Rally: the original motorsport
Photo Courtesy of Tim Winker
Photo Courtesy of Warwick Patterson
unknown photographer
Photo Courtesy of plaftaphoto
Boxed Lower Control Arms

You've seen or heard about the lower control arm failure on the classic 900. Here's what the rally guys do to prevent this. The application is not just for rallying though: Track guys report that turn-in is sharper and more controlled as there is less flex in the arms after this mod. In higher mileage 900's that see heavy spririted driving this would be a good preventative maintenance idea too.
The trick is to use thin metal for the main plates. About 1/16th or so. You do not need thick heavy material here. The strength comes from the 'boxing' of the triangle, not the thickness of the material used. The plate allows the loads to distribute evenly through the arm and resists flexing under loads. This diagram shows where to add plating and where we also strengthen the existing welds.

Do the 2 long welds first and then use a hammer to bang the plate down to the axis rod before welding there as well. It is a good idea to bolt in an old ball joint to minimize warping of the arm during welding and cooling. We plate both above and below. If you want to get fancy you can take a hole-saw and place some strategic holes through both plates in the middle of the triangle. When you do this make sure to hole both the upper and lower skin to allow drainage. If you want to seal the holes you can sleeve them to add back some stiffness. We did not bother to do this. After that it's a matter of laying weld down to reinforce the factory weld spots and joins.

Sometimes you will run into clearance problems between the upper skin and the lower loop of the driveshaft tunnel. If this occurs you should hole the skins as previously mentioned!

As always, click on the images for larger versions.
Solid Plate:

Holed Plate:

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