Rear Subframe to Body Connector using BMW e90 Parts

[DPDISXR4Ti]

I was working on my neighbor's 3-series and noticed the control arm between the rear suspension sub-frame and body. While the Bimmer rear suspension is more sophisticated than ours, at it's heart, it's similar. Wondered if a similar part could be fabbed, if not use this exact part on our cars. If someone wants to beat me at removing one from a junkyard e90 BMW, all it take to remove is a E18 External Torx socket and a conventional 16mm hex.

A couple pics of the part in question...

BMWe90ControlArm.JPG (969.74 KiB) Viewed 3023 times

BMWe90ControlArm-3.JPG (46.58 KiB) Viewed 3020 times

[DPDISXR4Ti]

Had a little time left before closing at the junkyard the other day, and with a 2008 3-series sitting in the air, I felt compelled to grab these....

BMWe90Arms-1.JPG (1.02 MiB) Viewed 2476 times

BMWe90Arms-2.JPG (994.93 KiB) Viewed 2476 times

No further progress to report for now.

[john keefe]

It looks like that connector is to stabilize the main bolt which attaches the subframe to the body/frame? Which makes sense because the "hub" of the subframe cross-member is very tall, and the torque applied to it under hard cornering would surely allow some wobble w/o that secondary bridge to another part of the frame. Or, am I missing something in the pics?

Just curious, has anyone tried transplanting one of the multi-link Series 3 rear suspensions into an XR? I know Nigel @ Draken swapped in a complete Supra rear suspension. Although he had some fabrication to make it work, dimensions et al from his photos looked like it was a reasonably close fit to start with.

[john keefe]

Just curious how you are envisioning this to be mounted. Although its not really a triangulation, the stock set up has the main mounting bolt as one point, and the forward-mounted plate-to-frame bracket as the second point. Not great, but stable enough, though I can see the subframe tube being able to flex a bit under hard loading. I think you'd need a much longer version of the BMW arm to attach inboard on the tube, and further forward on the frame (than where the plate bolts in) to get a true triangulation.

[DPDISXR4Ti]

Just curious how you are envisioning this to be mounted.

I've done nothing in the way of even hand-holding this up against the body at this point. That said, my envisioning would be similar to how BMW used it, from the single-bolt beam mount to the "frame rail" built into the floor. If you look at the Scorpio, a similar part already exists there. Offhand, I can't recall if it's welded or bolted in.

To use this part on the XR in that manner, you'd likely need to flush cut that nipple off that goes into the beam mount on the Bimmer. The XR beam mount doesn't have a recess like on the BMW. For mock-up, a giant fender washer could be used since you'd create a hole about 1.5" from the flush-cut. For the two forward-mounted locations, a SS rivnut would need to be sunk into the frame rail since those are blind (no access).


In summary, it may or may not work at all.

[john keefe]

I kinda' like the idea. Rather than flush-cut the nipple and have to bolt/screw into the subframe beam and the bottom face of the body frame rail, maybe rotating the unit 90deg., and mounting it to the side of the frame rail (if accessible) instead of the bottom plane. Cut out the nipple-end to match the radius of the XR beam so it looks like a large C. Bend the smaller end a bit so it can mount to the side of the frame rail at an angle. Mock fit, then install your SS inserts into the side of the frame rail, bolt that end in, and then weld the large radius end into the rear beam.

[my8950]

I've been looking at this recently, but I'm also easily confused.
Are you guys trying to fit the entire rear beam from the BMW into the XR?

[john keefe]

my8950, just figuring out a way to use that smaller stabilizing beam with the stock subframe beam for more strength, resistance to twisting (or, that's what I think Brad was getting at...). I think BMW really needed to have that additional beam because of how they chose to bolt the beam to the body. It kinda' serves the same purpose as the small plate the XR uses, but where the XR's beam end was flattened for a better bolt-in, the BMW terminates in the vertical "tube" and long, exposed bolt in shear, which allows for more twisting. Plus it's got a rubber insert, so the whole thing can float around and/or twist the bolt unless they added that mini-beam to stabilize the bolt to the frame.

I think the XR design is inherently stronger, but with only three points to mount the whole rear subframe assembly onto the body (i.e., two ends of the beam plus the differential mount), adding another, light element certainly can't hurt.

[my8950]

my8950, just figuring out a way to use that smaller stabilizing beam with the stock subframe beam for more strength, resistance to twisting (or, that's what I think Brad was getting at...).

Ok, makes a little more sense now, thanks for the explanation.
I am not familiar with the BMW so I have no idea.

[john keefe]

You can see the setup in Brad's first photo... the vertical "tube" is the end of the BMW subframe that bolts to the body. Since that's a long bolt in a rubber dampening core, lot's of movement under load. So, they had to tie the "tall" end of the bolt to the body with the mini-beam to stabilize it.

[DPDISXR4Ti]

What John said.

I'll report back when I make any progress; even if "progress" is just some imagineering hanging out under the car with these parts in-hand.

[john keefe]

I'm really curious if turning it on its side and welding to the subframe tubing will work. Looking at the pics again, seems now it might be a touch short to get a significant triangulation effect. Almost more like a gusset. But, gotta' be worth something.