Lessons from the Practice Carbon Composite Bicycle Frame

As a practice piece to simply prove out concepts, this mini carbon composite frame was quite successful. What did I learn?

For the tubular structure of a bicycle frame and the resulting transitional geometry, the best method to compact the composite layup and remove excess resin is by wrapping with an elastic knit polyester, no vacuum bag necessary. However, this does not work well for flat or concave geometry which needs to be avoided with good design or supplemental compaction is necessary.

I started these tests with the assumption that the composite cores used for the bike frame tube transitions would need to be removed in some way. However, after finding that very hard to actually accomplish, and understanding how little the cores actually weigh, the plan now is to simply entomb the cores within the composite and leave them there.

For instance, this transition for the top tube to the seat tube holding the secondary pivot for the rear suspension is 6.28 cubic inches in volume. When printed, I can fill the interior at only 5%, so the 2 parts weigh under 7 grams. I will however need to verify the hollow nature of the core does not allow resin to leak to the interior thereby trapping excess resin. If so, I'll need to seal the cores before applying resin.

My other hope when starting the project was the ability to use the cores themselves to provide enough dimensional accuracy upon assembly to preclude the necessity of a frame fixture, or jig. I now believe this to be overly opptimistic of the capabilities of the 3D printed parts. Fit between the printed cores and the tubes even when designed to be forceful and interference'd still allowed a high degree of movement between important components of the bike frame. A minimal fixture will be necessary to hold the hard points of the frame in their place while applying (at least the first) layups of the carbon composite.



I paid little attention to the design of the transitional geometry between tubes on these concept tests because the purpose was more to test the composite techniques. However the design of these shapes will be crucial to the success of this composite bicycle frame. In general, I must not focus on what the exterior of the bike should look like, but rather how the inside of the frame needs to be shaped for proper reinforcement to be applied in the appropriate areas.