Fill tires with foam using canned minimal-expansion rigid foam. This process creates tires that are much heavier than air-filled tires and puts more stress on the rubber. Use it with heavy duty tires on construction equipment, small tractors or riding lawn mowers.
Place the lawnmower on jack stands so the tire is off the ground. Drill a hole in the tire that is large enough to accommodate the spray tip of the rigid foam. Position the hole directly across the wheel from the valve stem. Also remove the valve stem insert. Place the tube into the hole you drilled and press the button to fill the tire. When the foam begins exiting the valve stem, the tire is full of foam.
Leave the tires off the ground while the foam cures. Depending on the brand you choose, this could take several days. Lower the equipment to the ground and use it as normal. If you notice a tire getting soft, add more foam sealant to fill the voids. You may need to add foam more than once.
To have your tires professionally filled with foam, contact a local tire shop. Most perform the service for between $30 and $50, as of 2015.