Tests the rotational “roundness” of the rim without a tire.
The rim is placed in testing flat, with a measuring device on the outer edge at the rails. Measurements are taken continuously as the rim rotates 360 degrees. The rim can only vary by no more than 0.4mm throughout the rotation.
If a rim comes out of the box with major imperfections in the roundness, spokes will not be able to correct and/or maintain the roundness of the wheel. A more perfectly round wheel means a smoother and more controlled ride.