FWIW I've seen a lot of those car rebuild youtube channel measure straightness by checking diagonal dimensions. Pick some symmetrical bolts and use a rigid meauring device to check the cross dimensions. You'll catch parallelagrams too. I'm sure there is info on how to do it out there, but looks way simpler than getting it on a frame rack.
In reply to Slippery :
Yeah, they're dirt cheap on eBay. I might consider that route, but only if photos for the car in question are shown and there's no rear end damage. If there's rear end damage on the cheap subframes they're selling, well, that would put me in pretty much the same boat that I'm already in.
I'll jump in with some of my experience as I just went through this on an 06 330i. The rear subframe in my car was rusted heavily with a large crack making it's way through on the drivers side so (past) time to replace. I got lucky, one of the local u-pull yards had about 8 cars to choose from, 7 of them the subframes were just as rusty as you would expect up here, number 8 was rust free (at least by our standards with just a bit of surface rust on some of the welds where the paint did not adhere well) and someone else had already gone through the trouble to remove the diff, half shafts and knuckles along with most of the mounting bolts! I pulled two more bolts, cut the rubber brake lines and dragged the thing out for $62 CAD! The 4 control arms I took with it cost more ($35 each).
If you have a local yard and you're not in the rust belt I think it is at least worth looking, you might get lucky and someone else has done a lot of the work for you. If you want a diff as well then that makes the u-pull even more appealing.
Some tips after struggling the last 9 days on this. The brake lines on the car are aluminum (from what I can tell) coated in plastic. They are a tight fit in the fittings so if you are planning on replacing your soft lines between the car and the subframe (now is the time) be careful loosening the fittings. Mine looked great but the plastic had swollen just a bit and when I loosened them off they twisted the line off too. Flaring these were not fun as the aluminum lines are a slightly larger diameter so it was a fight getting them to work with my flare tool. What worked on the front of the car when I replaced those lines was some heat from a Mapp gas torch just enough to get that plastic melted a bit and then the flare nuts turned fine. Leave the soft brake lines attached to the car and then reattach to the subframe side when you put it back in (there is very little access to the car side with the subframe in place). Hand brake lines are very easy to pull from the interior side once you have the centre console out, you won't need to touch the brake end or monkey with the adjustments afterwards this way. I found the rear sway bar links once attached were in the way of torqueing one of the control arms on the knuckle side so leave them out until that is done.
It wasn't a fun job but now that I have done it I know I could do it a lot faster with less struggles.
If you really feel like spending there are all those fancy aluminum rear control arms from an M3 you can throw in there too.
Good luck!
In reply to adam525i :
Thank you for the insight and tips! Although I have to ask: 9 days?!? Great Scott! I feel like I might have underestimated the labor involved in this job...
At this point, I'm strongly leaning towards picking up a new-to-me subframe from a junkyard if I can find one in good condition. My current subframe might be in perfectly serviceable condition, but I was thinking: if I'm going through the trouble of removing it, I might as well replace all the bushings- to include the ball joints, trailing arm bushings and control arm bushings with the stiffer M pieces. If I'm going to do that, I have to remove all of the control arms anyway, which kind of makes it a moot point. I would rather replace all the bushings slowly in my free time vs having the car sit on jack stands for days at a time and swap everything in as a whole.
I actually already have the fancy M3 rear wishbone arms on the rear of the car, as well as the fancy M3 pieces up front. I've been trying to document all the things in my build thread, if for no other reason than it helps me remember what in the world I've been doing!
https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/e82-128i-when-your-projected-track-car-turns-into-your-daily/258441/page1/
I guess it was 9 days of working on the car, 5 of those were evenings and I did a lot of other stuff at the same time. I was also fighting a car that's lived it's 330,000 km in Ontario driving through the winter. You'll be fine, nothing will go wrong lol
Theruleslawyer makes a good point about simple measurements that can at least get a rough measure of squareness on your current rear subframe.
On the one hand, you don't actually have to pull the rear subframe to install 2-piece subframe bushings, just drop it a bit. I did the UHMW ones, and was able to do the install without even removing the brake lines by turning just one (driver front IIRC) of them into a 3-piece bushing. Renting, borrowing, or building a bushing removal tool is highly recommended. People claim to be able to put something between the car and bushing and use the weight of the car to push it out. Perhaps I was doing something 'wrong', but I reached my sketchiness threshold before there was any movement. Luckily I found somebody local that had built the DIY tool, and was able to just borrow it.
On the other hand, if you have the space, money, and inclination, there is something to be said for your existing plan even if your subframe is perfectly straight. Pull the complete rear end (not just subframe) from an auto trans car with the 3.73 or 3.91 diff, replace (spherical?) all the bushings, LSD the diff, and have a fully-built direct-swap unit that's ready to bolt in whenever you are. Working at your leisure while also minimizing down time is a valuable combo. You might also be able to find a deal on one of these by keeping an eye out for locals parting theirs out.
I finally stopped procrastinating today, pulled the 128i into the garage and started measuring things. Cliff notes: I believe most of you are right and the stock rear subframe is just fine.
I didn't bother removing the aftermarket adjustable toe arms, I just tried to measure from the center of the bolt eye-hole on both ends. Sure, there could be some flex in the bushings, but the results speak for themselves: both sides were pretty much identical to the OEM toe arm I had sitting in my garage (right around 16 & 9/16th's of an inch). I tried to crudely measure as best as I could, but they all seemed to be within about a 1/16th of an inch of each other.
Comparison of the OEM toe arm to the aftermarket adjustable arm:
I then tried to cross measure the subframe itself as well. I used the furthest holes in the subframe on each of the 4 "arms". I'm sure it's not terribly precise, as I was laying underneath the car, using cardboard to shove into the holes (so they would extend down far enough for me to measure), but I measured both sides a few times while trying to get the cardboard as straight as possible and both sides appeared to have the same cross measurements, or at least close enough that I couldn't see a difference.
The previous theory that the car was likely lowered in the past: I believe that theory is likely correct. When I first got the car and threw it on an alignment rack (back around April 2023), even the front alignment/toe was WAY off.
So I suppose my best guess is that someone previously had coilovers or lowering springs on the car and removed them at some point without bothering to re-adjust the toe arms. Given that the photos from the accident make it appear as though it likely slid into a wall, well, I'm going to make some assumptions...
I have seen the subframe tab with the toe link eccentric bend from a curb swipe on many e8x & e9x cars. Normally a few hits with a hammer is enough to bring them back into spec
In reply to Caperix :
In comparison to the other side, the toe link tab looked the same- I didn't notice any bends or deformities. Given that the length of the adjustable toe links are pretty much identical to the OEM toe link and the rear toe is in spec, I think it's probably good to go.
From someone who did a lot of "what is bent?" analysis, measuring points and comparing one side to the other is simple. Bent parts usually have the paint flaked/cracked at the bend, or flaked/cracked surface rust.
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