Tests the “true” of the rim before laced.
The rim is laid flat on a granite surface. Measurements are taken continuously around the rim between the rim and the surface. The rim should remain less than or equal to 0.4mm from the surface throughout the rotation.
If a rim is not true when it comes out of the box, the spokes will not be able to correct and/or keep it in true. This means less maintenance on the rim when built up, smoother and more reliable braking, and a more controlled ride.