Bellcrank bushing (left) and fore chassis billet upright weld joint (right)
There has been some speculation regarding two potential weak points in the rear chassis; the aluminum bellcrank bushings and the weld joints between the 2” x 2” tube that supports the bushing and fore chassis billet upright. Endurance cars provide useful data points because they are subjected to more abuse than most cars, certainly mine, will ever see. After 180+ hours of intense racing (big slicks, large downforce, 1,200+ pound springs, being rubbed by many cars, off-track excursions, etc.) a routine inspection of one such car noted stress cracks at the aforementioned weld joints. The fix requires grinding and rewelding the joints and adding gussets. The good news is that the rocker bushings had no apparent issues.
There are two straightforward ways to mitigate potential issues in this area; brackets and gussets. Several cars, including the endurance one mentioned above, feature brackets that place the bellcranks in double shear. Whether the rocker bushings would have experienced wear without the brackets isn’t clear, but I fabricated a set in a previous post. I had been planning on adding gussets for some time and now that I’m about to modify a bunch of chassis tubes to fit my exhaust it made sense to do it now.
Temporary spacer replacing bellcrank, gusset tacked into place and preheating with torch
Welding a thick aluminum gusset to the 2” x 2” billet upright generates a lot heat which will melt nylocs, cook bearings and has the potential to warp the chassis. The first step was to remove the engine and everything that’s heat sensitive from the engine compartment. Once that was done, I reinstalled the rear chassis brace and rear roll cage legs with plain nuts (nylocs would have melted). Because the bellcranks have bearings I couldn’t leave them in place so temporary spacers were fabricated to allow the bellcrank support brackets to be torqued. The large hunks of aluminum also act as a heat sink.
Once everything was torqued, a temporary frame was fabricated to further lock everything in place. Four 1/4” thick steel plates were fabricated and bolted to the suspension suspension mounting points and 1” square steel tubing was used to connect them.
Temporary steel brace tying the fore chassis billet uprights together
I fabricated the gussets from 1/2” 6061. The hole reduces weight and the amount of heat required to weld it in place. I considered making the gussets larger, but Abe didn’t want to put that much heat that close to the bushings. The billet uprights and gussets were preheated with a propane torch. The amount of heat dumped into the chassis was impressive… even the aft billet upright got hot!