Originally Posted by
Chandne
Like I said, the carbon fiber does not seem to break down but the resins do...the resins that are the glue, essentially. Now, I do not work with carbon but am close to the composites made for the aerospace industry (by hexcel, for example). In the bike industry, they use similar composites but often much more resin. The resin can break down over time (and with heat).
Resins used in carbon composites remain stable for a very long time. They do soften at high temperatures, but the temperatures that bicycle components experience are nowhere near high enough to induce this softening.
The one exception: carbon rims with rim brakes. Extended braking downhill can generate enough heat to raise the rim temperature above the resin softening point, and the rim will fail. To avoid this, manufacturers now use higher temperature resin in rims.
Originally Posted by Chandne
Now the alloy thing is also from personal experience with some softer alloy rims that were great at first but eventually after a couple of years, started to feel a bit softer and needed more truing. I did not examine the alloy at a molecular level, obviously. When I called Notubes, they said that the lighter rims would break down and soften up over time and to stick with the Flows or stiff carbon rims.
Notubes was mistaken. The metal in those rims rims wasn't softening up. The rim was probably failing. Lightweight aluminum rims usually fail from cracks near the spoke holes.
Originally Posted by Chandne
My wheelbuilder (who may have read the same "soft alloy" discussions) said basically the same think.
Your wheelbuilder was also mistaken.
What happens from a crystalline structure level is obvious in metals when you heat and bend them but in rims over time/beating...does the crystalline structure slowly change too?
No. The crystalline structure of aluminum alloy does not materially change from stress cycles. And even if it did, the stiffness of the metal wouldn't change.
But cracks do form in aluminum rims, and grow, leading to failure.