Unstable over 80 mph

[bmwfnatic]

Worn out RTABs can make it feel terrible at highway speeds.

Had me constantly correcting the cars trajectory and would even make my steering wheel go off center under WOT (bushings were totally shot when I just got the car).

[Obioban]

Worn out RTABs can make it feel terrible at highway speeds.

[Leafsrule531]

Absolutely could. Probably getting super hot at highway speeds. The tell tale sign is the tire wears out super fast.

Let's summarize this. You have crappy mismatched tires with potentially mismatched springs and your car is handling poorly. I'm shocked.

Hahah very true. Either way, you’re a genius. I swapped out the tires and the car drives perfectly now how it used to. No stability issues whatsoever.

Im shocked those tires (lexani lx20) had such an impact tho - they must’ve been defective. Either way - lesson learned, always spend up for good quality tires and don’t wait for sales 😂

[bigjae46]

Just checked the load rating on those tires ....website says its Load Index. 1047 lbs (82).

Quick google search is telling me that the minimum load index for the e46 m3 should be around 88.... is that true? does this mean the tires are the issue?

Absolutely could. Probably getting super hot at highway speeds. The tell tale sign is the tire wears out super fast.

Let's summarize this. You have crappy mismatched tires with potentially mismatched springs and your car is handling poorly. I'm shocked.

[Leafsrule531]

Just checked the load rating on those tires ....website says its Load Index. 1047 lbs (82).

Quick google search is telling me that the minimum load index for the e46 m3 should be around 88.... is that true? does this mean the tires are the issue?

[bigjae46]

Check the tire load rating. If its not high enough then it would definitely cause issues. Sounds like there are a couple of different janky things going on here...

[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.

[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]

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 ?

[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.

[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.

[9kracing]

Post your alignment sheet if you can

Improper toe can exhibit darting at high speed

[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!

[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

[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.