Sexy_Lugs

Good morning all,

I noticed a small crack (around 2-3 inches) in the seat tube heading into the bottom bracket area (under the carbon fiber overlay) of my bamboo bike. It does not seem to have created any issues in the carbon fiber, but I cannot tell how far it goes down. Do you think this is repairable with some cement clue and good brace? what would you do to fix this?

Attached is a photo for reference, and as always, thanks for all the help.

unterhausen

I think I would wrap it with more carbon

Sexy_Lugs

How would I do this? Sorry, I have no idea how the process works in this type of situation.

Sexy_Lugs

What I am currently thinking of doing is drilling a small hole at the tip of the crack, stopping further spreading, then epoxing it with a boat filler, then wrapping it in carbon. do you think this would be a good solution?

jadocs

Do a YouTube search for "Boogaali Bikes", they build bamboo bikes out of Ghana and some of the videos show how they make joints. It may give you some ideas to use same method to wrap/reinforce the bottom of that seat tube.

unterhausen

I thought you had made the bike, I take it you purchased the frame?

stop drilling is a really pointless in bamboo. There is no good reason to stop drill in metal either, it's just something people have heard about and done without knowing why. They certainly don't stop drill cracks in guitars, which are highly loaded structures.

Batstar

I’m not familiar with bamboo but I have built a couple of wooden kayaks one from cedar strips and the other from marine grade plywood. For joinery, I’ve often used filets of thickened epoxy (resin/hardener) combined with fine sanding dust from the build or silica powder. This creates a non-slumping mixture which can be pushed through into the crack creating an extremely a strong joint. The joint can be sanded smooth and the small section of the seat tube glassed all around. Working with carbon fiber can be finicky and the dried strands can be hard to remove afterwards but when fully cured it is more rigid. Fiberglass, on the other hand, is quite forgiving to work with and dries clear after wetting out with resin.

West System offers complete small handy repair kits for this very purpose.

Canaboo

You can just fill it with suoerglue.

Boo bicycles is starting to run into cracking problems with some of their frames and even have a how to repair splits with minwax pencils and nailpolish.

unterhausen

So Boo is suggesting cosmetic-only repairs? Or flow supeglue first?

On cracks in guitars, repairmen usually use some method to force glue into a crack. They often use a small suction cup used as a pump. There are plenty of videos

fooferdoggie 

yes though I would use a regular epoxy thats thickened a bit. so it soaks into the bamboo.

Canaboo

The repair pencils have some sort of bonding capacity.

With a crack like the one pictured an adequate amount of epoxy will flow in with just gravity and wicking. Unless the split actually goes entirely through. Then some thickener will stop it from flowing right through.

I am a bit surprised that Boo can charge close to $4000 for a frame and people won't ask for a replacement if the frame cracks.

alo

A couple of other people have already suggested epoxy. I would glue it with epoxy. You need to ensure the epoxy gets into the joint. Lie the bike on its side when you do it, so the epoxy does not run down onto the moving parts.

Canaboo

Lie it on its side and build a barrier around the crack with play dough or similar before you pour the glue.

Batstar

It will be a much easier applying an epoxy with a higher viscosity akin to a peanut butter-like consistency. Fine sawdust works great as a thickener. Superglue will not be thick enough to adhere to the walls of the crack unless there is some backing inside the seat tube. A small spatula will suffice to force the mixture into the aperture. Bear in mind though, the more epoxy you apply, the more sanding is required. One could leave just enough so that it stands a bit proud on the surface of the crack. Glassing around the tube with CF or fiberglass will also add rigidity to the structure. Dealing with a small crack of this size, you won’t require many materials.

jadocs

I would apply epoxy and reinforce the exterior at that location.