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How to assess a cracked carbon frame

cracked carbon frame
Stu Bowers
13 Apr 2020

If you've discovered a hairline crack on your carbon frame, is it the paint or a ruined frame?

Perhaps you’ve been in a crash, or maybe your bike simply fell over in the back garden. You check the frame over and there’s a suspicious mark that wasn’t there before. Maybe it’s just the paint or lacquer that’s been compromised and the carbon is intact.

But how will you know for sure, and is it still safe to ride? Even taking it to your local bike shop for assessment might not help, as no matter how experienced an eye is cast over a frame, some levels of damage are simply invisible.

Your ears, though, may be able to tell you more.

'Carbon usually has a very crisp sound to it [when tapped] and when it’s damaged the tone changes completely,' says John Hansell of Yorkshire-based Fibre-Lyte, which offers a full inspection and repair service for carbon frames.

Hansell says, 'An experienced ear can tell you a lot more than just looking, although we do sometimes use an endoscope [internal camera] to see how a frame looks on the inside to back it up.'

Others are less convinced by the tap and listen approach, however, suggesting that while this may work on very uniform tube shapes (such as boat masts) there are too many complexities within the lay-ups involved in bicycle frame design to get clear indications from sound.

The use of x-rays to establish a frame’s integrity is also something frequently suggested, but Hansell is sceptical, saying, 'Using x-rays is a bit of a red herring.

'It’s used in aerospace but it would likely cost more than the frame, and even then it takes someone highly skilled to interpret the result clearly.'

An equally sophisticated but perhaps more plausible approach is employed by Volker Carl of German company Carl Messtechnik, a leading specialist in non-destructive materials testing (carbon-bike-check.com).

The process uses a combination of heat, ultrasonics and hi-resolution thermal imaging, and Carl says, 'I’ve tested around 2,000 frames now and found great success with this method.

'You can be sure if there is damage, even if you cannot see it with your eye, the heat flow will be affected and this will show up on the camera.'

That’s costly, so it pays to start with a bit of common sense. Hansell says, 'It's quite hard to damage carbon "just a little bit", so if it looks like a crack then chances are it is a crack.

'Also, the elasticity of carbon fibre is usually less than the paint, so if the paint is cracked, there’s a high chance there’s been some kind of trauma to the carbon too.'

The jury may still be out on the most reliable method to detect the extent of any damage, but one thing all agree on is that taking chances with carbon is never a good strategy, due to its propensity for catastrophic failure.

If you want peace of mind, the advice is get it checked thoroughly by an expert.