Building a Carbon Fiber Bike Frame – Part 1

When I was a senior in high school, I made a bike out of bamboo joined with wrapped carbon fiber thread. For various reasons, including my own impatience, inaccurate measurements, cut corners, lack of an income, and just a general ‘winging it’ attitude towards the project, I failed. While it looked nice, the bike broke at the head tube lug on just the second ride. I had made some questionable design choices, including but not limited to sanding the carbon fiber lugs after every carbon fiber wrap (basically ruining the strength of the carbon fiber every time I wrapped it), and not having any sort of strategy with respect towards orienting the carbon fiber wrap in an optimal way. I never squeezed out any epoxy, so the carbon fiber was very loosely wrapped on the lugs, and leaving much of the strength of the frame dependent on the epoxy. I was 18, didn’t know very much, and I never tried again.

I’m 25 now, and after a lot of research I know a tiny bit more about working with carbon fiber, and a lot more about working with bikes. I have an income now, and am arguably more patient with myself. So over the last few months, searching for a distraction from the gloom that is the world outside my apartment, I picked up framebuilding again. I proceeded to get way in over my head and drew up plans to create a whole frame out of carbon fiber.

At various times since my failed bamboo attempt, I’ve researched and picked up various tips from hobbyists across the web about working with carbon fiber, only to reach a barrier and not have the time or energy to actually pursue the project. I don’t want to exclude anyone understanding this process, but there are much better ways to learn the details of how to handle carbon fiber layups and epoxy than in this blog post. I’m not going to give an exhaustive description of the epoxy that I use and why (West System), and a lot of things will just, for the sake of hopefully making the post a more interesting read, be presented with the understanding that anyone looking to replicate this already knows about working with composites. Feel free to email me if you have any deeper questions.

The main barriers to this project are the process of making moulds for the carbon fiber, and the compression of the carbon fiber layups into those moulds. Without accurate moulds, it’s hard to get any accuracy. Without a solid compression method, it’s impossible to get any strength. Unidirectional carbon fiber (not the weave we typically associate with carbon fiber), comes in a roll of fabric. This fabric is either pre-preg (epoxy is already impregnated in the fabric, and is then heated to activate) or raw (liquid epoxy has to be impregnated into the dry fabric). My process is a composition of a few different techniques, sourced from a few different methods.

Making the moulds is traditionally for hobbyists a very intensive process, involving positive moulds made out of styrofoam, wood, or other substances, casting them in plaster, making a silicone or fiberglass negative, and eventually laying up the carbon fiber and making the parts. Compression is usually achieved through vacuum bagging. The bike manufacturing industry typically uses metal moulds that take months to design. Moulds can be made of any size, and frames can be made all at once, one half at a time, and then glued together.

Hobbyists and sailors often make simple carbon fiber tubes for use in things like kayak paddles and sailboat masts. This is usually done on a mandrel. The carbon fiber is wound around a pvc or other plastic rod, and pressure is applied. This removes the involved process of making a mould, but limits the shapes that can be created.

I do not have a vacuum bagging system, and have no desire to undertake the traditional casting method. Instead, my process relies on 3D printed moulds (both positive and negative) and clamps for applying pressure. The 3D prints are easier to produce, albeit somewhat weaker than the traditional metal or fiberglass moulds. So, I reinforce the reverse side of the plastic moulds with chopped strand fiberglass impregnated with epoxy.

At this stage, having created my first short tube, I created a longer mould. Since the printer is small, I created the mould in four separate pieces. Again, held together by a strip of chopped fiberglass reinforcing the rear of the mould.

The moulds were sort of a process in themselves. Fiberglass reinforcement was key, because making a mould that is rigid enough using a limited amount of PLA filament turned out to be a dilemma in itself. Although the filament isn’t particularly expensive, chopped fiberglass is cheaper. I wanted to find a design that was rigid, but wouldn’t use up a whole roll of filament per mould, and could print relatively quickly. No 10 hour prints for just a quarter of a mould. I eventually settled on ~2mm thick 100% infill PLA at .25mm layers.

I think my most complex mould at this technique used up about 4-600g of filament. That was for the rear triangle, which I’ll get to later. The tubes were probably quite a bit less. 300g or so for the whole thing. Between the epoxy, filament, and fiberglass, I’d say each mould probably cost about $20-25 in materials. It was a good technique, and the moulds are definitely strong enough to be reusable.

With the tubes created, it was time to move on to the triangle, and more complex geometries. For these, I printed positive moulds to wrap the fabric around, in addition to the negative moulds.

It is possible but difficult to remove the positive mould from the final part. Acetone would probably help, but I didn’t actually try. I used some mould release on the positive mould, and was able to get the piece free around the edges.

Again skipping ahead a few steps, four rear triangle pieces (bad fits), a broken dropout, and a poorly-fit framebuilding jig, I used fiberglass and epoxy to join the pieces together into something that looks more like a bike.

I am currently at this stage, debating in my mind ways to wrap carbon fiber onto the lugs. I am apprehensive of this stage, because it’s where I failed last time. So I will probably end up over-reinforcing the lugs and making it heavier than it needs to be. I also ran out of carbon fiber, so this is where I will end this post until the next shipment comes in from China.