I cannot find where I talked about this before, but Comet was using the pets at home wheel (which I didn't like). He could still fall out of it flinging himself around. I tried him with the wheel Ria recommends, but it was even worse, so I gave him back the pets at home one.
When I first got him I tried him on the 9 inch silent runner, but he couldn't/wouldn't use it.
Anyway after asking people on Instagram, I decided a bigger wheel had to be the way to go, so I tried him with a 12 inch Trixie wheel, which he just ignored and went and hung out in his tree (see previous posts).
So, last night I tried again with the 9 inch silent runner and oh my goodness he is not off the wheel now, never seen him use a wheel like he was doing last night - i really fell awful about it, but at least it's all sorted now and he can't possibly fling himself right round in it.
So, if you ever have an issue where your hamster is flinging itself around in its wheel and flying out, try a bigger and/or slightly heavier wheel, it may just work.
Comet the Robo on his 9inch Silent Runner Wheel. - YouTube
My conclusion is that he must have been still too small to use it when he first arrived as he was only 19g then and now he is about 34g.