Speed rings
What is Speed rings in skateboarding?
Speed rings are the thin axle washers that sit between the nut and the bearing and between the hanger and the bearing, cutting friction.
Extended definition
You run two per wheel, and the mounting order is speed ring, wheel, speed ring, nut. They keep the nut and the hanger from rubbing straight against the bearing. Built-in bearings come with the speed rings and spacer integrated, so there are fewer loose parts to lose.