[zuluZero]

I'm experiencing "squirrely" behaviour when passing on the highway, and searching Google / threads, still not sure how to isolate the weakness without trying expensive parts and hoping it works....

e92 335i full bolt on, so ballpark 400t/hp.
6MT. Stock sport suspension, 60km / 40K miles.
18" 235/265 PSS rubber, getting worn, but still 5mm or more
Traction control on, partial, or off (tried all 3)

I've never noticed any similar issues in normal driving, straight line acceleration or very limited track time. Only when passing under full power (which is different):
Smooth 2-lane highways, cruising at 90-100kph / 55mph. Drop to 3rd, use full power, then switch to left (oncoming) lane to overtake. Stay on the power until past. Although smooth, the highways are a little "crowned" in the center.

During this maneuver, the car feels very unstable, like I'm taming a wild bull. I'm having to counter steer and fight to keep the car on target. On the rare occasion when you have to overtake quickly for safety, I don't want my suspension letting me down.

Could it just be wheel spin, causing me to drift at that speed? Seems unlikely, but then I guess I'd wonder if LSD would be the first option?

Could it just be so much torque, as the car accelerates at the same time as switching to left lane? Can M3 suspension sway/bushings handle this?

Has anyone seen and solved the same use-case? (or better searcher than I)

Thanks!

[6ixSpd]

Did you check your tires? Alignment? Bushings?

I'd start there before thinking about an LSD... sounds like your rear toe settings are off and/or you have worn the inside parts of your tires.

[juld0zer]

As suggested, get a full alignment and check the rear tyre condition.

How are your rear brakes? That may affect your DSC performance too if you can still get the car fishtailing

[erttuli]

I sometimes seem to have the same issue. Road surface affects it a lot.
Best way to avoid this is to change lane while NOT going full power.. use that power when you get on the lane. Its not a great idea imo to go full power while changing lanes. Especially if the road isn't good or its wet etc. Just my experiences.

Originally Posted by zuluZero

Stock sport suspension

Issue found.

Even though you have relatively low miles, the dampers are the weak link here. Swap those out and even on stock sport springs you'll notice a huge difference.

[zuluZero]

Originally Posted by FCobra94

Issue found.

Even though you have relatively low miles, the dampers are the weak link here. Swap those out and even on stock sport springs you'll notice a huge difference.

Thanks for the suggestions. I'm planning to get new rubber next year, so I'll recheck the alignment at the same time, and have the bushings inspected (I have no idea how to do that).

Not hammering the power while I change lanes? Sounds smart, but it will be darn hard for me to relearn that skill. I'll try. I should also take out an M2 for a test drive to compare ...

For dampers, I assume you mean a reasonable change, like to B8s? I wasn't planning to lower.

Does anyone find that M3 bars & bushings would help in a case like this?

Originally Posted by zuluZero

For dampers, I assume you mean a reasonable change, like to B8s?

Does anyone find that M3 bars & bushings would help in a case like this?

Yes and yes. All options/mods/suggestions have been discussed a million times with respect to your requested goal. Have a read through some recent threads for more info.

[ECSTuning]

Another point to check, rear trailing arm. If they are damaged suspension geometry get's very "funky" during high load situations (full power lane changes), and causes all sorts of fun handling characteristics.

So you know what you're looking for, stock photography of the rear arms.