Why put a lot of work into a body if it doesn’t last? Follow these tips for a Lexan shell that will last for an entire season.

Why do polycarbonate bodies split and crack in the first place?  Usually, we can trace the problem to one of three causes: fatigue, impact, or vibration.  Lexan is pretty strong stuff, especially when it’s used as its inventors originally intended.  When installed as a thick, vertical screen, it stops a bullet without shattering (it’s the stuff used as protective shields in taxi cabs and police cars).  Molded into a complex curve without any sharp corners, it makes a terrific cockpit cover for jet airplanes.  So you’d think with qualities like those, our bodies would simply last forever.  I mean, we don’t ask our body shells to take a bullet or travel at the speed of sound.  However, manufacturers of radio control body shells incorporate a significant amount of realistic-looking detail into their designs, along with folds and abrupt corners that weaken the overall structure. 

In case you didn’t know, Lexan bodies aren’t injection-molded like plastic or nylon parts.  Instead, they start out as a flat sheet of .030 inch polycarbonate, which is attached to a frame -kind of like a picture frame- and heated in a convection oven until it becomes soft.  Then, it’s laid over a female mold made from a plug that looks like a solid model of the final product, and the air between the mold and the soft Lexan is vacuumed out.  The most complex molds are made out of several pieces of metal, which permits the designer to curve the final body shape more aggressively than if only a single piece were used.  When the polycarbonate cools and hardens, it’s pulled out of the mold and set on the shelf.  The edges of the sheet are then sliced off with a device that looks a lot like an industrial paper cutter (you know, the kind with the big lever on the side), and the body is packaged in a box or bag with the appropriate decals.

The result is a crystal-clear body that has a significant amount of detail included on the exterior.  But it also means that the soft polycarbonate sheet gets pulled, stretched, and as you might imagine, thinned, as it follows all of the folds and corners- especially around the grill and rear end details.  Where is the body the thinnest?  Unfortunately, things get thin where they really need to be the thickest- around grille details, headlights, and body post holes.  Things get especially nasty around the rear body mounts on off road trucks.  So, a lot of what needs to be done to preserve your new body involves strengthening it where the polycarbonate is the thinnest as well as where it tends to get the greatest impact or receives the most fatigue.

Gummy and Gooey

One way to strengthen the body where it’s been folded or drawn around a sharp corner is with a material that’s adhesive (so it will stick to the paint) but doesn’t affect the integrity of the material itself.  So stay away from CA or super glue, which hardens and clouds polycarbonates.  Instead, use a silicone RTV (room temperature vulcanizing) product like Shoe Goo or Marine Goop.  These products come out of the tube as a very soft substance, which means you can get it into all of the tight nooks and crannies, yet they harden overnight into a gummy, rubbery substance that’s almost impossible to remove from the surface to which it’s been attached. Perfect!

These room temperature vulcanizing silicones can work wonders on your Lexan bodies.

One good example of how well this technique works is Tamiya’s Toyota GT-One body.  This GTP shell is considered by many serious racers to be the ultimate on-road handler.  Unfortunately, that beautiful sculpted front end comes apart if you look at it too hard, let alone clip a corner or two.  Even worse, the rear wing separates where it folds into ninety-degree angles to bolt into the uprights.  At over fifty bucks a copy, anything you can do to ensure that one of these Toyota shells lasts for a couple of months is well worth your time.  With a little effort and some RTV, you can extend the body’s useful life to cover eight or ten race meetings.  That’s a full summer’s worth of fun!

Start by washing and painting the body in the usual fashion, but give yourself an additional two weeks of prep time before you intend to use it.  Allow a full week for the paint to cure, preferably in a warm dry place like a sunny window, with the body turned upside-down so that all of the solvents can fully evaporate.  Then run a bead of RTV along each side of the front wheel wells and into each of the interior folds.  Keep a paper cup of water close at hand, and dip your finger into it every once in a while to keep the RTV from sticking to your finger as you work it gently into each crevice.  Don’t try to fill the spaces all at once, because the RTV solvents will cause the paint- especially flourescents and metallics- to discolor or flake.  Instead, put one layer on and let it dry for two or three days before working a second coat across the surface.

See how the RTV gets worked into the folds and crevices to support the Lexan.

That’s fine for the GTP crowd, but how about the sedan guys?  Take a look at one of the most popular bodies on the asphalt today, Tamiya’s Acura NSX.  It’s beautifully molded, without a lot of overhang on either end, and even has a lovely hard plastic rear wing to keep the back end planted.  But the front end, especially around the grille area, has a number of folds and creases that make it more fragile than we’d like it to be.  Check out the pictures to see how the same Shoe Goo application techniques can keep the body from cracking and splitting.

See the similarities between the Toyota and Acura bodies? All of these folds and crevices will crack and split unless supported by RTV.

Shaking and Shivering

Fatigue is another major cause of early body failure.  Some bodies have large, flat sections that simply shake a whole lot, even when the body is firmly pinned down in the proper locations.  What can you do to prolong the life of a body like this?  Change the body’s harmonics by altering certain areas of the shell after it’s been painted.   Along with a support material like light Lexan or fiberglass tape, RTV does an excellent job of helping the body shrug off the shakes.  

As an example, let’s look at Tamiya’s wildly popular Mercedes DTM body set.  Seen on TA02, TA03, and TL01 chassis, this body has been a multiple concours winner and favorite of racers since its introduction in 1994.  But check out any DTM that’s been run for five or six weeks, and you’ll probably see stress cracks appearing in the area immediately in front of the front wheels.  The body doesn’t have a lot of folds or creases, but the panel is fairly flat and lies at the very end of the support arch offered by the front body pins.  Look closely, and you’ll also see the front wheel wells on most of these DTM bodies cracking right at the top of the arch, in line with the body post holes.

This flat area in front of the front wheels can be supported with RTV and some fiberglass tape.  Plus, the wheel arches need RTV to keep them from cracking.

You can help your DTM body and similar styles live a longer life by adding support in both areas.  Use RTV along each side of the front wheel well arches, running a thin bead on each edge, and then flattening it out with a wet finger.  Spread a thin layer along the lower forward panel and place a section of fiberglass fabric cut to size on top of it. Since this is used by the airplane crowd all of the time, it’s pretty easy to find at Hobby Works.  Work some more RTV into the tape, smooth it out nicely, and give the whole thing a few days to completely cure.  The body will be slightly heavier, but it will last a whole lot longer.  Can’t find any fiberglass tape?  Try a piece of thin Lexan instead (I save old wheel well cutouts for this purpose).  Or use drywall tape, the woven kind that’s used to patch over cracks and seams.  Find yourself in a situation where you don’t have the time for RTV to cure?  Then use thin mylar tape- like the brown stuff sold by 3M as packaging tape- or gray duct tape, both of which are easily shaped and formed, and which hold up well under impact.

Foam Bumpers

When you’re trying to keep the front end of your car looking more or less like the day it came out of the mold, there’s nothing better than a foam bumper.  It supports the front end of the body so it doesn’t tap on the pavement, keeps it from deforming under impact, and slows down the common wheel arch cracking caused by impact.

Tamiya makes a set of dense foam strips with adhesive on one side that can be built up in layers to fill the space between the bumper and the inside of the body.  But there are also many companies - such as HPI - who market foam bumper blanks meant to be carved and shaped into the proper form for your particular body.  Just make sure that you completely fill the space between the body and the plastic hard bumper at the front end of the chassis.

See how the area between the bumper and the body is filled with foam?  This keeps the front end from deforming under impact.

Can’t find a foam bumper? Check out the dense foam used to pack computer monitors.  Although they tend to be funny colors like light blue and pink, they’re the same closed-cell foam material used in the more expensive bumpers and can be easily carved and shaped to fit correctly.  Some tuners - like me - prefer to attach the foam to the inside of the body rather than the bumper.  That allows the bumper to be tuned to the specific body rather than the chassis.  If you tend to swap bodies around to find the perfect aerodynamic combination for a certain track on a given day, this method probably works better for you.  One thing to consider when gluing the foam to the inside of the body is discoloration of the paint caused by RTV or contact cement.  Use a layer of wide masking tape over the larger areas to protect the paint.  The foam glues on just fine, and the outside of your body will look as lovely as ever!

If you can’t find foam bumpers, try the packing material used to ship computers or televisions.  It’s the same stuff, just a different color.

Special Problems with Nitro Cars

So far, the techniques we’ve discussed are pretty universal, but gas cars have one additional problem that needs attention: the motor’s exhaust pipe.  This source of heat, vibration, and unburned fuel/oil mixture is particularly difficult to deal with.  First, ensure that the pipe doesn’t tap and vibrate on the Lexan and that the hole for the exhaust pipe is large enough that it doesn’t interfere with the shell.  Double-check for pipe placement when the motor is revved or when the throttle is suddenly chopped, because that’s when the pipe moves around the most. Alternatively, you can slice a length of fuel tubing and cover the edge of the hole with it to provide some cushioning from the vibration.

To deal with exhaust pipe heat, line the inside of the body around the hole with heat-resistant foil tape or a thin layer of contact cement and aluminum foil. On the outside of the body, a layer of clear 3M packaging tape will keep your decals protected from the oily residue that comes out of the exhaust pipe.  This area can be regularly cleaned with lighter fluid or alcohol, and the tape can be replaced when it becomes tattered or discolored.

Take a few moments to look over your favorite body style and see where it cracks or chips. I’m sure you’ll be able to make the next one last a lot longer, just by applying some of these body saving and money saving tips!