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Originally Posted by The Toecutter
Do it, especially if it is new. Your site has a lot of good information that is easy to miss.
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I had the opportunity to make another HPV body in Coroplast, the same Polypropylene extrusion resembling corrugated cardboard that was used for the front and back of the Car-Cycle X-4. The specifications emphasized low, low weight, and a vague resemblance to the Space Shuttle Columbia. I decided that extreme ruggedness, within the weight allowance, would also be a great virtue. At 15 lbs., the body came out a lot lighter than expected, and after it was rolled onto it’s side on grass without causing any visible damage, I decided that it was tougher than I had hoped.
The first step was to convince my plastic dealer that a lighter gauge of Coroplast could be had. The factory calls it “fine flute,” recognizing the closer rib spacing; the sellers say it is “2mm,” for the thickness of the specimens that they soon found in 4’X6′ (120 X 180 cm) sheets weighing only 2 lbs, or half the usual weight. Other weights and thicknesses, from either of the two standard dies, are mentioned in the factory brochures. You can probably find free samples and/or full sheets of the standard stuff at a sign shop, and some lumberyards also have it. It comes in translucent, white, yellow, sky blue, and several muddy “background” colors. It will seem pretty floppy at first, but it stiffens up quickly as curves, ribs, bends, or extra layers are added. You can get a good idea of the possibilities and techniques from cardboard shipping boxes and liners, and from the cheapest furniture.
Once I had the lightweight sheets, I made up test pieces for the Coroplast reinforcing ribs until they were VERY hard to break by brute force and ignorance. By suppressing my hard-won ability to design things that are light and stiff, I found that I could make them a bit less geometrically rigid, so that by buckling they would be able to bounce back from severe distortion instead of stiffly resisting and then fracturing. This is the same principle that allows grasses to co-exist with feet. Then, since the structures were getting a bit thick and heavy in developing enough stiffness so as not to appear alarming, I added a bit of fiberglass, and tested again. A few ounces of glass did wonders for the strength and stiffness, and then would break away, letting the Coroplast buckle and bounce back as before. Often the thin sheet of delaminated fiberglass could also buckle independently, and snap back ready to be glued back on. If the glass broke, it would only need a small patch for restoration.
I wound up using several different types of Coroplast beams, according to the space available. For the window sills, six layers of heavy gauge Coroplast were glued together with the ribs in each layer at 90 degrees to the next, and the length of the beam at 45 degrees to both. The open ends of this honeycomb-like assembly were trimmed to the body contour. A single layer of unidirectional fiberglass was sandwiched between this and the body side. A second strip of ‘glass could have been used on the interior side to add stiffness and finish. The ribs at 45 degrees are optimized to work as the shear web of a beam.
Ahead of these reinforcements, a single sheet of 2mm Coroplast was used as a dashboard, with the back edge folded and glued to make a tube. The wheelwells were outlined with layers of 2mm Coroplast stepped back to lay in a single plane yet support the curve of the body. In order to conserve material and keep the grain roughly paralell to the opening, these layers were built up from short arcs with the joints staggered. Some parts of the wheelwells had an inch-wide strip added on edge to make a stiffer “T” section where it didn’t interfere with anything.
Behind the rider, where there was more room available, a deeper, more efficient section was used, with a single hoop running all around the inside of the body and connecting to the body mounts on the chassis. Here, two layers of 2mm Coroplast were used for a box section about1″ inch wide and 3″ deep. The layers were set at 90 degrees to each other, and 45 degrees to the length of the beam, making an efficient shear web. A single strand of fiberglass roving was epoxied between layers in each fold line. This produced a deep C section and the open ends were then trimmed to the contour of the inside of the body. Then a 1/2″ wide strip of unidirectional ‘glass, a bit rich in epoxy, was folded over the open edges. Next a 2″ wide strip of 6 oz. bi-directional fiberglass tape was laid up on the inside of the body panels, and the still-wet beam clamped on. The small gaps were adequately bridged by the unidirectional strips and epoxy. The joints in this beam were reinforced with fiberglass and epoxy.
In places where two pieces of Coroplast were joined flat, or a just a bit of extra stiffness was needed, a backing strip was fastened with double-sided carpet tape. The factory recommends thick, foam-center tape to press into the spaces between the ribs better, but I got away with the lighter, thin stuff. The factory is also unconvinced of the reliability of cyanoacrylate (crazy glue) or epoxy. My good CA didn’t stick, but I trust the Gougeon Bros. WEST epoxy after the bonding surface has been cleaned with lacquer thinner. It will peel, but not too easily. If the surfaces have not picked up any fingertip oil or other contamination, a dry wipe may be best, as the factory gives each sheet a strong charge of static electricity to attract paint. I had also used water base contact cement with good results on the Car-Cycle, but apparently it can shatter at temperatures below freezing. Regular clear Silicon Seal gives the best bonds of all on Coroplast, the colored silicon is probably fine, but the water-base, paintable stuff is useless. If you don’t mind a bit of extra weight, silicon can be built up into very tough butt or T joint fillets. Always remember, though, that only more silicon will stick to a surface that already has even a hint of silicon on it.
For the single-plane curves (conic sections) of the body, I used the technique of slitting the Coroplast on the inside, bending it to shape in a minimal fixture, and laying on a single layer of 2 Oz. fiberglass cloth with WEST epoxy resin. If necessary, these curves can be run up to 45 degrees off the grain direction by ending each slit at the edge of the area to be bent. The thin glass cloth is easy to get from model aircraft suppliers (browse while you are there). I found that there is a noticeable difference in the thickness of the faces of thin Coroplast, so I slit the thinner side. The tool of choice for slitting is an Olfa knife with a blunt tip produced by a few attempts to slash through a concrete floor or a whetstone. The knife is held at a compound angle so as to slice cleanly through the plastic, leaving slanted edges that will ride over each other as the sheet is bent. The blunt tip keeps the cut at the right depth. For gentle curves, not every cell needs to be cut to produce an acceptably smooth surface. The final product is reasonably tough and resistant to anything except a pinch. To guard against that, you could use 6 Oz. cloth, more Coroplast, or tiny reinforcing ribs.
In areas where a compound curve would be ideal, such as the nose cone and front of the roof, I used several strips of Coroplast, giving a shape similar to cloth stretched over a framework. The first few strips were bent over cardboard ribs, and the rest were formed by support at only two or three points. The strips were cut with an unmodified Olfa knife roughly parallel to their ribbing, and it took some practice, low cunning, and often several passes to get a smooth line cut. Each strip was then held in alignment to it’s neighbor with masking tape on the outside. Next, the seam was backed up by cross-grain strips of Coroplast glued on with Silicon Seal. The cross-grain strips were cut almost across at regular intervals, making a chain of approximately square blocks held together along one edge. This made them easy to handle en masse and able to follow a curve and stick to it with just the wet silicon. After the silicon had dried, the masking tape was replaced with vinyl tape to give a finished surface. This tape is sold in small retail lots as 3M #190, and wholesale in big rolls (see below). Many different widths and colors are available. This is the only tape I know of that will stretch nicely over a curve and then stay put instead of shrinking back. Duct tape also works, but is rougher and less reliable, especially around solvents.
For the windshield and side windows, I got some generic polycarbonate, .03″ thick X 4′ wide from a roll. It seems to be much better than GE’s Lexan brand polycarbonate. I used a cheap strip heater to flange the vertical ends, and glued the windshield in with silicon seal, taking a chance with the effect of acetic acid on this very tough clear plastic. The body shape as a whole was set by a 1/4″ plywood profile and a number of cardboard formers. The third body mounting point is at the front, with everything between a self-supporting monocoque. The door function is performed by hinging the roof up at the back and removing a side window.
I am very satisfied with the finished product, and with the ability of Coroplast to hold its shape against the wind, while yielding and bouncing back from most other encounters. In this application, toughness is far more important than strength or stiffness. I also think that it can be vacuum formed into smooth compound curves. I did a little sample in my kitchen, so it shouldn’t be too hard to do it industrially, making even better fairings. Making custom fairings this way is rather time-consuming, but it has the advantages of using very little extra material in the process, and producing finished parts early on in the process. You can also build right onto the chassis, designing the details as you go.