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Originally Posted by DampRobot
First, there can be actual physical stretching in the chain, in the sense that there's some very large spring constant to the chain. You can probably stretch chains just a tiny bit as you're putting them together, but barring any extreme loading, this is a very small amount.
Second, the chain itself will wear over time. Grease in the tiny bearing surfaces in the chain links will dry up, and the chain links will physically wear so that the link to link distance increases very slightly. Unlike component one, this is a long term effect, and isn't reversible.
Third, the sprockets around the chain will wear. This doesn't have anything to do with the chain itself, but will manifest itself in the same way. Over time, the steel chain will wear aluminium sprockets, and the chain will become looser as the sprockets become very slightly smaller. In the absence of hard evidence, I would guess that this is the dominant effect behind long term "chain stretch."
In any case, I can't see a way in which a c-c design would stretch chain any more or less than a tensioned system set to exactly the same tension. The only difference is how you compensate for the stretch. In a sliding bearing block system, you just re-tension and you're good to go. In the case of c-c designs that asid was talking about, the wall of the tubing retains the chain, which keeps it from coming off or ratcheting on the sprockets, which makes the stretch less of an issue.
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I don't beleive the sprocket wearing is the main cause of chain stretch. Our elevator and drive chains both stretched over the course of the season, and in both cases, the black anodize coating wasn't scratched off or worn at all. Also, I've noticed that replacing old chain with new chain causes a pretty large increase in tension.
It appears to me that he chain stretch comes predominantly from the fit between the pins and the bushings loosening up over time. If you play with a new and old length of chain, you can feel the difference in flexibility.