packed full of rubber would actually balance anything. Rubber would lose it's shape rotating at those speeds right? Rubber helps absorb vibration so I'm more inclined to think it's made to dampen cabin vibration for comfort. If it was made for actual crank balance, wouldn't it be solid? Lastly, I've never heard anyone say that their UDP resulted in bearing failure. My bearings failed with the stock pulley so it kinda kills the theory for me.