This SAE Standard applies to upper coupler kingpins for commercial trailers and semitrailers in the unladen condition. See Figure 1. A 90 degree ± 1 degree angle extends (in all directions) from the centerline of the kingpin to the upper coupler plate surface within a 48.26 cm (19 inch) radius. The upper coupler plate surface should not bow downward (convex) more than 0.635 cm (1/4 inch) within a 48.26 cm (19 inch) radius or more than 0.3175 cm (1/8 inch) at a radius of 25.4 cm (10 inches) from the kingpin. The upper coupler plate surface should not bow upward (concave) more than 0.15875 cm (1/16 inch) within a 48.26 cm (19 inch) radius. (See Figure 2.)