Mower wont move after you changed rims?

Quite a few mowers have round shafted axles with a groove cut into them. The rim also has a groove cut into it. There is a small square piece of metal called a keyway that slide into the groove in the rim and axle. Once you have the rim on the mower spin the rim til these grooves line up with each other. Then slide the square keyway into that groove. It wedges between them and makes the axle turn the rim.