The Rear Subframe is Coming To Life

It's been three years since the last post, but there has been progress so let me try to remember and document it here. Reviewing my own last post, I had come up with a CAD model for a rear subframe that would carry the rear suspension and tie it forward to the engine and the tub.

That meant that it was time to cut metal and start welding. What you need to know now is that I've never really welded before. But in the whole spirit of this project, that didn't stop me for a second. I had taken three welding classes from various places around town and I got good enough that I could get two pieces of metal to stick together. My thinking was that I was good enough to tack weld everything together and then when I was finished, I'll find a real welder to do the final welds. With that plan, I bought myself a decent Hobart MIG welder and an Evolution Metal Cutting circular saw. They have both been great and I highly recommend them, especially the Evolution saw.

I apologize that I didn't take any pictures during the fabrication process. I guess that I was so focused on what I was doing that I just didn't think about it. In any case, it really was tedious. The cutting and welding was pretty straight forward. Most of the effort and the difficulty was making sure that everything was level or plumb or parallel or perpendicular depending on what I was doing. I was able to make it work by just taking my time and measuring everything fanatically. That and lots of levels and triangle squares and plumb bobs.

I want to mention here that there were some metal pieces that I had designed that were going to be too difficult for me to cut accurately on my own, so that's when I relied on a company called Send Cut Send. If you haven't heard about Send Cut Send, they are an online fabrication service. The way it works is that you send them your digital design file and they will CNC cut it out of whatever material you want and then send the part to you. It only takes a couple of days, it isn't much more expensive than just buying the material and if you're willing to pay a little more, then they'll do the work in the US, which I always do.

So here's where we're at as of this week. I've got about 60% of the frame done and that's enough to reattach the rear suspension. And I got the engine front mounting plate cut (by Send Cut Send) and installed in place of the test spacers that I had been using.

Figure 1: Rear subframe so-far.

Figure 2: Engine front mounting plate.

Figure 3: Everything connected to the tub.

This is a fairly major milestone because the car is back together enough that I can reattach the bodywork. That had to happen before I could finish the frame because I didn't have an accurate 3D model of the bodywork when I was designing the frame. Because of that, I'm pretty sure that I've designed in interference in a couple of places down low.

Figure 4: The floor waiting for installation.

The next step is going to be the installation of the aerodynamic undertray a.k.a. the floor. With the floor in place, I can check my lower frame rails for clearance and make any necessary adjustments.

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.

The Engine Fits Under the Bodywork!

This weekend’s project was to re-attach the floor so that I can take measurements under the engine and bellhousing.  I mounted the engine as high as possible while still fitting under the existing bodywork to give me the most possible ground clearance.  That is the opposite of what you normally want to do in a race car where you’re trying to lower the center of gravity, but that’s not my primary concern at this point.  The current plan is to keep costs down by using a wet sump oil system (rather than an expensive and cumbersome dry sump system) and a Corvette C5 transaxle (rather than a big-buck racing transaxle) while not modifying the upper bodywork and minimizing the amount of cutting to the floor.

Figure 1: The floor is reattached.

With the floor in place, I was hoping to be able to measure under the block for the oil pan.  One nice thing about a mid-engine layout is that there isn’t a crossmember to deal with so I have a lot of freedom with regards to the sump design… front, rear, whatever.  But I ran into a snag in that my prototype wooden front engine plate has started to sag.  There’s a noticeable deflection between the back of the tub and the front of the engine block that is causing the floor to not fit properly.  So I’m going to have to redo at least a part of that and get everything lined up before I can start taking the final measurements and picking an oil pan.

Figure 2: Long block in place without an oil pan.  If I did everything correctly, there should be about 5.5" clearance at the front of the engine and a little less in back.  I need to solve the aforementioned mounting problem to get the final numbers.

Just for fun, I went ahead and set the rest of the bodywork in place to check clearances on top of the engine.  I’m withholding final judgement until I get the front engine mounts straitened out, but so far everything looks OK.  The truck intake manifold is definitely going to need to be replaced by a lower car manifold and there might be some interference with the oil filler on top of the valve cover.  I was already planning on relocating the ignition coils to the side of the engine so I’ll probably look for a set of nice smooth topped aftermarket valve covers.

Figure 3: Bodywork sitting roughly in-place.  The engine basically fits so I don't see any showstoppers as of yet.

The (non) structural integrity of plywood

So it turns out that I may have overestimated the structural integrity of plywood.

I made the prototype engine mounting plates out of plywood because it was cheap and easy to cut.  And it was good for test fitting and verifying my dimensions, but it was not strong enough to actually hold up the engine.  Once I got everything bolted together and tried to jack up the car to remove the dolly from under the engine, the two piece mid-plate between the engine and bellhousing creaked and flexed in a major way.  It was not going to hold.

So then I had to come up with a Plan B.  Eventually that materialized as two 3" x 1.25" x 24" aluminum bars.  One bar runs horizontally across the top of the bellhousing where it bolts to the top center hole in the block and the top hole on each side of the bellhousing.  The second bar also runs horizontally and bolts to the two lower holes on the back of the engine and the middle holes on each side of the bellhousing.  With those bars in place and the wooden front plate still bolted to the tub, I put a jack under the transaxle case and started jacking up the back of the car.  There were a couple of groans from the front plate at first, but it held and I was able to jack the car up high enough to get the engine dolly out.  Success!  The two halves of the car were joined again and it would roll.  

Figure 1: Engine-to-bellhousing mating with 1.25" thick aluminum bars.  

It was a nice day so I rolled the car out and washed it.

Here is a close up of the engine, bellhousing and the rear of the car.

Figure 2: Engine, bellhousing and transaxle.  Notice how there is no rear subframe, the rear suspension bolts directly to the bellhousing and the transaxle case.

Now that that's sorted, it is time to reattach the bodywork to measure how much clearance there is for all of the peripheral systems and probably a different transaxle.

Engine mounting plates are underway

Mounting the Chevrolet LS street engine in the car is going to be a challenge because the original race engine was stress mounted, that’s what formula car designers say when they mean that there isn’t any frame between the front of the car and the rear suspension.  The engine block itself acts as the structural member and carries all of the static and dynamic loads.  The front of the engine bolts to the back of the tub and then comes the bellhousing and the transaxle.  The rear suspension then bolts to the sides and top of the bellhousing and transaxle.  It’s all very compact and elegant, but not really re-producible with the street car components that I’m trying to use.

I’m not even sure if an LS block is strong enough for stress mounting, but I know it’s been done in other Indy cars while using a lower supporting brace so that’s what I’m going to do.  So at this point, I am fabricating drag race style front and mid plates to 1) bolt the front of the block to the back of the tub and 2) bolt the rear of the engine to the bellhousing.  I’ll figure out something for the rear suspension subframe after I figure out what transaxle I’m going to use.

I started with the engine-to-bellhousing mid plate because it was easier.  It was easier because there are several good dimensional drawings of the back of an LS engine block out there on the Internet and these drawings gave me the positions of the six mounting bolt holes.

Figure 1: I found this on the Internet a couple of months ago, but I forgot where, so I'm sorry that I can't acknowledge the original author. 

Then it was a matter of determining the positions of the six mounting holes for the bellhousing.

Figure 2:

This is when I learned about transfer punches. 

Figure 3: I bought a set of these from Harbor Freight.

I figured out pretty quickly that I was never going to get good measurements of the bellhousing bolt locations by trying to measure with a tape measure from the center of one hole to the center of another hole.  Luckily a little bit of web surfing turned up information on how to use transfer punches on drilled holes and I was able to make a template of the bellhousing on a piece of scrap plywood.  Then I combined the six hole locations for the back of the block with the six holes for the bellhousing on my mid plate prototype (a ¾” piece of plywood sandwiched together with a piece of ½” MDF because I couldn’t find any material that was the needed 1 ¼" thick).  With that done, I was able to marry the engine to the bellhousing.

Figure 4: Mid plate installed.

At this point I should say something about the thickness of the two plates.  From the best that I can tell with a hand tape measure, I have approximately 22 ½" from the back of the tub to the front of the bellhousing.  I measured my LS block at 20 ½" from the front to back mounting faces so that means that I have two inches to work with (That’s what she said!).  I could have just made each plate one inch thick, but I decided to make the front plate ¾ of an inch thick in order to give myself the chance to use a stock mechanical water pump.  This is an LS engine specific topic that will come up in the future.

The front plate to join the engine to the tub was a littler more work.  Unbelievable to me, I couldn’t find a dimensional drawing of the front of an LS block anywhere on the Internet.  But once again a little Internet searching led to the discovery of threaded hole punches.  Like the transfer punches, they help you locate the center of a hole, but these are little inserts with spikes right at the center of the hole that screw into threaded holes.

Figure 5:

I screwed the punches into the six holes around the water inlets and outlets, the eight holes for the timing cover and the one bracket mounting hole that there is on the lower driver’s side of my Gen III LS truck block.  Then I took a hammer and transferred the hole locations to a piece of scrap Melamine by whacking my template with a hammer at each hole locations.

Figure 6: LS engine block front mounting hole locations.

Figure 7: Close-up of the dimples made by the threaded hole punches.

The six mounting holes for the back of the tub were relatively easy because I had the old bulkhead panel to use as a template.  Those are drilled holes so I used the transfer punches again.

Figure 8: Original bulkhead panel that separated the engine from the tub and the fuel cell.

You’ll notice that I left a lot of extra expanse on the sides and that is on purpose to help me figure out where I can attach a future rear subframe and the middle supporting brace.

Figure 9: Front plate prototype with six mounting holes for the tub and nine for the engine: six around the water inlets/outlets, the two lowest timing cover bolts and the lower accessory bracket mounting hole.

These prototypes are made of wood, but eventually I’ll put everything into a CAD drawing and have the plates made out of metal.  For right now though they are going to allow me to marry the engine to the car and start measuring for some transaxle options.  Oh, but first I have to test fit the body work…