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On topic: His wheelchair company should use his story as proof of the rugged construction of his chair. I know every time I got around 20 mph on a bicycle, the wheels would start vibrating out of control, because they were out-of-round. I can't imagine going 50.
Or maybe his chair's wheels weren't touching? It looks like they were. I can imagine the chair's wheels getting caught up and forcing the chair into a position such that the kid was pitched out of it. It's a good thing that didn't happen.