How to tell if carbon frame is knackered

[Yellow Saddle]

Are carbon frames solid?

Yes and no. They have thin-walled, solid-wall tubes.
Some carbon wheels are filled with hard foam, so technically they are solid, I suppose.

 

[Yellow Saddle]

When it gets all technical and scientifical like this I can't follow. Is the plumber under the sink there to observe the crack in my arse or am I to study his?

You've got this arse about face. The clue lies in the word observe.

 

[Globalti]

Well, it is easy to figure out. Firstly, carbon doesn't crack in little bits like metal. It is either cracked or not. Cracks will happen not mid-section but at intersections and a quick inspection of the paint will reveal all. Secondly, a cracked carbon frame creaks. Thus, remove all wheels, chains and other semi-loose bits. Now find two strong blokes to manhandle the frame. Bend it this way and that way, pull on the fork, push on the fork etc etc and listen. Then, since it was rear-ended from the side (if that makes sense), put the wheels and chain back on and put the cranks in the 6 'o clock position, stand next to the bike and push hard on the crank with your foot, in an attempt to make the frame flex sideways.

If it passes this test, it is OK to go.

This is spot-on. The resin that makes the shape of a carbon frame is relatively weak and easily damaged by impacts from the wrong direction. However if you stress it from the intended directions, as designed, it is massively strong in the same way as concrete reinforced with steel rods.

The notion that you can x-ray a frame is twaddle - what would an x-ray show and where would you find someone with a machine they were willing to allow you to use? The hard glossy paint used on bike frames cracks easily and will show any damage.

This is my cycling buddy's right hand chainstay, which was crushed after he had an unfortunate meeting with a car. The bike bounced off the plastic front of the car, flew over my head and landed on top of a high hedge ten feet behind me. The brittle plastic has fractured but the carbon has held it together. Squeezing it shows that it crushes easily and is clearly a write-off:

 

[Yellow Saddle]

This is spot-on. The resin that makes the shape of a carbon frame is relatively weak and easily damaged by impacts from the wrong direction. However if you stress it from the intended directions, as designed, it is massively strong in the same way as concrete reinforced with steel rods.

The notion that you can x-ray a frame is twaddle - what would an x-ray show and where would you find someone with a machine they were willing to allow you to use? The hard glossy paint used on bike frames cracks easily and will show any damage.

This is my cycling buddy's right hand chainstay, which was crushed after he had an unfortunate meeting with a car. The bike bounced off the plastic front of the car, flew over my head and landed on top of a high hedge ten feet behind me. The brittle plastic has fractured but the carbon has held it together. Squeezing it shows that it crushes easily and is clearly a write-off:

That's a nice example. However, it is quite easy to repair actually. I've fixed plenty of such breaks with excellent success. Carbon is very forgiving to work with and no different from fibre glass in that aspect.

 

[Globalti]

My buddy kept the frame and we keep talking about repairing the chainstay this winter. It's not a massively stressed area and the inside half of the tube is intact. The difficulty will be in achieving the tapered oval shape. We could just sleeve the entire tube like a broken limb making it stronger than before.

 

[jiberjaber]

That could be quite an easy repair to do invisible... use the outside as a former to layup a carbon/glass 'tube'. Slice off the tube when cured, cut a hole in the frame where the existing damage is and slide and epoxy the new 'tube' inside - even better if you can get a bladder in side to press the 'tube' to the frame inside (perhaps through the BB?). Fill the hole with an epoxy/micro-balloon mixture, sand down, prime and paint. It would be an almost invisible repair. This would be subject to what the inside is like of course

Some ideas here: http://www.rcgroups.com/forums/showthread.php?t=880336

Repair of carbon stuff isn't all that hard, but needs a fair bit of planning and thought about the workflow.

 

[Globalti]

I think I understand that! The only concern is that the rear derailleur cable runs through that tube.

 

[Pumpkin the robot]

I wonder if this can be fixed?

 

[Mr Celine]

What no one has mentioned is that you must try to locate any cracks immediately, otherwise the moisture in the atmosphere will cause the carbon to melt together again. Unfortunately this usually only covers up the crack on the surface, disguising the unsafe flaw underneath.

To check for cracks in a frame that has been exposed to moisture after an accident have your frame looked at closely by this chap -

Then get an opinion from this mob.

If the cat scan and the lab report are OK the frame is safe.

 

[themosquitoking]

What no one has mentioned is that you must try to locate any cracks immediately, otherwise the moisture in the atmosphere will cause the carbon to melt together again. Unfortunately this usually only covers up the crack on the surface, disguising the unsafe flaw underneath.

To check for cracks in a frame that has been exposed to moisture after an accident have your frame looked at closely by this chap -

Then get an opinion from this mob.

If the cat scan and the lab report are OK the frame is safe.

I can't decide if this is the funniest thing i've read this week or if you tipify everything that is wrong with the internet.

 

[Yellow Saddle]

I can't decide if this is the funniest thing i've read this week or if you tipify everything that is wrong with the internet.

The latter. It has humour and lore blended into one deadly package.

 

[Yellow Saddle]

I think I understand that! The only concern is that the rear derailleur cable runs through that tube.

Engineer a re-route or, insert a little tube and do a single bypass operation.

 

[Yellow Saddle]

I wonder if this can be fixed?

No, don't be tempted. It will be difficult to align the bike properly and it may walk like a crab afterwards. But more important, that area is under a lot of stress and a failure is catastrophic. A failure on a chainstay or seatstay is survivable.

 

[jiberjaber]

I think I understand that! The only concern is that the rear derailleur cable runs through that tube.

Shouldn't be a problem, if you have a look through the link I posted, the repair to the booms on gliders also house the control push rods too, so if you used a bladder, then you would still have a void for the cable to run in.

*edit to clarify*
You would obvs need to remove anything running in/through/around the area the repair was being made... the bladder is inflated to ensure there is something pushing the 'tube' against the inside of the frame and is deflated and removed once the epoxy gluing the tube has cured sufficiently.

I just use a old inner tube, but you can use quite a few things depending on the dia you need... even a condom if you can rig it so you can inflate it.

In terms of the 'tube' you are in effect laying up some carbon & glass fibre and curing that in situ on the bike, then removing it (kind of like cutting a toilet tube lengthways) and because it should be oversized compared to the ID of the are you are repairing, you should be able to trim it to spring against the inside wall of chain stay. Its not an issue if it overlaps a bit - you are after longitudinal support here, but too much overlap may reduce the ID too much.

 

[Pumpkin the robot]

No, don't be tempted. It will be difficult to align the bike properly and it may walk like a crab afterwards. But more important, that area is under a lot of stress and a failure is catastrophic. A failure on a chainstay or seatstay is survivable.

One company I contacted thought it would be ok to fix it, the rest would not entertain the idea. I decided against it!