Originally Posted by Kimmo

I understand there are a number of high tech and quite expensive methods of investigating the structural integrity of composites, which provide more information for an engineer to make a theoretical assessment.

But someone without the inclination or means to afford such techniques can perform a practical assessment to a pretty reasonable degree of confidence, albeit insufficient for legal purposes.



In addition to the usual visual and tap and squeeze inspection, do the following:
Put the front wheel against a wall and give it a shove. Lean the bike over with the brakes on and stand on each pedal, give it a bounce. Put both wheels against the base of a wall, a hand each on seat and stem, and twist the frame. Put both hands on the seat, bend over so your belly is resting on your hands, and shove down. Any bike worth half a damn, except perhaps a tiny handful of bespoke ultimate weight weenie machines, can easily withstand such tests from all but the brawniest dudes.

Demonstrate the tests on the lightest bike in the shop first, tell the customer how any bike should be strong enough to survive such tests, and if their bike doesn't, you've saved them from it failing underneath them. If the bike passes without making any creaks, it should be good to go, but you can't give the customer a guarantee; all they have is witnessing the frame withstand forces extreme enough to simulate pretty much anything it'll cop during a ride, and the decision to trust it is up to them. If it does creak, try to eliminate the usual suspects before testing again, because that's probably more likely than frame damage.

Even if the frame has visible damage, depending on how overbuilt it is, it could still be serviceable. I have an '08 Tarmac I scored for free because it has a bit of impact damage above the FD. There's a crack maybe half an inch long across the right side of the tube, but because it's a relatively heavy frame at 1.2kg, there's plenty of redundancy, and I can't get a sound out of it. Theoretically, stiffness is compromised, and asymmetrically at that, but it's undetectable via seat of the pants. Been riding it for years without any visible change in the crack.

Carbon is bloody strong stuff. If in doubt, try to break it. If you can't break it by hand using leverage and all your strength, do you think it's suddenly going to fail when you're riding it?