Have air valve so you can change pressure with an air pump. Chrome plated hubs are two piece so you can hide a wheel collar inside. These are VERY strong and will support heaviest airplanes. But they are very light -- from 1.8 oz to 3.5 oz. each. 6mm axle holes can be drilled to 1/4" or shimmed with 7/32" brass tubing (KSN130) to 3/16" axle.