are those 15's? you've got a huge sidewall, so it'll have more give in it. lot harder to do that on lower profile tyres.
the way tyres are designed these days with dual locking bands etc, its a lot harder to pop em off.
hell i've run my tyre at 12PSI in a lap around a track and it didn't pop off. did get told off for not checking tyre pressures before goin out when I came into pits and complained about car not handling properly