Since moving to California, I’ve found it more difficult to access machining resources like the ones I used in Ann Arbor. However, I still had a stack of carbon cloth and all the tools. I decided to build a few tube to tube frames, starting with a gravel frame for my Fiancee. Afterwards I built a carbon frame for myself – then promptly ruined it by both over-tightening the FD clamp and crashing it on a Bott dot. I decided the effort to sand, repair, and repaint was more than just making another- plus I wanted to open up the tire clearance for 30’s if I could.
I really liked the geometry I’d used in the last frame, so for the upcoming build I just fit the frame into the jig and used that as the basis of the next frame. The following is a picture of my super crappy jig with an old frame in it for reference.
Having the geometry figures in a spreadsheet, the next step is to miter (cope) the tubes. I should have gotten more pictures of this step, but the general idea is to get the tubes to sit against each other with as small a gap as possible. I do all my mitering freehand with a dremel, then finish with a fine tooth round file and sandpaper.
After mitering, tubes go into the jig, and a structural adhesive is applied to the joint to “tack” the tubes together into the final frame shape.
After the adhesive dries, alignment is checked on the glass table to make sure everything is straight. At this point, if things need adjustment it is still possible to sand away the adhesive at the joints carefully and separate the tubes for another try. Luckily, I have only had to do that once ever, as the adhesive is VERY strong.
The next step is to do a wet layup on the joint. If I had the room for an oven, I would use prepreg as it’s significantly easier to achieve a good surface finish directly from cure… but life isn’t perfect so here we are. Steps are:
- Prepare the surface by wiping with acetone, sanding, then acetone again (clean surfaces ALWAYS when using composites!)
- Cover any surfaces you don’t intend to bond to with electrical tape to prevent drips, errant fibers, etc. + fill holes with foam to prevent the bag from popping
- Cut dry carbon cloth into the approximate shape of what you need, but a bit larger
- Saturate cloth with 2 part laminating epoxy
- Wrap cloth around joints after waiting about 15-30 minutes- this is important or the cloth will just slide off the frames. The extra time lets it develop some tack
- Cover the joints with peel ply and breather fabric, then put the whole thing into a vacuum bag and pull down the pressure slowly, checking for bridging, tight areas, and leaks
- Assuming the bag holds pressure, wait 24h and then remove. Voila!
After this step, all that remains is to clean up the joints with some light sanding and rivet on the cable stops, bottle bosses and front derailleur mount (I learned! No band on clamps!). Once the frame is done, my Fiancee gives it a wild paint scheme and we’re off to the races. I’ll post the build once it’s complete and include a ride report.
OTB Engineering 