John in CR said:
After another read-through I've got more questions. I've been sitting on a bunch of carbon, both cloth and uni-directional, and really want to do some form of composite layering like you've done. Under 50kg is great! Can you go into detail about your layering of the plywood and carbon with resulting thicknesses and how you joined them at the corners?
Sorry John, I missed answering this one.
I can give you basics, but I don't actually recommend you follow my process. An aluminum frame would be lighter in the end. My method required a lot of work and had some pretty weird steps. I'll give you a basic expalnation but I'm not going to make a how-to book or anything like that.
To make each side panel:
Start with a smooth waxed surface to do your layout on. It has to release the cured epoxy later
Lay down the basic shape of your frame profile + a couple of inches in two layers of 8 oz carbon bi-directional weave cloth. I cut the cloth to match angels and used tow layers of cloth tho make sure I had cloth over-laping cloth on the joints.
Mix up a big batch of slow cure epoxy and saturate your two layers of cloth
Lay one sheet of 3 mil aircraft birch ply over the mess (don't loos track of where your first layer is)
Put down the basic shape again, making sure it line up onto of your first layer. This time using a carbon/Kevlar mixed cloth, again two layer
Mix another big batch of slow cure epoxy and saturate your two layers of cloth
Lay your final sheet of 3 mil aircraft birch
Place mastic material, release cloth and vacuum film and vacuum clamp it all together.
Now do it again for the other side. Each side panel ended up about 1/4 inch thick
Once both sides are cured, peal them up, line them up and stick them together flat. now you have to put down a paper template of your shape and using a jig saw, cut it out of your plywood shapes. Cutting thought the kevlar is a smokey, nasty job and should be done outside. You will need several blades.
I built my headtube and pedaling station as a unit, by taking the front end of a Norco Torrent which has really thick tubes, chopping it off and laminating it to plywood using multiple layers of ply, kevlar and carbon. The headtube/pedaling station is the width of the inside of the frame and the side panels are attached to it.
the other spacers that separate the two side panel were made from strips of 1/2 inch ply, accept for bolt points which were made from one inch hickory board.
I then epoxied and screwed the side panels to the spacers and pedaling station/headtube, keeping the screws 1/4 inch away from the edge so I could round all of the corners with a sander.
Then, finally I wrapped the would thing in two layers of carbon cloth folding the inside edges over inside the frame and holding them there with cut Styrofoam. saturated it all with epoxy and vacuum bagged it. upon taking it out of the bag much sanding and finishing ensued.
Ok, that said, I suggest that you do your frame in aluminum. It's much easier, and because there was no way to know if what I was doing was going to be strong enough I overbuilt it by quite a bit.