Originally Posted by EricH

Ah, but that is a bit of a stretch. Belts and chains both stretch over time, especially for the first little bit right after they're put on as they wear in. Having no provision for dealing with that is asking for trouble. If you design for the final distance, the initial belt tension is going to be a bit tight, running the potential risk of a catastrophic stretch. If you design for the initial distance, you're going to be fine at first, but a little bit sloppy/slack later. Split the difference and get both sets of issues. It won't be much, but it will probably be enough to be noticeable.

There are a number of ways to deal with this--sliding bearing blocks come to mind as the typical WCD solution--but if you don't want to design tensioners in, probably the easiest solution to add later is the "floating idler" tensioner--drop a slightly larger-than-the-rest pulley/sprocket into the middle with no anchor and let it find a spot where the tension is right. Second easiest is a small spring-loaded pulley; install and use the spring to push the top of the chain or belt upwards a touch to increase the tension.