I think this was answered a few hundred pages back, still searching, but what is the cause of the belt slipping off the pull under hard breaking?

I'm not braced but it's happening on carpet (track carpet not living room) on flat ground. I replaced the tensioner hardware which took some of the wobble out, but they didn't have the little wheel at the LHS.

I'm trying to remember if someone said the Al wheel worked or not, I think I remember someone saying NOT to use it for some reason.

Anyway, do you guys think it's just the wheel or should I check slipper or something else (need to soften my breaking power as well)