Hi maggs_43 and welcome to the forums!
I think there are 'anterior counterweights' that can be placed on the chair to balance things out. I tried a search and found mention of them, but no real information to pass along. Maybe someone else will have better luck.
I seem to remember reading once that the 'die-hard' wheelchair sports participants don't want the tump wheels on their chairs. I suppose it might hinder those tight wheelie turns. (?)