The next step was to apply the carbon fiber/epoxy reinforcement. I wish I had taken more photos of this process. But unfortunately, like most of "my" tools, the cameras I used to document the process were borrowed, so I did not always have a camera available. Also, it's hard to take pictures when your hands are covered with epoxy. Anyway, here goes:
These images show the carbon fiber "sleeves" that I used and which I highly recommend. Carbon sleeves are essentially carbon fiber woven into cylinders, rather than plain flat fabric. Using carbon sleeves makes it very easy to apply carbon fiber to long cylinders (like bike tubes!). You just slide the sleeve over the form and then pull on the ends, as I've tried to show in the two images above. They act just like "Chinese handcuffs."
The sleeves are available in a variety of weights and nominal diameters, and any length you want. And any sleeve will fit over forms with a wide range of diameters. For example, a "3 inch" nominal sleeve will fit over forms ranging from about 1.5 to 3.5 inches in diameter.
A great thing about carbon sleeves is how much easier they are to use than regular carbon fabric. Before finding the sleeves, I bought some fabric and tried making some simple tubes with it just to get some practice with composite fabrication methods. Working with the regular fabric was a real headache. It took a lot of cutting and was very difficult to get the size just right. All the cutting created lots of fiber dust (not good). Applying the epoxy was difficult too, and resulted in more distortion and fraying of the fabric.
Working with the sleeves was so much easier. Just cut to length, slide over the form and work in the epoxy. Very little cutting (and hence little dust), virtually no fraying. No seams. Virtually no waste. So easy.
I used sleeves with nominal diameter of 3 three inches and a weight of 19.9 oz/yard (674 g/sq m). The estimated thickness (final) was 30 mils (0.76 mm). I purchased the carbon sleeves from Soller Composites. And "No," I do not work for Soller Composites, I'm just a very happy customer.
These images show the frame after the carbon and epoxy was applied. Yes, I know, I skipped a few steps. Here's what you missed:
Before making the frame itself, I did some practice. I made up a short hollow plywood section with the same cross-section that I planned to use for the actual frame. Then I laminated layers of carbon fiber sleeves and vacuum bagged it just as I will describe below for the actual frame. The main purpose of all this was to become more familiar with the handling of the carbon fiber sleeves, the application of the epoxy and the use of the Foodsaver vacuum bagger. The practice was very much worthwhile, and by the time I was done I felt competent and confident in the basic process.
When my practice test piece was finished, I set it up on blocks and jumped up and down on it. It felt really stiff (in both orientations) and was apparently pretty strong as well. Under my weight, there was no discernible deflection, no bounce, no creaking or cracking or any other worrisome sounds or movements. True, the practice piece was short, so this was not a true test of the load bearing capacity of the structure. Still, the stout feel gave me some degree of confidence that the frame would hold up well.
These two pictures show my practice piece after I cut it in half. It gives you a good idea of what the actual frame looks like in the fully hollowed out sections.
Adding Epoxy, Peel Ply and Batting
I used a two part West Systems 105 Epoxy resin with 207 "Special" Hardener. This epoxy was ideal because it is specifically formulated to adhere to wood. The 207 hardener is a "slow" hardener that allows some working time and also includes a UV inhibiter. I purchased the epoxy from "Aircraft Spruce" and also at a local boating supply store.
Before getting on with the carbon fiber application, I applied some wood stain to the areas of the frame that would be exposed after the carbon sleeves were applied. I'm not certain of this but I think at this point I "painted' a layer of epoxy resin over the entire frame. The reason (if I really did do it) was because I knew some of the epoxy would soak into the wood. I wanted to make sure that when I later applied the first layer of carbon that too much epoxy would not soak into the wood, creating an epoxy starved region at the interface between the wood and carbon fiber.
After this layer of epoxy cured, I cut a carbon fiber sleeve to the desired length and slid it in place on the front end of the frame. Next I mixed the epoxy in small batches (one "pump" of each component per batch), poured the epoxy onto the sleeve,and then spread it with a plastic scraper. Finally, I worked and spread the epoxy by hand (wearing latex gloves) starting in the center of the sleeve and working in each direction, squeezing the excess epoxy out by squeezing the "tube" between my hands.
I then began preparing the frame for vacuum bagging. First, I wrapped the epoxied sleeve with "peel ply". I used 2 inch wide peel-ply tape. During vacuum bagging, the peel-ply (Dacron fabric) allows excess epoxy to seep through into the batting, and does not adhere to the epoxy, so the peel-ply and batting can all be removed after curing. Wrapping with peel ply is a bit like applying handlebar tape, overlapping each wrap a bit at you work your way along. I found it easier to precut the pieces of peel-ply to a length of about 3 feet. After the whole sleeve was covered with peel ply, I wrapped the area with a layer of batting. I purchased both the peel-ply and the batting from "Aircraft Spruce"
For vacuum bagging, I used a "Foodsaver" system designed, as the name implies, for preserving leftovers in your refrigerator. I actually borrowed the Foodsaver from my grandmother-in-law. I don't think she understood exactly how I planned to use it, and I think it was best that way.
Using the Foodsaver for carbon fiber bikes is exactly the same as for leftovers, only the bag is a bit longer. First, you cut the bag to the desired length and use the Foodsaver unit to heat-seal one end. The bags are 11 inches wide and come in rolls 18 feet long. After sealing the first end, you slide the pierogies, leftovers, or carbon fiber recumbent bicycle frame into the bag. Finally, you use the Foodsaver unit again to pull a vacuum on the bag and heat seal the other end. So that's exactly what I did.
After allowing the epoxy to cure overnight, I removed the frame from the bag, and peeled off the batting and peel-ply. This step was not quite as easy as the name "peel-ply" implied. But after sufficient pulling and poking and a few choice expletives the stuff started coming off, and once started it came off relatively expletive free.
I repeated this same process two more times. The next time was to apply a second layer of carbon fiber to the front end of the frame, and the final (well almost final, anyway) time was to apply the first (and only) carbon layer to the seatback portion of the frame. I made the second layer on the front section a bit longer on each end than the first layer, and before adding the second layer I sanded down the ends of the first layer of carbon so there would be a smooth transition from the carbon reinforced area to the bare wood.
Then I repeated it two more times. First to add a "strap" of carbon fiber along the bottom side of the frame directly under the spot where the swingarm would be attached. I had not planned on this originally, but figured it couldn't hurt. Finally, I added a small patch of fiberglass on the bottom side of the frame where the headtube (aluminum) would rest against the frame. This was to insulate the carbon from the aluminum to prevent galvanic corrosion. The images above show the frame after all these steps were completed. Note that the carbon layers now covered over the hole drilled for the headtube. I reopened the hole by drilling a series of small adjacent holes through the carbon around the circumference of the headtube hole, then cleaned up the rough edge using a half round file.
At this point the carbon fiber work was complete, at least from a structural perspective. However, it did not have the cool "3D" carbon fiber appearance. The peel-ply left behind a slightly rough, matte texture, as well as a few stray Dacron fibers. Getting the appearance I wanted was a lot more hard work.
First, I pulled out as many of the stray peel-ply fibers as I could. Then, I painted on a new layer of epoxy. After letting it cure, I sanded it down until I started getting down to the carbon fibers in some areas. Then I added another layer of epoxy, cured, and sanded again. As I repeated the process, the epoxy filled in the low spots and I sanded down the high spots and the texture got better and better with each step. Eventually, it started looking really good. I don't remember for certain how many times I repeated I repeated I repeated this, but it was at least three, and maybe even as many as five times.
I experimented with lots of different sanding media. The most effective were drywall sanding screens, which worked on the flat surfaces. For the rounded corners I used those abrasive sponges. I had a series of coarse to fine grits I used for each round of sanding.
For painting on each coat of epoxy, I used the little yellow paint rollers also visible in the images (way) above. I used a lot of them. To spread the epoxy around I rolled it on just like you would for paint. This evened out the coating thickness but left little air bubbles on the surface. I found that dragging the roller over the surface without letting it roll eliminated the bubbles.
After applying and sanding the last epoxy layer, I did some additional "wet" sanding using a series of automotive sandpapers. Finally I sprayed on a few coats of clear gloss polyurethane finish.
This recent image shows how it turned out in the end, and also how well it has stood up over time. It's been almost two years (and several thousand miles) and it still looks good.
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