KingDarius

I'm looking for some advice regarding the repair of an Argon 18 Krypton 3000 ht which I bought off of craigslist. This was my first time buying a road bike so when a full carbon one popped up on craigslist, I thought why not 🤷‍♂️😬? When buying it, I checked the frame for cracks and gave it a test drive to make sure everything worked fine. I did notice that it took some effort to shift to the big ring, but I brushed it off as something that could be fixed by a quick tune-up. Anyway, once I began to do some maintenance on the bike, I realized that the the front derailleur moved around quite a bit, and when I tried to tighten it, the RivNuts that were holding the hangar to the frame they popped out revealing what seemed like a crack in-between the two RivNut holes. I took it to a local bike shop and the gentleman who worked there told me that it could probably be solved by a couple larger RivNuts. I also took it to another repair shop that specialized in repairing carbon fiber and they also told me that I should try with larger RivNuts before attempting to do a full on carbon repair because it would cost a lot more ($600). I was thinking of fabricating a metal backing plate that would support the area and the new RivNuts from the inside of the frame but i'm not sure if that's a good idea. Please let me know your thoughts on how I should move forward with the repair. I can't add any pictures to this post because I'm a new user so I made a post on Imgur - imgur.com/a/FcVLIGw

zandoval 

I don't have any experience with carbon repair; however, I agree with you that an accurately fit metal backing plate would certainly help reinforce the area.

And for me 600 bucks is really allot of money...

KingDarius

Thanks for the advice Zandoval. It's good that the backing plate idea has some potential. And yes, $600 is more than half of what I payed for the whole bike so it's almost completely out of the question.

cxwrench

I've re-riveted a bunch of derailleur mounts and bottle cage riv-nuts in carbon. Any Trek store can order the oversize ones or you can get them from someplace like McMaster. Reinforcing from the inside is an interesting idea and might work pretty well.

HillRider

If there is a crack joining the two original riv-nut holes, oversize riv-nuts are likely to make the situation worse. I agree an interior metal backing plate is a very good idea. I'd use a piece of aluminum curved to match the inside contour of the seat tube and inserted through the bottom bracket shell. It could be drilled to match the original riv-nut diameter and the tab derailleur fastened with longer riv-nuts or drilled and tapped and the derailleur tab fastened with machine screws.

KingDarius

Thanks, HillRider. Should I use adhesive to attach the aluminum plate to the tube and if so, what type? Also, could galvanic corrosion be an issue between the aluminum and the carbon or would that be solved by the glue? Another thought that I had was to use carbon patches instead of the backing plate and re-drill the holes after I put on the patch. This guy does something similar - youtu.be/jaLUZ4kwNU8 (still can’t hyperlink because I’m new)

Andrew R Stewart 

Some bike companies have suggested DP-460 for repairs. I have used some a few times with good success.

I will second the suggestion to reinforce the area before reinstalling the RivNuts. I would also use the epoxy when installing them.

I'll end with my usual- one more reason to not buy carbon. Andy

HillRider

I agree with the use of an epoxy as the adhesive to fasten the reinforcing plate in place and galvanic corrosion in that location will be a non issue.