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.

Corvette Transaxle

In keeping with the low buck nature of this project, I never intended to use an XTrac 195 gearbox, which was the original transaxle when the car was raced. The car came with an XTrac case, but it was empty inside, no gears, nothing. For years I've been trying to figure out what to do. I considered other racing transaxles, but there was nothing cheap that could handle the torque of an LS Chevy.

I then went on a years long Internet journey to find a street car transaxle that would work. It would have to physically fit within the car, be readily available and be able to handle a lot of power of a torquey V8. The short version of the story is that I settled on a C5 Corvette transaxle. Those in the know will point out that it is not a real transaxle per se, it's really a regular Borg-Warner/Tremec T56 gearbox with a special tail housing that lets it bolt directly to a differential (see Figure 1.). It looks like it is going to be a viable solution as it fits my criteria with only a couple of adjustments to make. So full of optimism, I bought a used Corvette T56 off of the Internet and a differential from LKQ.

Figure 1: Corvette C5 transaxle. Notice the shift linkage coming out the front of the gearbox above the input shaft. Image courtesy of Oards Automotive Hub.

The Corvette transaxle is not a bolt-in solution though. As the Corvette is a front engine car, the front of the T56 is not set up to bolt directly to the engine, it's configured to accept a torque tube. And because the driver is so far forward, there is a specific shift linkage that comes out the front of the gearbox, through the area where the clutch normally sits (the Corvette clutch is up front attached to the flywheel), and runs back up the torque tube to get to the driver. I have already started attacking the first hurdle with the front of the trans. This is actually a pretty easy fix because there is so much interchangeability with the different versions of the T56. In this case, you can replace the front plate on the Corvette trans with the front plate from a Camaro. Then you just need a regular bellhousing to connect everything to the engine (see Figure 2).

Figure 2: Transaxle mated to the engine.

I haven't started working on the shift linkage problem yet, but I have a solution in mind using parts from the Cadillac CTS-V configuration of the T56. I'll talk more about that when it's closer to actually happening.

So there is everything bolted together for the first time. The whole contraption looked really long when I was putting it together, but as you can see in the background, the Corvette trans is actually much shorter than the original XTrac gearbox (see Figure 3). And importantly, the half shaft location is within an inch of the original location.

Figure 3: Corvette transaxle (foreground) vs. XTrac 195 transaxle (background). 

Now for the elephant in the room... how the heck am I going to attach the rear suspension?

Almost two years?

I’ll blame the lack of progress on COVID. And I’ve been working full-time while going to community college to get a degree in AutoCAD. More on that later…

I have actually managed to get some work done during this time. The most visible change is the removal of almost all of the old decals and helicopter tape. What a mess! The car hasn’t been changed in twenty years so some of that decal glue had plenty of time to harden. A lot of the decals came off with just a little heat, but others took Goo Gone and/or acetone and a lot of rubbing. But the decals were nothing compared to the helicopter tape! And there was a ton of it.

The race teams use helicopter tape on all of the body panel joints to streamline the car and reduce turbulence. But these guys also used it as a clear bra to protect the bodywork. It was everywhere: all over the nose cone, the leading edges of the wings, on the sidepods, even on the leading edges of the A-arms. And it would not peel off because the plastic tape part had completely broken down. By the time I applied enough heat to the loosen up the adhesive, the plastic tape would tear and melt. So what followed were uncountable hours with the heat gun trying to find just the right temperature to melt the adhesive and scrape it away without getting everything too hot and damaging the paint and carbon fiber underneath. Then more Goo Gone, acetone and rubbing. But I did find a trick, a 3M Eraser Wheel. It's a great big eraser on a shaft that you put in your drill to remove some of the residual glue. They say that it will work on the decals themselves, but that didn’t work for me. I removed as much of the decal and glue that I could with heat and then I went back with the eraser wheel to knock off a lot of the remaining adhesive. It really reduced the amount of final cleaning with chemicals, so I recommend it if you ever have a bunch of adhesive to remove.