I woke up in the middle of the night thinking again, because I had to use the bathroom. While I was sitting I started thinking of how I was going to make the splitters, because they had to be somewhat thin and fairly strong because the aero loads would get rather high at highway speeds and faster, and because people would probably stand on them getting in and out of the car. Then I had a flash from the past back when I was building ultralights (lightweight single-seat airplanes that don’t require a pilot’s license under Civil Aviation Code Part 103) and I sat in on a composite construction clinic. During the clinic we watched a video on making a solid core wing with fiberglass skins using vacuum bagging after wet lay up. I realize that the preceding sentence read like Star Trek technobabble for most of you, so I will explain by describing how I’m going to make the splitters and how that changes the way the rest of the car will go together.
First of all, the core. I’m going to make the splitters and the floor of the car as a single piece, using blue styrofoam insulation board, from the nose to the start of the diffuser at the rear. This is our solid core. Now I could get all high-tech and use carbon fiber to remove every last gram possible, but that would be both expensive and overkill. I will make use of different layers of material for different purposes in the lamination, but the most exotic will be the Kevlar under my butt and the gas tank. Now the layers outside in from the ground up. First will be some light fiberglass mat that will basically be there as a sanding layer and to prevent “print through” from the woven layers. Next will be three layers of fiberglass cloth oriented in different directions to make the splitter stiff no matter how it’s loaded from people standing on it. Then the core of insulation board, then another three layers of ‘glass like on the bottom and the ‘glass mat. Then under the driver and gas tank compartments 9 layers of Kevlar oriented like the ‘glass cloth in the previous layers. The purpose of the Kevlar is to protect my butt and other parts from road debris like what broke the engine on my last car. It goes on top of the mat because the Kevlar is inside the car and it is easier to lay up that way and the mat is damned cheap so I can just run it all the way across without leaving any gaps.
Now the “wet lay up”. This means I mix up batches of very slow curing resin and completely saturate the various layers with the resin before laying them on the core using things like paintbrushes and rollers to make sure every fiber is covered in resin before it goes on the core. The reason slow curing resin is used for this process is applying the many layers is time consuming and exacting, because there can be no gaps or wrinkles in the lay up. Now why wet lay up is simply cost, buying cloth and resin separately is much cheaper than buying pre-preg which is the cloth and resin all-in-one but not yet cured, because it is made below the kick-off temperature of the resin and shipped frozen to prevent the resin from curing until the cloth is molded to the desired shape.
Now “vacuum bagging”. As the name implies there is a bag involved that has the air removed from it after the part is placed inside. Now between the bag and the part are 2 more layers of materials that just keep the bag from sticking to the part. First is what is called the “bleeder layer” that just exists as a path for the air to escape from around the part so atmospheric pressure can squeeze against the part and the layers and get everything really close together. Next is the “peel ply” that goes next to the part and gives the excess resin a place to go and is the real reason for vacuum bagging a wet lay up. The process squeezes excess resin out of the layers which then goes into the peel ply where it is wicked away from the part. Resin is the heaviest part of a composite part, my 47 pound bucket body probably has less than 20 pounds of glass fibers and the rest is resin, in fact from looking at the actual body I would say it is closer to 15 pounds of fibers and the rest resin. So leaving just enough resin to hold the fibers makes a major difference in weight. You can see graphically how much resin is squeezed out by weighing the peel ply and bleeder after unbagging the part. The outside of the bag is polyethylene sheet like what is used as a drop cloth for painting.
And there you have my major objection to vacuum bagging, the amount of waste left after the process. The bag, the bleeder, and the peel ply are all garbage after the part comes out of the bag. The only thing in the process that is reusable is the fitting between the bag and the vacuum pump, and that vacuum pump. In my case I will be using a shop vac so I will be getting sub-optimal results compared to using a real vacuum pump, but I will be getting a flatter, smoother surface by leaving the part to cure on the garage floor. And then throwing away a full garbage can worth of bag and bleeder and peel plies.
Now the real thunder, I can use this process to make an entire monocoque frame and only have the roll cage made from tubing. And in the process reduce the weight of the frame by half over welded tubing.
Witch on a Bicycle 