Tests how much vertical load a rim can withstand before failure.
The bottom half of the wheel is secured into a gauge in a vertical position with the valve on the side. Vertical pressure is then applied to the top of the rim, 90 degrees from valve, and pressure is increased until crack appears in rim. Max load and type of crack are recorded.
This is a good measure of the overall durability of a rim. The more pressure it can withstand, the longer it should last on the road.