Carbon bike frame repair
I bought a cheap Chinese carbon 29er hardtail frame to build up with 1x11 for muddy winter rides. I was extremely pleased with it considering the price paid until I noticed a massive crack in the top tube!
It was obviously not worth paying someone to repair. After encouragement from folks on the SingleTrackWorld forums (and inspired by bigdean's full carbon home build!) I decided to try repairing it myself.
I bought a kit containing the essentials including carbon fibre material, epoxy (resign and hardener), shrink wrap with release coating from eBay.
The main time consuming part was frame preparation. I started by sanding the area around the crack by hand. After I revealed more of the crack it seemed the frame had cracked along a bonded joint because the fissure was basically straight across.
Sanding by hand got boring quickly so I used a sanding attachment on an electric drill and finished the trickier parts around the seat post by hand. I did it outside and used my Respro cycling pollution mask as a lot more dust was generated.
The accepted wisdom seemed to be completely remove the damaged part round the crack. It was much bigger than I expected. It seemed the area didn't have enough epoxy generally as the broken part was quite flakey.
I used polystyrene to fill the gap so the original shape of the tube would be retained . I must admit it took two attempts to get something I was willing to proceed with.
Then I made a template for cutting out the carbon fibre material (actually to be honest I started this after the first polystyrene filler thing that I discarded, then I made a better template).
Then I made my first bad mistake- I jumped straight to mixing the epoxy rather than preparing all the pieces of carbon. As soon as the hardener and resin are mixed you are against the clock. The carbon took longer to cut than I imagined. The strands at the edges came loose. After the first layer I was convinced this project was a stupid waste of time and money. I compacted the layers down with plastering gauze tape, which also kept the edges of the carbon material from lifting. I alternated layers at 0 and +\- 30 degrees (approximately). By the third layer I felt more confident. By the fifth (final) layer I thought maybe building my own carbon frame would be a fun project!
I took no photos of the layering because I was wearing epoxy-covered nitrile gloves (came in the kit) and felt under time pressure.
After layering I wound the shrink wrap tape with what I hoped was the release-agent coated side against the fibre (another mistake- should have checked beforehand). I poked lots of little holes in the tape to set the excess epoxy escape.
At this point I was feeling pleased with progress and granted myself the pleasure of admiring the mess I'd made.
I had a quick tidy and moved back into the shed. It was time for a another big mistake. I didn't check what temperature to use with the shrink wrap tape. The lowest temperature on the heat gun (50) didn't seem to do much so I just whacked it up to 400. The tape quickly contracted and glue oozed out as expected. However, parts of the tape just melted away to nothing before I realised what was happening. Also one part of the polystyrene got compressed down so there is now a narrow part in the tube. More worryingly the epoxy curing process may have been compromised by the heat (parts already seemed rigid after the heat gun treatment).
After a day (~demould time) I peeled off what tape I could and then sanded off the melted parts. The surface is more uneven than I hoped for (especially where the polystyrene got squashed) but overall it looks reasonable as far as ghetto repairs go.
I waited patiently for the rest of the day pondering whether to spray the frame luminous green or orange while it was completely stripped. Then, suddenly I couldn't wait to rebuild. I figured gently bolting on a few components wouldn't stress the repair. This was a silly thing to do, not least because there was then overwhelming temptation to take it for a quick spin, which I later did! So there are now two potential flaws in the curing- application of excessive heat and premature stress. I henceforth will refer to the bike as DeathTrap.
A few weeks later after more test rides I put clear lacquer on with UV protection (Autotek Professional Clear Lacquer).