Belt tightness? (2 replies)

I have a CoreXY printer that's a few years old (this design: [www.thingiverse.com]). It has worked very well, but lately I'd noticed that the carriage was getting stiff to move around, especially in the Y axis, and print quality was deteriorating. So I decided to investigate. I found that the rods that run on each side for the Y axis were no longer parallel - I believe that this was due to the belt tension pulling the front-right bearing support inwards. It was only a tiny amount, I could only see a sliver of light through the gap, but it was enough to make it impossible to set the X axis rods accurately to allow a smooth movement in the Y axis. I have repositioned everything and now both X and Y move very easily... but now I have to address various design aspects, mostly to do with stopping the front pulley supports being pulled out of line by the belts. I might have to redesign a couple of parts, but that isn't too significant a problem.

What I really need to get to grips with is the correct tension for the belts - I believe that a major part of my problem was over-tightened belts. Not knowing how tight they should be I had set them up so that there was a significant note when I strummed the belts but, on reflection, I think they shouldn't have been so tight.

So, my big question is, how tight should the belts be on a CoreXY? Obviously they should be tight enough that they don't jump the teeth on the motor pulleys, but I have no idea what 'correct tightness' is.

Can anyone advise?