If you make the arm tight against the motor and the sheeve isn't perfect [they never are] the belt will stress the rear spokes and often break several if the sheeve is off center or out of round. The spokes don't like the belt going loose & tight and often snap the head off of the spokes at the hub. I would guess I replaced at least 30 or 40 spokes in different wheels before I learned this lesson. The vintage Whizzer clutch arm & bushings [which the current company tried to copy] used the spring to compensate for the rear sheeve, and to keep the belt tight, and I have never had any spokes break on any vintage Whizzer ever.
There is a lot of difference between a smooth, secure, & well thought out vintage clutch as opposed to the current setup. For example, most current clutch arms are secured by passing the bolt through the motor, then the hole in the arm and then a nut. I always turned this around on all the bikes I sold because if the nut works loose [and it has happened] the arm can simply fall off, whereas if the bolt is run through the arm then the motor, then the nut, loosing the nut isn't as destructive because the long bolt will drift out and be stopped by the clutch pulley. It is important to note that a properly aligned clutch works about 200% better than one that is loose, or too tight, or running at an angle.