6″ azusalite made of space age glass reinforced Zytel nylon combines very high tensile and compression strength and good impact resistance making them worthy of comparsion with similar die cast aluminum. Black in color, has precision ball bearing which is top quality for higher speed with no lubrication. 6″ wheel has 1-3/8 od, 5/8″id, 5/8″ bore, 1.671 lbs per wheel, 2-3/4″ width.