Cracked Carbon Fix

Bump...sorry but I need more help....

Unless this is just a ding in the finish, there is no such thing as a non-structural crack. A crack through a carbon layup can't be repaired with any certainty. There is no way the bike shop can guarantee it won't fail, unless it truly is just the finish.

I just had an accident in which the top tube was cracked. Even though I didn't see any damage to the carbon handlebars, I'm replacing them as well...it's not worth risking catastrophic failure. Although a crank arm failure is not as catastrophic as handlebars or front forks, I would replace the cranks if they were cracked.

Remeber when bikes were made of steel?

Now you need a torque wrench to attach a bottle cage.  Oops my bike fell over in the garage while I was cleaning the chain and now I need a new frame.

Is it any wonder the entire bike industry has moved to push carbon bikes...  no longevity. 

 

this...

Carbon can be fixed, but not a lot of people do it. We're lucky that we have someone local who does: Robert's Sailboards

He's fixed a lot of frames and wheels for the cyclists around here.

For a crank though, I'd just replace it.

Edited by jeng 2010-08-16 6:52 PM

Yeah, I mean it's not like carbon fiber has any performance increases to make it worth it.  /sarcasm

Yes, there is no such thing as a non structural component of a crank.  THe entire crank is under stress.  

A carbon monocoque typically cannot be repaired.  Carbon fiber layups can have its life extended sometimes by laying more carbon on the non cosmetic side (the side that isn't shiney).  In this case its not a complete repair and it will fail again.

Carbon fiber wheels were outlawed in MOTO GP racing because stress fractures in carbon are hard to detect, and once a few micro fractures develop, the wheel will fail catastrophically.    They banned CF wheels after a handfull of accidents which were caused by undetectable microfractures at the rear wheel.

 

even though its just a crank arm, get new ones.  Contrary to popular belief carbon fiber is incredibly strong and takes much more of a hit than most people think to break down.

Carbon fiber derives its strength from the layers of carbon fibers layered over each other.  The layers are bonded together with some kind of binding agent.  If I'm understanding your description correctly you have a crack down one of those directional layers.  It will eventually propogate and the crank arm will break down, and it will most likely happening when you stand to hammer up a hill.

Step lightly my friend. 

Okay....I really didn't explain that well....It is a metal crank arm covered in carbon to make it look cool...the load is handled by the underlying metal structure.  If the crank arm was only made of carbon fiber it would have failed under my massive power (I wish I had that kind of power)

So all im looking to do now is seal up the crack so water, dirt, etc don't get in there.  It's been mainly re-glued to the metal but there is still a crack.... 

lol why didn't you just say so!!!   Glue away.

Mix 2 part epoxy, spread evenly.  Nail polish won't do SH17 

Edited by funkyd04 2010-08-16 8:23 PM

can you post a pic...i am having a hard time picturing what you are talking about...sounds like it could be ok...

Okay....it almost the same as this one....

http://www.fullspeedahead.com/fly.aspx?taxid=53&layout=product

So you can see that it's solid metal construction but it has a carbon "cover."  It's that cover that cracking...so the bike shop injected epoxy into the crack to adhere it back to the metal crank arm.  It did a great job.  

Now there is just a small surface crack remaining that needs to be sealed up...the major cracking has been fixed....