Each skateboard wheel is mounted on its axle via two bearings.
With few exceptions, the bearings are the industrial standard "608"
size, with a bore of 8 mm, an outer diameter of 22 mm, and a width of
7 mm. These are usually made of steel, though silicon nitride, a
high-tech ceramic, is sometimes used. Many skateboard bearings are
graded according to the ABEC scale. The scale starts with ABEC1 as the lowest, 3, 5, 7, 9.