Carbon Fiber Battery Tie-Down

Every year at Carnegie Mellon University, part of the Carnival is the ever-popular "Buggy" Competition. Teams compete to design and build unpowered wheeled vehicles that can be pushed uphill by team members and can roll, very quickly, down some pretty long hills in a fixed course around Flagstaff Hill. These are manned, steerable buggies, (actually, most of them are womanned buggies, since the pilots must be very small, and lgiht). Most entrants are fraternities, but there are a number of other organizations that build buggies. My friend Tim is a member of a group called Fringe, which focuses the majority of its energy on creating and racing some very advanced buggies. Modern buggies are made almost entirely out of carbon fiber...not the bling bling stuff that you tape to your dashboard, but honest-to-god naked carbon fiber, sheets of the stuff, that you have to build molds and mix epoxy for; and that is as strong as steel while being about 10x lighter. After building a buggy out of carbon fiber, they had a bunch of scraps lying around....Tim and I talked about making stuff for the cars out of it. Since it's expensive, we wanted to keep it small, so we could use the otherwise useless scraps. And since they don't have a giant pressure-cooking vacuum oven, we had to use more standard expoxies that are not rated to insane temperatures (like brake duct blank-off plates...oh, if only).

Tim and I decided that a battery tie-down would be a good first attempt, small, relatively simple, and not terribly mission-critical, but still pretty cool. And since the stock ones corrode violently after only a few years, replacing one with a non-rustable materal would be a preemptive strike against under-hood uglies. We took my stock tiedown, measured things, and cut some wood into the basic shape of a mold for the CF tiedown. We sanded the mold down as smooth as possible, and then covered it with plastic-wrap to prevent the expoxy from making the wood a permanent part of the creation. The side facing the plastic wrap is the only part that gets a bling-bling quality, since the surface is smooth and the epoxy sets with a shiny finish. Tim decided six layers would be more than sufficient, so we used four weave layers (some laid diagonally to the others, for increased strength), and two unidirectional layers, which are basically long strands of carbon that increase the strength even further. These unidirectional layers are in the middle, with two weave layers on each surface. The process is very messy and time-critical, so i don't have any pictures...basically, you mix your epoxy, use a credit card to smear it into each carbon-fiber layer, and then lay the layers on your mold however they need to be. Once the last layer has been placed on the mold, a layer of fabric designed to let excess epoxy through and still peel off the finished product is placed on top, and then a patch of felt that absorbs the excess epoxy is the final layer. The whole mess is then placed inside a large ziplock bag, with a hose from a compressor set to "suck" stuck in the bag. An impressive vacuum forms in the bag, I think it peaked at around 17 inches of mercury, and you leave it overnight for the epoxy to set in the vacuum. When you return after the night, you get this:

You can see the vacuum hose, the green is the felt that absorbed the excess epoxy.

It is a complete pain in the ass to get the fabric layers off on something this small...many tools were tried with varying levels of success.

Once you've popped the carbon-fiber off the mold, it's time to trim away the excess. Here, we've clamped our creation to make dremeling easier...dremel rocks carbon fiber about as hard as it rocks everything else.

once the desired shape was dremeled, some coarse sandpaper helped smooth out the edges and finalize the shape.

drill a couple of holes where they need to be, and you've got a CF battery tie-down!

this is the face that had the fabric blotter on it, much less bling bling, much more hard core. This side will rest on the actual battery.

real carbon fiber is kick ass stuff. This piece weighs about as much as an envelope, but I couldn't break it with my hands if i tried.

This is one of two from this mold that exist in the world so far...and no, you can't have one, so don't ask :)

Worth a good 2 tenths in the quarter mile, I'm sure.

Thanks again to the Fringe folks for letting us use your scrap CF! Carbon-fiber hood, here we come!
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