They are made of rubber, and plenty of it. I hauled many 44,000 pound loads of synthetic rubber from the refinery at Beaumont to the Michelin plant in Greenville SC. Good Lord do they crank out that rubber, it comes in blocks wrapped with cellophane, weigh 73 pounds each, made from oil. Looks like vanilla taffy. Every ten seconds another block comes down that conveyor belt, and must be loaded by hand into the crate. Good workout for a young man.
They blend it with natural rubber at the tire plant.
