Heat......
The worse belt I had only lasted for 50 miles.
Find a good source of belts and buy them three at a time. The cheaper the belt, the more variable the life-span.
If you aint smokin these cheap rubber pocket bike CVT belts, then chances are your motor hp and riding style isnt aggressive.
My rod did 22-25mph stock, 30-33mph with CVT. 43 mph with IGH jack shaft. Same engine.... 43 MPH is what Im getting now at the engines 'end of life' phase. there are a few thousand miles on the piston and jug....
I sure had a great time mod'ing the CVT's though befor I gave up. Cooling fans, air flow, seals, killer gear oil... The IGH was 85$ and is thousands of miles ahead.
The racers with CVT's got me beat off the line, but where they top out I shift into third.