When and why to replace synchronous spindle belts.

I am looking for concrete facts or evidence that replacing older but otherwise geometrically perfect belts will do anything to improve noise, finish, etc?

I read where everyone seems to recommend "change the belts" but I have not once replaced a sync belt that did anything better other than make me walk lighter (cash). Some here recommend replacement for spindle noises, vibrations, etc but I have not found a sync belt reference yet that replacement actually did anything and I sort of suspect any improvement might be more from belt tension adjustment. Indications from Gates is that the belts won't stretch so unless there is wear or damage to the belts, replacement is not needed.

The reason I ask is I have a couple with a little memory in them from sitting. not perfectly round. However, they lock into the sprocket teeth like new. Even though they are old, I am trying to justify why I would replace them. ?

In my experience, replacing an old belt that has taken a set will reduce noise sometimes. I've never heard of or seen any case where it improved a finish issue.

When I got my Mori, it had been in storage for about 9 years. The belt was so set that it was kind of hard to rotate the spindle by hand out of the "set" position. It also made a rhythmic "thrumming" noise when running. It's still in there today, 2 years later. Runs quiet and rotates freely. Just needed a bit of exercise to relax the set it had taken.

If yours show signs of cracking or have any "flappers" then I'd advise replacement. Otherwise go make some parts

One thing a lot of people forget, once you have worn out a couple of timing belts of more or less any flavour, especially in a dusty enviroment, its just like a chain, you got worn sprockets and there going to need replacing too. other wise noise and issues will increase badly!

Most belts have a set when new - sat for a while, once used for a hour or two - warmed up there fine again.