This problem was beat to death on the old HPI forums. Hesky is correct, the rocks get thrown up by the tires. Usually the holes in the chassis allow the rocks to fall back through. Usually the cars stops suddenly when a big enough rock gets into the belts. When this happens, stop. Don't gun the throttle to try and force it forwards. Walk to the car and gently roll it backwards a few inches, then foward. You might have to go back and forth a few times. If its really stuck you might need a probe or small screwdriver to dislidge the rock. I stopped shreading belts when I stayed away from parking lots with decaying asphalt. If your local track uses a lot with a surface like that, your only solution is to keep 2 or 3 extra belts and extra pulley gears with you. I also try to check my all my gears between heats because sometimes crud or pebbles get lodged in between the teeth of the gears.