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 in) radius. The upper coupler plate surface should not bow downward (convex) more than 0.635 cm (1/4 in) within a 48.26 cm (19 in) radius or more than 0.3175 cm (1/8 in) at a radius of 25.4 cm (10 in) from the kingpin. The upper coupler plate surface should not bow upward (concave) more than 0.15875 cm (1/16 in) within a 48.26 cm (19 in) radius. (See

Figure 2 .)