Some may find the following reason enough to avoid a belt driven PSRU.

I just completed the most painful part of my annual - tracking the belt on my PSRU. Many thanks to my son, a commercial pilot, for sitting at the controls and managing the engine.

I have a Soob EA81 with a Reduction PSRU. The belt has not been tracked for 70 hours (since installed) and there were minor signs of misalignment towards the back limit. The
belt is in great shape and I wanted to keep it that way.

Some of the issues we encounter:
- when the engine warms the aluminum back plate on the PSRU expands tracking the belt forward
- RPM matters and dramatically changes tracking
- the Reductions manual states fine adjustment -- just pressure on the adjustment nut and they mean it
- tracking without load (prop) does not work.

On my third attempt: After tracking was perfect at 1500 RPM I ran it up to 2500, and all was lost. The belt moved forward. Finally we tied the plane down and ran it up to 3500 - me standing 1ft away from the prop (my head about 8 inches) playing with the adjustment nut. Yes... one “adjustment nut” playing with another: Not a great feeling and darn cold to boot but the tracking seemed fine. The belt moved to the back limit at 1500 but at 3800 it seemed to be sitting off the front limit. A higher RPM made both of us too nervous to say the least. But, we did need to make sure since loosing a belt on departure at 4800-4900 RPM would be nasty.

4000 RPM -- My son came up with a brilliant idea and set up a spotter scope 20 feet away and zoomed in on the belt. Sitting at the controls he ran it up to 4000, the plane buffeting, the dust flying, and I watched from a safe distance slightly offset from the prop (just in case). To my surprise the belt moved forward to the front limit (just slightly off it) at 3600 and at 4000 started to track back leaving a 1mm gap. Good enough for me and we tighted all the bearing block bolts down. One more quick test confirmed all was well. I marked the front edge of the belt with a white grease pencil which will allow me to monitor the wear on the front side, just in case.

If anyone has a better way, I would be intrested. Otherwise - Happy tracking.