3 camber anymore. Reduced it to -2. Toe is still the same.

[Leafsrule531]

I think I realized what the issue might be and why I’m feeling the instability. My friend test drove the car and he mentioned that the front moves first then the back is moving which is causing the whirling / instability. I realized that I have swift linear springs in the front which have a higher spring rate and progressive springs in the back which came with the kw v2 Coilovers.

could the mismatch in spring type and rate result in the back moving in much slower than the front and causing instability ?

[Tbonem3]

Yeah, certainly doesn't help. Don't see how it would cause big instability, but certainly a strange feeling and possibly unexpected behavior closer to limits.

[Leafsrule531]

At high speeds post 70 mph, even a little bit of a turn to change lanes makes it super unstable as if the front moves first then the back.

embarrassed to say this but I also put cheap Chinese tires in the rears (temporarily - my rears were shot!) because I was waiting for a sale on the continental tires which I have in the front. He mentioned that the mismatch in tires can also be causing this. I do lose traction very easily.