Tech Talk: Can Carbon Fiber Frames Go Soft?

Dear Lennard,
If there is no external damage, can carbon fiber components be assumed to be structurally sound? If not, how can the component be effectively inspected for damage? Do carbon fiber frames and components ever go "soft"?

I realize that the modulus of the carbon fiber elements is unlikely to change, but was wondering if fiber-to-fiber bonds within the matrix can be compromised by stress, altering load distribution and potentially strain characteristics. I have frames and forks that I swear grew soft over time.

- Jim

Dear Jim,
Inspect all tubes for cracks, gouges, buckles, dents, and paint stretching or cracking, especially near the joints where stress is highest. With a carbon frame, use the "coin test" to check for damage to underlying carbon layers.

Tap on the tube with a coin in questionable areas and compare it with the sound on other tubes, in surrounding areas, and on the opposite side. If you have delamination or cracking in underlying carbon layers, especially in central areas away from the joints, you'll be able to hear the difference; the damaged fibers deaden the nice "clack" sound you hear when tapping on an undamaged tube.

If in doubt, take it to an expert for advice. Carbon structures that look good on the outside shouldn't get softer unless the layers start delaminating, and the coin test may be able to detect such areas.

- Lennard

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