u/Critical-Error-415

I have a BMW E90 which uses a McPherson strut and a double pivot front suspension. I changed the standard tension arm (the most front bit) and the rear control arm to BMW M3 E9x parts.

I changed these parts in steps and changed the front suspension’s rear control arms last because it’s the part which definitely needed alignment. Everything was fine until I changed this last control arm. It’s made so that it’s slightly longer than the standard control arm so that it pushes the bottom of the wheel outwards therefore introducing more negative camber.

Alignment got done. Now at low speeds (<15mph) the steering won’t self centre but self centres fine at higher speeds. At low speeds the steering also kind of wants to keep turning. At very high speeds on crap roads the harsher bumps make it feel like the road could turn the steering wheel, a bit like bump steering but you can maintain control of the car with little but not negligible effort.

Isn’t what I’m experiencing a symptom of too much scrub radius? Is it possible due to these longer lower control arms introducing more camber they have increased the scrub radius? Even though the kingpin or SAI angle should’ve moved with the contact patch centre line. The kingpin axis on this type of suspension is simply top strut mount to lower ball joint of lower control arm, so by moving the lower ball joint of the lower control arm more outwards, it should’ve adjusted this angle correctly so that it maintained the original scrub radius.

The thing which confuses me is, after the change, my toe-in was nearly 2 degrees and the steering was even worse where when you were turning the steering really put good effort into wanting to keep turning. It sort of points me towards toe being the issue rather than scrub radius?

Do I have a remedy other than changing the control arms back to standard? This is a popular mod to improve handling and I don’t see many people complaining about it. It has definitely helped a lot in cornering. Usually people get -1.0 to -1.5 degrees camber but somehow my car has ended up with -2.0 degrees camber.

Any of you here put M3 arms in the front of your 335i? I only did the front and my car is great in cornering but the road really affects the steering. What’s your high speed feel like? I’m talking about 160-200kmh (100-120mph) the steering feels kind of unstable like the road can turn it when the surface is old and uneven. I’m thinking of switching back unless something is wrong with my setup. I have about -2degree camber on the front. I don’t know if the M3 has a different steering ratio which negates this or what’s up with my car.