I was just investigating the cause of a non working VCR, and found the belt had gone rather saggy with age... Fortunately I managed to track down an already sized part, but the thought occurred to me, is there design guide or rule of thumb for rubber belt selection?
Knowing the two pulley sizes and their separation, its easy to work out the circumference of the belt path, but presumably one needs one a bit shorter so that it stretches a little when applied.