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. 6″ wheel – 1-3/8″ od, 5/8″ id, 5/8″ bore, 1.671 lbs. per wheel, 2-3/4 ” width. Use 5/8″ axle.