Rear Trailing Arm Design

Pictured are the parts that have been ordered. 1/4" steel plating, 5/8" female reverse thread heim joint with a steel double adjuster, and a 5/8" tube adapter, and the spherical bearing assembly. 

First step was to build a simple jig of the BMW trailing arm. What's important to locate here is the trailing arm bushing mounts. I bolted on some plates out of scrap metal, to be welded on to another plate. With these in place, I was able to take off the trailing arm and I will know exactly where I need to mount the heim joint and the spherical bearing. 

Next step was building the heim joint assembly. Beforehand, I marked the center of the plates, and drew a circle the same diameter of the smaller end of the tube adapter. Next step was to weld the tube adapter in place. After welding the tube adapters, I was able to bolt on the double adjuster and the locking nuts, and then the heim joints.

This step is tricky, and it is worth it to take your time on this one. Next was to cut the outboard arm eyeball, but be careful to only cut the inner diameter, and leave the outer diameter intact. After grinding and cutting and shaving the inside to expose the inner diameter, it was a matter of prying it out. After the eyeball was removed, the spherical bearing will go in its place.

Next step was to weld the spherical bushing in its place. It is important to note that you want the nut facing inboard, as you would not be able to take it out and replace the bearing inside without pulling the trailing arm otherwise. You may need to do some shaving to the outer diameter of the mount to have a better countour to the spherical bearing, which allows for a better weld. After this, you are now done with the inboard arm (camber side) of the trailing arms.

Next step is to weld the heim joint assembly into the outboard arm (toe side) of the trailing arm. It is very important that you take the proper measurements and cut slightly shorter and trim down as necessary. Using the jig, you are easily able to locate where you need to make your cut and have the heim joint assembly in to where you don't alter the factory BMW suspension geometry. After figuring out where you want to cut, it is a matter of cutting straight through, and welding the heim joint assembly in. It is also important to weld in some reinforcement plates and this will be your weakest point of the setup.

Your trailing arm setup is now complete. Here's how it looks after completion. Not pictured, you will need to make some spacers to go on the sides of your heim joint when it bolts on to the subframe. I used some old bushing inserts and cut them to size and they worked perfectly. After putting the arms back in the car, you are now able to adjust your toe on the fly. You adjust your toe by taking an adjustable wrench, and turning the middle nut insert. As you extend the threaded rod, the arm will push itself out, creating your toe out. If you want toe in, retract the threaded rod, and the arm will pull itself in. The reason you want a spherical bearing on the camber side is because as you adjust toe, the trailing arm rotates, and rubber or poly bushings will bind itself after even the slightest rotation. 

Pictured here is the trailing arms in action. This design presented itself with an issue, as you extend your threaded rod out to bring toe out, your caster will be adjusted at the same time. In my second version of this design, I will shorten the length of the inboard arm (camber side) to correct the caster issue. Since the left trailing arm was bent before doing this design, you will run into unique issues with alignment. The right trailing arm was not bent, and the alignment is perfect on that side. Some adjusting may be necessary according to your preferences and situations. These arms held itself up very well, withstanding an entire day worth of drifting, and driving home afterwards with no issues. After some tweaks to the design, this will be much more refined, and will allow more opportunity for more refined suspension setup on an otherwise non-adjustable rear end.