Sudden jolts to the belts are what tears up the rubber in harmonic balancers....so high rpm 4spd shifts, nitrous hits, even turning the A/C on & off, will all wear out the rubber. I got a Summit SFI balancer on my Mustang and after about 3 yrs. the battery wouldn't stay charged & I couldn't figure out the problem (neither could 3 other mechanics & 1 Mopar dealership mechanic)....turns out the balancer rubber went bad and the outer part of the balancer was still spinning (where the pulley is on a Mustang) but just not as fast as the center part of the balancer. I was surprised it didn't squeal like a pig.