E30 trailing arms are known for bending on cars that are tracked or rallied. Last rally season we bent three control arms. We ran the car quickly, but had not major impacts that I felt deserved bent parts. SnoDrift was the first rally for us in 2016. Snow rallies often necessitate the use of snow banks to keep your car on the road. To prepare for this event we analyzed our previously bent arms and set out to engineer/fabricate reinforcement that will prevent this from happening in the future. Our solution is a mixture of common fixes that are used in the community, resulting in effective BMW E30 trailing arm reinforcement.
Arms typically bend from one of two forces. A load caused from a side impact (like sliding into a berm or curb) is most common. This can happen on street cars and track cars alike. The second failure mode results from a large vertical force when a vehicle has high spring rates and sees excessive road force (ie a heavy landing from jumping). Both types of bent arms result in a rear wheel that is obviously cambered-in (negative). The extra camber combined with E30 rear geometry that tilts a lot during travel will result in tires wearing on the inside very quickly when driven hard. RCR did the following to reinforce our arms. After two days of snow banks and jumps at Snodrift trailing arms are straight.
We opted for a combination of several methods used to strengthen the arms. A thin walled tube was added to the front section of the arms near the mounting points to avoid the hub from being used as a bending fulcrum with heavy horizontal blows. This is similar to the reinforcement that comes factory on M-Coupes and is a common upgrade. Our arms had this modification and were bending torsionally so this modification is obviously not enough for rally. To combat torsional twist we boxed in the body of the arm. Thin sheet metal (16-18 ga) can be used for this, just be carefully to align the filler pieces so they are on plate with the top and bottom of the arms. We solid welded the top to keep water out, but only stitch welded the bottom. This will allow for drainage if needed.
Another common reinforcement point is the rear shock mount. The stock location of the mount is in single shear and is certainly not ideal. We have developed a dual shear mount that we run with our coil over setup that uses the brake caliper mounting points as additional attachment points. This event we ran soft suspension so it was not installed. The reinforcement shown is not to help with shock loads, but instead to combat side impacts from lateral hits (in fact these gussets do very little to help the shock mount out). This addition IS very effective in resisting impact bending. Combined with our custom Bilstein coilover mounts these should be quite strong.
Powerflex polyurethane bushings will finish off the job. Our rear trailing arms our now properly reinforced. We have also added weld nuts and a steel pencil rod loop for attaching HDPE armor sheathing with zip ties. This will be installed prior to next gravel event to have added protection for the control arms. We may improve on this design in the future, but feel it is a good first step based on real world parts that have failed in rally use. Next up for the car will be front control arm reinforcement. Look for an article on that in the near future!
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