We retrofitted a Gates Carbon Belt to our Co-Motion Primera two years ago and have been very happy with it. Everywhere we read, "the Carbon Drive lasts as much as 10 times longer" than a timing chain. We are nearing 10,000 miles with the belt.

My question is, how do we know when it is time to replace it? What indicates that the belt needs to be replaced or is about to fail? So far, we have not heard of anyone replacing their belts. (We have bought a replacement belt - just haven't installed it.)

Yesterday on a 200k brevet, we started the ride and during the first ascents heard a new noise when the captain stood up in the pedals. A random clack sound. It took us a couple miles to diagnose it as the timing belt slipping (we've never had any problems with it before). What helped us diagnose it was our pedals became out of phase. The eccentric was very tight so we don't think it had loosened. Does that mean the belt stretched - which we didn't think was possible. Could this be a sign?

Could the teeth on the belt being wearing down so it doesn't mesh as well with the rings?

Has anyone else replaced their belt, and what were the signs or reasons to do so.