I’ve been getting requests for the dimensions of the car from around the world. This should help. Some of the aerodynamic details have changed since this 3D model was made, so don’t take this drawing too literally. If you click on the image above you’ll get a PDF file with both top and side views dimensioned and in high resolution.
SCCA FB rules require a metallic or composite front impact attenuator. Can’t have cars running around on the track with a battering ram on the front… My impact attenuation structure, or crash box, consists of a carbon-fiber and honeycomb sandwich laid up directly on the inside of the fiberglass nose. The carbon fiber varies from four layers at the front to eight layers around the rear attachment points so that it will crush progressively from the front to the back. Cylindrical aluminum inserts are used in the honeycomb as hard mounting points for the wing to the nose and for the nose to the chassis. This area is designed to be strong enough not just to absorb impacts but to allow lifting the front of the car by the front wing.
I had not realized how many small brackets and things need to be fabricated and welded onto the frame before it can be painted. Weeks of work…
The nose mounts are so strong because the car will be lifted by a nose jack under the wing in the pits.
The limited-slip differential is a torsen or quaife type made by OBX, imported from the USA for an American-style Honda Civic. The differential ring gear on American Honda Civics is mounted with left-hand threaded bolts, so I blithely go down to the auto recycler here in Thailand and buy a differential for donor parts. Hmmmm… these bolts don’t fit. So I check carefully and find these differentials are sold in Thailand with right-hand threads! I go back to the auto recycler and ask for left-hand threaded bolts. They just look at me with that “crazy foreigner” look. OK, all we have to do is order some American-style bolts from Ebay US… There is exactly one listing on all of Ebay, and they don’t ship to Thailand! Plan B: drill the suckers out and use through-bolts.
This is where I find out the differential housing is made of some ultra-hard tool steel, or maybe kryptonite or something. Wow, are these holes difficult to drill out. Solid carbide end mill, highest speed on the milling machine, lots of lubrication, and wait. And wait. And wait…
Next we had to drill a matching hole pattern in the rear sprocket, then cut it in two halves for quick changing at the track. This sprocket will be for static test only as the new hole pattern wasn’t compatible with the old one, leaving thin aluminum in some places. I’ve since ordered a blank rear sprocket from England which I will cut with only the correct holes.
Time for a photo update showing how I built the cockpit, tube by tube.
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.