I was lucky enough to find some 7075-T651 aircraft aluminum plate, still with the Martin Marietta markings on it. Very useful for miscellaneous parts like this ECU mount. That stuff is strong! Almost as hard to bend as steel, but it makes for a very stiff end result. So I just cut this blank out of the aluminum plate, bent it in my hydraulic press, and added lightness using the milling machine. It mounts to the frame with aluminum clamshell mounts purchased on Ebay.
A couple of videos posted to YouTube. #1 gives an overview of the chassis as it stands, and #2 talks about the chassis jig table.
Now it’s time to test one of the big unknowns of this project: can the top rails be formed in the shape of a complex 3D spline, and can the left and right sides be made to match? The CAD software won’t even allow a structural member in the shape of a spline, requiring them to be composed of straight sections and arc sections. Other exoskeleton cars have been built, but as far as I know always with a single curve in the main frame members. And of course a tubing bender is designed with the assumption that it will be used to form constant-radius segments. I believe this is the first car to be done this way, so I’ve put a lot of effort into the chassis jigs to get it right. The top rail spline is a key to the beauty of this design. To make a long story short, it is in fact possible to bend a 3D spline on a tubing bender. It just takes a lot of trial and error, patience, and about a day of work per rail. And luckily, overbending can be corrected by running the tube back through the bender, clocked 180 degrees.
“The pricking of my thumbs” is not entirely rhetorical. In fact I almost cut off my left thumb while building the top rails, so a word about safety. A tubing bender is a SERIOUS PIECE OF EQUIPMENT. It won’t even notice bones and flesh being fed into its’ rollers. I had already turned off the bender and reached to grab the tube as it was twisting on the way into the bender. The bender caught my glove and started pulling my thumb in, stopping at the last possible millisecond before inflicting permanent damage. It hurt and left a mark, but I was insanely lucky. So, 1.) Never wear gloves while using a tube bender. They protect you about as much as Saran Wrap, and it’s better to have your fleshy appendages dangling about unprotected to remind you of the danger. 2.) NEVER, NEVER touch the tube on the side being fed into the bender. Always handle the side being fed out. and 3.) Before pushing the “on” switch, stop, think, and say to yourself “I’m not going to become an amputee on this bend.”
To get optimal suspension geometry and aerodynamics I’ve designed the car with a front keel under a raised nose. This gives the longest possible lower front A-arms, minimizing the angle changes of the front suspension as it goes through bump and jounce motions. The raised nose clear airflow around the front wing. My computational fluid dynamics (CFD) studies show airflow around the front wing is extremely important as the wing operates in ground effect and generates downforce all out of proportion to its size. I spent a considerable amount of time trying to increase the downforce generated by the underbody and rear wing to match that of the front wing, even though those elements are far larger.
The front keel will use a stressed skin of aluminum formed to shape and riveted to tabs welded onto the frame tubes. This is the highest-stress area of the entire chassis, as under braking something like 2800 pounds of force will be transmitted through these members. You can visualize the car supported vertically on the front keel, with two more cars stacked on top of it, so this needs to be really strong.
I’d been poking around local tool stores for a while, looking for tubing benders. I’d found and purchased a 3-wheel tubing bender, but so far no mandrel-type benders. I need both types. Finally, someone suggested I try a shop that builds headers and exhaust systems, and bingo, success, not 100 meters from my daughter’s school. They have a huge floor-mounted motorized unit with all the necessary mandrels. Of course, on the same day, I found a shop selling mandrel-type benders but it appears I don’t need one now. So the header shop says, sure, they can build the rollover hoop, although I’d have to wait about 10 days as they’re booked solid until then. The price was right, however. Really right.
They don’t have much (any?) experience working from drawings, unfortunately. When I went to pick up the two samples, the boss mentioned they’d bent the top section 15 degrees forward. I pointed out on the drawing, “You mean like here, where it says 10 degrees?” Oops. “We’ll fix that for you by tomorrow afternoon.” So I go back the next day and everything looks fine. I take them back to the shop and lay them out on the full-size drawing I’d given to the shop so they couldn’t possibly get it wrong, and, well, they’re not even the same as each other. One is about 15 mm too narrow. The other needed a half hour of work on my tubing bender to make it match the drawing, but we’re on our way!
I’d like to meet all the SCCA rules for Formula 1000, but sourcing the required tubing in Thailand is difficult. The prototype will have to be an adaptation of the US rules to fit tubing sizes and types available locally. Where SCCA rules require seamless or drawn-over-mandrel tubing for the roll cage, I’ll be using ERW (electrical resistance welded) tubes. I’ll also be modifying the required diameters and thicknesses a bit. That means this first prototype won’t be able to be homologated under SCCA rules for racing in the US, but realistically that wasn’t going to happen anyway. I visited a shop called “Boon Racing Pattaya” and talked to the owner about tubing. He said everyone in Thailand uses ERW for roll cages and recommended I visit Chinatown in Bangkok to buy any unusual sizes or types.
I’ve been gathering the tools to build race cars since before I realized that was what I wanted to do. Before I finished building the house, I bought the really big machines at an open-air market in Bangkok and had them delivered through an opening in the machine shop wall that was later sealed up. Back then the truck could drive through the front fence and back right up to the front of the house, making delivery much easier. I’ve since lost track of the Bangkok open-air machine tool market, and would appreciate any info on how to find it again. Bangkok is a big place.