How to Build a New Unibody

Fabricating a Subframe from Steel Tubing

Copyright 2008 by Morris Rosenthal All Rights Reserved

The Omni Project

If It Jams Home

I should start with the warning that you can't rebuild a unibody to manufacturer specs unless you have a terrific shop and skills. This rebuilt unibody would certainly perform differently in an accident. Our new subframe member is the longest piece we need to build, which runs from back of the driver side floor pan up to the front-end crossmember. The first challenge for fabricating the piece is getting the angles right at the joining surfaces. The floor bumps down behind the driver seat, which is where the original subframe was welded up. I plan to attach the new subframe with a bolt through the formed structural member that runs across the passenger cabin right under the front seats, so I need to cut that angle on the piece, or shorten it. The finished end is shown to the right, I'll make up and leftover space between the edges and the floor with Bondo.
The more challenging fabrication is where the subframe intersects an old repair at the front of the car, just past the crossmember. The repair was a large piece of angle iron, about 3" on side, which was welded into place by a friend seven or eight years ago, when there was still a subframe to weld to. I cut the angle into the piece at the left, and then bent the flap over, as shown in one of the later videos. While welding the flap closed would have helped solidify the piece, I decided against welding anything for the time being, and later cut the flap off in an attempt to fit the piece better to the old subframe. It's not really possible to build a new unibody unless you start from scratch, I'm just trying to patch up the old one.
The picture to the right shows our new subframe member installed, though not bolted up yet. You can see the crossmember (above the black jack) to the right, the upper surface of which is about an inch higher than our new subframe element. As I type this, I realized that if I'd turned the steel tube on end, I could get that inch without having to do anything else, though I might have to cut a slot in the side to accommodate the remaining subframe over the stub frame. It would in the subframe running an inch lower when it get back to the floor, which wouldn't be great. Another option would be to cut a two inch wide slot in the end of our newly fabricated piece and bold in a a few inches of the tube turned on its side. The videos below show a few steps in the rebuilding process, nothing too fancy.

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