Aluminum caps with rubber seals that I use to replace all plastic tire caps on trailers, hand trucks and vehicles. Whenever I buy tires, I install metal air intakes. and it usually costs me nothing more or maybe $1 a tire. This is to prevent cuts and leaks in the tire's rubber core. I also attached these metal caps for better protection. I even use these metal caps on my garden carts, handcarts, trolleys, golf carts, vans, pickups, cars - anything with a tire. Plastic caps dry out in the sun, crack and fall off. I would suggest using them when working off-road or on construction sites. I found them expensive at $22+ for 100 - almost a quarter of each. I used to buy them for about 10 cents each.