The Stress of Stress-Mounting

One of the most elegant things about modern formula cars is that they don't have frames. The front suspension is attached to the monocoque, the rear suspension is attached to the transaxle and the engine block acts as a structural member tying both ends of the car together in an arrangement called "Stress Mounting". It makes the car lighter, narrower and really easy to work on. Now that's all well and good when the engine and gearbox are designed by really smart guys to carry the anticipated loads, but what happens 20 years later when some yahoo decides to replace the original transaxle with one from a Corvette that doesn't have the necessary attachment points nor the structural rigidity to carry the stresses?

So that is the problem du jour. Or more like du année. I'm going to need a rear subframe.

Fortunately for me, this has been done before so I know it will work and I can get inspirations from other guy's ideas.

Figure 1: Tubular rear subframe. Supports rear suspension loads instead of  the transaxle. Image courtesy of blu808.

For my project, I just needed to figure out the locations of all of the rear suspension mounting points from the existing transaxle and come up with a design that reproduced those locations in 3D space. In an earlier post I mentioned that I had been taking AutoCAD classes and I was able to get approval to use this exercise as my Capstone project. So to get all of the necessary measurements I dug out several tape measures and rulers, straight edges, triangles, a plumb bob, my digital caliper, a tilt meter and I don't remember what all else to reverse engineer both the old and the new transaxle. It would have been nice to have a 3D scanner instead, but I got it done. 

The following series of figures show my design process in coming up with an initial design. I'm sure that there are going to be many modifications as the project evolves through the fabrication phase, but the exercise showed that there weren't going to be any major interference/clearance issues between the transaxle and the rear suspension.

Figure 2(a): I started by building the major components in AutoDesk Inventor. The models were built using the necessary precision to check for interference.

Figure 2(b): Once the original XTrac transaxle was constructed, I isolated the positions of the sixteen required mounting surfaces. 

Figure 2(c): Upper image shows the sixteen mounting surfaces projected in 3D space around the Corvette transaxle. This confirmed that there were no major interference issues. Lower figure is a draft version of the rear subframe around the Corvette transaxle. 

Figure 2(d): Rear subframe. Design uses only square tubes and sheet material to simplify fabrication.

 

Now the project is getting real. I'm hoping that after the holidays I can get enough of the fabrication started to make the car a roller again. Then I can refit the bodywork to make sure that I don't have any clearance issues there and then nail down the final design.