COROPLAST HPV BODY CONSTRUCTION

by Bob Stuart

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 effecient
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 paralell 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 in big rolls by wholesalers as 3M #471. 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.

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