RJRacing

The team was hard at work this weekend putting the finishing touches on the car in preparation for the opening round of the Western Endurance Racing Championship on April 25, 2009.

After the catastrophic hood pin failure at California Speedway, the team needed to replace and repaint the hood, replace the front windshield, and replace the rear windshield. Since the team is no longer running in the Spec Miata class where a completely stock hardtop with glass rear window is required, we chose to install a polycarbonate rear window (this material is commonly referred to as Lexan even though technically Lexan is merely polycarbonate which is made by G.E.) Not only is polycarbonate impact resistant, it also weighs half as much as a comparable glass replacement. Our polycarbonate rear window ended up weighing just over 5 lbs, whereas a glass rear window weighs 11 lbs.

A polycarbonate sheet was procured measuring 24″ x 60″ in 1/8″ thickness. We chose standard polycarbonate as opposed to abrasion resistant polycarbonate, because standard poly can be cold-formed, as opposed to abrasion resistant poly which must be heated in an oven to form it. Since the team would be competing in numerous endurance races where the lights at night from other cars are extremely bright, a lightly tinted polycarbonate sheet was used. Competitors can still see through the rear window, but it will still cut the glare. This polycarbonate sheet cost just under $60. A clear sheet in this size would have cost around $50. Poly can be procured from most local plastic suppliers, just do a search for “Plastic Fabrication” in your area and start making calls.

Forming the window took a significant amount of work. Normally when polycarbonate windows are made, a template can be made from the original window which is to be replaced. Since the team’s “template” was shattered into a million pieces, we went back to our friend John Mueller and used the rear window off the #13 Weekend-Racer.com Spec Miata as a rough template. Significant trimming was still necessary, but it made the job much easier. We used a standard jigsaw with wood-type blades which gave excellent results, producing a smooth edge and cutting relatively quickly. Then came time for attaching the window to the fiberglass hardtop. For this we used 3/16″ diameter pop rivets with a 1/2″ grip range. Since we would be attaching the window to fiberglass, aluminum rivet washers were used on the inside of the hardtop to prevent the rivets from pulling through.

Trimmed Polycarbonate Window

The old adage “Measure Twice, Cut Once” became “Measure Five Times, Drill Once.” To drill the holes in both the polycarbonate and the fiberglass we used a titanium tipped drill bit in 3/16″ diameter. Some claim that drilling through polycarbonate can cause the polycarbonate to crack, and to prevent this, you should drill slowly to heat up the poly. We found that there was virtually no way to crack the polycarbonate by drilling. Using a test piece, we drilled numerous holes at various speeds, angles and locations, trying to see if there was anything we could do to crack the poly, and all that we got were perfectly smooth 3/16″ diameter holes. Some of the test holes we drilled were a mere millimeter from the edge of the piece (where cracking would most likely occur). We suspect that those claiming that poly will crack by drilling were using the term “polycarbonate” generically to include any type of clear plastic sheet, such as acrylic, which is very prone to shattering.

Drilling the polycarbonate - the shavings are from the particle board backing we used to avoid drilling into the concrete

Back to the drilling. The first two rivets are the most important, as they locate the final placement of the entire window. The first rivet was placed at the center of the top of the window. Since the window was not yet adhered to the window, and any minor movement of the window while drilling could cause fitment problems, we chose to first drill the window by itself, place the window in its desired spot on the hardtop, and then using the hole in the poly as a guide to drill through the fiberglass. This method worked perfectly. The first rivet went in like butter, the window placement was verified, and then the second rivet was placed.

After this, it was just a matter of drilling holes about every 6″ progressively further away from the center. Once we got to the curvature of the window, we began placing rivets every 3″. One thing to be careful of though. On the bottom edge, there is a molded fiberglass bracket on the inside of the hardtop about a foot from the centerline. Make sure to drill your rivet hole on either side of this bracket, because if you make your hole where this bracket is located, you will not be able to put a washer on the back side of the rivet.

Now that the rear window is installed, we will lay a think line of black silicon caulk around the edge to make it water tight. We are very pleased with the results. The optics of the polycarbonate are great. Only around the curves and where the pop rivets have been placed does the window look like it’s not made of glass (Note: this statement is merely a comment on the optical quality of the polycarbonate and is not an encouragement for Spec Miata competitors to install a polycarbonate window illegally–the pop rivets would be a dead giveaway that you didn’t have a stock rear window anyway). There is a bit of flexing of the poly at the center of the window. This may required installing aluminum window straps, but we will be checking this after the coming race weekend. It should also be noted that there are a number of different colored rivet heads to suit your preference.

Polycarbonate is more scratch-prone than glass, so it does require a little extra care to make sure it does not become marred. Alcohol-based cleaning products should be avoided (such as Windex). The manufacturers recommend using a light detergent like dishwashing liquid diluted in water. And believe it or not, the manufacturers actually specifically reference Palmolive and Joy.

Special thanks to www.targamiata.com for publishing the basics on a polycarbonate window install and getting us on the right track to making our first polycarbonate rear window.