The steel tire is made about one-eighth-inch smaller in diameter than the wheel onto which it will be mounted. The process begins by heating the tire with propane torchs until it expands. Then the tire is pounded onto the wheel using sledge hammers and tolerance gauges. As it shrinks, it grips the wheel tightly and requires no additional mounting hardware. Rivets were used at first but as they wore down, tires slipped off wheels and derailments resulted.