The failure modes of carbon are such that there can either be minor or major indication of some sort of "trouble spot" on the frame. That's not to say that the frames are fragile, because they really aren't (at least P2's and P3's aren't), but a small ding in the outer layer could be an indication of fiber failure below the surface, or it could just be a cosmetic issue. If it looks bad it probably is bad, but it takes a substantial amount of directed force to do significant damage to a frame, so if substantial portions of the frame aren't all effed up and there aren't any marks that look worse than just the handlebars or a shoe tapping the top-tube or a rock dinging the underside of the frame there shouldn't be a whole lot of damage...

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Look for cracks (obvious). If you see them try flexing the frame and seeing if the crack widens. Some carbon bikes are notorious for cosmetic paint cracks at joints. Feel along the tubes and beware of dents.

Bike shops don't really have "experts" on this sort of things.

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Styrrell

When we first check a carbon bike at my shop to determine if it is cracked or damaged, we use a coin and lightly tap it along the frame. A normal frame should have a full sound when you tap it. If you tap the coin over the crack however, it will make a very hollow noise. It is a very distinct noise. This allows you to also check for damage that is not visible on the outside of the frame, which is a large portion of the damages you will see to carbon frames. Definitely an easy test for someone to do on their own.

Back in 09 when I got hit by a car whilst riding my SLC-SL the fork, handlebars and front wheel got totalled. I took it to a bike shop where the guy specialises in carbon bikes (build/repair etc). I don't know what it was, but they applied some sort of paint which gets absorbed into cracks and shows up under UV light (think CSI and male bodily fluids!). I think I was told it's very hard to find any other way.

You can actually use an ultrasonic sensor to "see" cracks that have formed - this is how they do airplane inspections - but I doubt anyone that's not in the carbon repair business would have one of those. Unless you make some friends with Materials Science grad students at a local university.

ride the bike really hard for a couple of days. if it breaks, you had a crack, if not, you're probably okay.

Just joking.
good question that I too was looking for the answer to; thanks all for the replies

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I thought it was the opposite - hollowish sound on a carbon frame = normal. Dull/dead = bad.

It depends on the frame. Different areas of a frame have thicker and thinner carbon and different area may or may not have foam or other filler inside.

Tapping a frame is about as good as tapping walls to find studs. Some people have a knack for it but its not highly accurate.


Most modern frames from large manufacturers will show visual signs of damage in the paint if the carbon underneath is damaged. Its very unusual to have the carbon weakened and not see some indication of impact if you look carefully. You are much more likely to find paint issues where nothing is wrong with the carbon than vice versa.

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Styrrell

This. In my former job our NDI division was part of a study which assessed the efficacy of various field-expedient inspection methods like coin tap. Don't remember the exact #'s but what I think I remember is that it took a delamination on the order of several square inches to ensure a 90%+ probability of detection. Across multiple operators, in uncontrolled environments, etc.

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Carl Matson

Ride it into the ground then!

Ultrasonic detection for damage on a carbon composite bike frame is overkill, and unless you know exactly how the frame was designed and laid up, what you think might be a void is just a tapered tube, with a thinner center section and thicker butted ends.

We find the most effective non-destructive inspection method is still just using a quarter and tap testing. A proper tube makes a distinct "click", a damaged tube makes a dull "thud". It's really easy to tell the difference. Occasionally we also use an endoscope to scan the inside of the tube (when possible), because sometimes a tube can be damaged on the inside with no visible problems on the outside.

Hope this helps.

Sincerely,

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Kurt Gensheimer
CarbonFrameRepair.com - Your source for quality, professional carbon bicycle frame repair
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Interesting, I did not realize this. At my shop, we used the coin test as a preliminary test to give a customer possible immediate feedback if there was something obvious, however many times, we would have to take a closer look at the bike, many times sending it back to the manufacturer. Its funny though because Trek was the company that told us to start doing it.

Carl, Do big manufacturers have more expert ways to gauge damage that they use to evaluate warranty claims?

it's not that it can't be effective, nor that it's inappropriate for bikes. it just shouldn't be considered a definitive method for determining the existence or extent of damage. it's extremely operator dependent, and in a structure on the scale of a bike (vs say, a helicopter blade) with more challenging curvatures etc it's not going to be as easy to tell if the change in sound is due to damage, or you've simply come across a ply dropoff or a hidden joint of some sort, or a patch of bondo, etc etc.

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Carl Matson

it's possible. it's probably also not likely to be very common to have it in-house. $pendy. and, anticipating the question: no, I can't comment on what we might have.

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Carl Matson

You're talking about Flourescent Penetrant Inspection, which work nicely for finding surface craks; it will not however detect underlying flaws\cracks.
The quick and simple coin tap test plus a good visual inpection [bring a 10X magifying glass] is your best bet, The tap test is most accurate if you have a similar serviceable frame side-byside, that "ring" of the coin should be the same for both bikes, if you tap a part of the frame that sound flat, it may have damage.

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res, non verba

Fill the frame up with water. See if anything squirts out. BAM. Crack detection.