RE: Muzzy zero effect for roller guard?
Don:
I analyzed you situation yesterday and was able to come up with a possible cause/effect. If you have the nocking position too low, there is the possibility of the main arm and hook weight 'pulling' the cable out of the roller at the instance of the shot. This will cause a "slapping" of the hook into the back of the riser. This would be more prevalent on a higher poundage/higher speed/lighter arrow application. You can demo this by just pushing the cable forward where the attachment is made. You'll hear a click, which would sound more like a slap at high speed.
There are two possible fixes. First, if you don't want to change your 'tune', install a Sims Mini Limbsaver on the back of the riser so that the 'hook' wouldn't be allowed to move as far. I checked it out and this is a perfect thickness. Second, change your nocking point to make it higher.
As far as the main arm hitting your thumb, Muzzy and I discussed this at length. I was promoting more space but there are reasons why this can't happen. Therefore, if it were mine, I would put a nice radius on the inside of that main arm. If you radius it nicely, it will not affect performance or strength and will relieve your problem.
Hope this helps.