4″ wheel , 5/8″ id., 1-3/8 od. Made of space-age glass reinforced Zytel Nylon combines very high tensile and compression strength and good impact resistance making them worthy of comparison with similar die cast aluminum. Black in color, has precision ball bearing which is top quality for higher speed with no lubrication. 4″ wheel: 1-3/8″ od, 5/8″ id, ball bearing. .957 lbs per wheel. 2-3/4″ width, uses 5/8″ axle. For ultralight use only. Cannot be used with Azusa brakes. Use with Azusa axles AZ-582, axle nuts AN365-1011A, and 5/8″ washer AN960C-1016 which can be ordered separately.