I try to post updates only for completed projects, and since I’ve had several projects in progress it’s been a while since I’ve blogged. The diffuser is finally finished, so here are some pix and a video that explains it all:
Building the undertray started with building a surface large enough to hold it. It’s larger than it looks, so we had to laminate decorative plastic laminate onto two 4×8 foot sheets of plywood that had been trimmed to about 3×8 feet each. Then wood to form the side air dams was screwed down, and the radii filled with auto body putty. Next we cut plastic honeycomb and plywood pieces to fit, with the plywood located to pick up the attachment points on the frame and to protect the radiators on either side of the car. I built a hot-wire foam cutter from a tree saw handle, a piece of guitar wire, and an automobile battery charger, which I used to cut foam profiles for the leading edge of the floor. Then we laid the whole thing up with epoxy and two layers of fiberglass on the bottom and one on the top, and vacuum bagged the whole shebang. Vacuum bagging was made more difficult by the random tiny holes in the plastic sheeting, which we expediently fixed by adding a whole second sheet on top of the first.
After debagging we painted it and found out the hard way that you can’t paint enamel over fresh epoxy in a humid climate. It never dried, and had to be scraped off like tar. Epoxy paint worked much better. We then mounted the undertray on the car, drilling mounting points through the plywood in the correct places. We had to fabricate a mount for the front of the undertray, which was a little tricky as we didn’t want to remove the fiberglass body panel under the driver’s legs so everything had to be done from the outside. We fabricated a small pylon from aluminum sheet and pop-riveted and epoxied it to the bottom of the body.
For proper protection in a crash, the driver’s head surround needs to be filled with foam. I placed an aluminum panel where I wanted the bottom of the foam to be, covered everything with plastic sheeting and poured two-part urethane foam into the cavity. The foam generates considerable pressure as it expands and cures, necessitating many iterations of trimming and fitting. I then sat in the car with the HANS device on, followed by many more iterations of trimming and fitting. Once the foam was cut to shape, I covered it in a single layer of fiberglass and epoxy, then painted it.
Time to lay up the first set of body panels. In some photos you can see the joggles laid into the molds with duct tape so the panels will overlap smoothly. Nine coats of mold release wax and there were no problems releasing parts from the molds, although at times I did have to work a bit. Each mold required about a day of finishing work to remove ripples due to waviness in the body buck. As I’ve said before, don’t build a body buck the way I did it. Instead, immediately after completing the X-Y grid of cross sections, lay about 3mm of fiberglass on top to give a good solid surface, then use body putty on top of that. You’ll be finished in half the time it took me. The only place you should use foam is where actual carving is required due to the complexity of the shape, like the sidepod air inlets. Yes, I know the main roll hoop forward braces are still not there. Patience…
One problem I found out the hard way is that a chemical in some brands of duct tape inhibits gelcoat curing. In the end, gelcoat that had been in contact with some kinds of duct tape never fully cured and had to be cleaned out with acetone. Also, the joggles formed with duct tape were too sharp for the fiberglass mat to conform to, resulting in bubbles under the gelcoat that have to be scraped out and reworked. Gelcoat is probably more trouble than it’s worth given its weight, so next time I’ll just prime and paint the body panels to finish them. The sharp joggle corners need to be filled in with fiberglass roving before laying mat on top.
Since this is the last of the molds, in the photos below I’m showing each of the layups, for a total of three. Target thickness for the molds is 4.8mm, three times the expected thickness of the parts to be molded.
Probably the main thing to explain here is how I create the overlaps in the molds. The edge of a mold is marked on the body buck with duct tape, which will leave an impression in the mold for later trimming of the finished part. Then after removing the mold from the buck, I lay in a strip of 1″ duct tape touching the existing tape, then another strip touching that one. I then remove the first two strips of tape, leaving the edge for the next mold with a 1″ overlap.