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      04-09-2020, 02:00 AM   #20
msendit
First Lieutenant
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Drives: M240i
Join Date: May 2018
Location: San Francisco, CA

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Stage X: (in progress)

End parts/specs:
SPL rear suspension links
Tires: RE71-r 275/35/18 square
12 mm spacer front
Front spring: 90 N/m 170 mm

This is where things are now, a few big bits still largely untested. I'll keep updating this thread if / when there's other major "stages".

As I mentioned, the biggest issue with the previous config was that 285/30 tires didn't have enough sidewall strength and kept failing on track, and 275/35 (which presumably have enough sidewall) didn't fit in the wheel wells in the front. I swapped the front spring with a 30mm shorter one to get a bit more suspension clearance. Also, /35 tires had the worst clearance problem at the back of the wheel well -- not just a light rub, but full on jacking. Turns out the actual contact point is not in the frame itself, but in a stiffening brace that's bolted to the frame. The black bit here. Even better, my shop had a new M2 in for cage work and they noticed it didn't have that brace at all! (The M240 vert doesn't have it either.) So I just took it out -- like this -- which gained a good 2-3 cm of clearance. Also, it was a pretty hefty piece -- about 15 lbs on each side. I then trimmed the plastic fender liner at the front of the wheel well just a bit.

So, now 275/35 fits easily with a 12mm spacer. Haven't tested this out on track yet -- if the chassis ends up flexing too much (doubt it), we can come up with a much more discrete brace.
This actually ended up much easier than expected. Before we realized the clearance problems were due to a removable bit, the thinking was to add an adjustable thrust arm and way more caster to pull the wheel 10-20 mm to the front, then trim whatever's in the way upfront. Seems like we can get away without that.

The other largely untested bit right now is at the back of the car. Late January, I noticed that my rear suspension was starting to wander. Over a few days, I'd see pretty uneven tire wear especially on RL. Grip in right-handers was somewhat unpredictable too. Once back from the track, we'd put the car on the alignment rack and everything would look fine. This smelled like busted bushings on the rear suspension -- I was still running the OEM rubber bushings, now with more than 100 hours of track time on them. So we took them out and replaced the 4 links with SPL adjustable parts with solid ends. The old bushings looked worn but not outrageous. So far, I've only taken the car out once with this rear and it felt fine -- needs more testing to see if that was the real issue.


Last edited by msendit; 04-09-2020 at 04:29 PM..
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