October 30, 2002

Building a new rear suspension cross member

The stock rear suspension cross member weighs 24.5 pounds. Four bolts through rubber bushing locate the cross member to the chassis. In this picture the stock rubber bushings that locate the cross member have already been replaced with solid metal bushings. Blue arrows indicate these points.

The cross member supports the rear differential on two rubber bushings (white arrows indicate the differential holes)

Stock rear crossmember with stock rubber bushings replaced with solid bushings

and locates the ends of four suspension arms.

rear crossmember with lower suspension arms
There are three changes we would like to make to the rear cross member: First, we would like to remove the suspension slop.
Second, we would like to make the cross member lighter.
Third, since we are lowering the car a great deal, we would like to raise the differential to keep the half shaft angle small for efficiency.

This would involve cutting most of the top (shaded area) of the cross member off to fit the diff.

Crossmember marked to be cut to fit raised differential
While this would certainly make the cross member lighter, it also would make it quite a bit weaker. We decide we will end up with a stronger and lighter cross member by building a new one from scratch.
The first step is to build a jig from the old cross member This is a simple matter of bolting on bits of angle iron to the points we wish to duplicate and then welding the angle iron together in a way that will allow us to remove the old cross member The jig
We take some previously fabricated bushings (left) and use a one inch hole saw to help get the tubing to fit nicely. bushing (left) and tubing drilled to fit (right)

We bolt the bushing to the jig and weld a bar in place.

rear bar in place
We remove the rear bar from the jig and finish the weld. The more of the weld that can be done in the jig, the more chance it will fit back in the jig.
The next step is to bolt the rear bar back into the car and try and decide just where we want the differential. This involves much checking that the diff is square to the bar and the centerline of the car and guessing the half shaft angle at ride height. Amazingly the diff's studs end up in the middle of the bar. It is a fairly simple matter to drill holes through the back bar and then tack weld bushings in the holes held in place by the diff.

We remove the rear bar from the car and finish the welds for the diff holes (marked with white arrows). Having had a close look at how the differential will have to sit, we know we need to bend the front cross member bar to clear the diff. We put a nice bend in each end and fit it to it's own bushings (front bar highlighted in yellow). Four cross tubes add stiffness and strength.

top of rear crossmember still in the jig (white arrows mark differential holes, yellow highlights the front bar)
Now we need brackets to hold the lower suspension arms. We are replacing the old lower suspension arms. The new ones are lighter, adjustable in length, and use rods ends instead of rubber bushings. rubber bushings vs. adjustable rod ends
We make the brackets out of rectangular tubing. The brackets are drilled then cleaned up using a grinding wheel. cutting out a bracket
The brackets are bolted into the jig and then pipes are fitted to support the brackets.
Green arrows point to the brackets. brackets bolted in the jig (green arrow to brackets)
After much cutting and fitting and welding, the new piece is done. It is 15.6 lbs or 8.8 lbs lighter than the original. It is also much stiffer.
Green arrows point to the suspension brackets. White arrows mark the holes for the differential. Blue arrows mark the bushings that bolt to the car frame. The new crossmember
When it is all assembled, it looks like this. Rear crossmember in place

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