I was have the same problem with mine. Was going through belts every few weeks. I was running out of ideas what was causing it. After stripping my kf down to fit a new belt one time I checked every thing I could check. It turned out I had a fault with my chassis. The holes at the back that hold the gear box in place where not the same distance apart. There was a mm difference so it was causing the gear box to not sit straight and faucing the belt to pushed over to one side and rub on the top deck.
Hope this helps you out. It's worth looking at just to check.
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