pretty sure they discontinued the belt drive system and have gone back to a shaft drive setup under the "Apex" series if I'm not mistaken.

With my SC18, I have found it necessary to use my dremel to file down the set screw in the pinion to prevent the belt from rubbing. If the pinion itself is rubbing, then you want to go with a larger spur/pinion to minimize the risk.

I think I'm currently running a 17/66 in my current setup... I think some folks were running 60T spurs at one point and were having issues with their belts.