When measuring the downward deformation of the gooseneck tunnel, what reference point is used?

The correct choice is the top face of the bottom side rail because this point serves as a critical reference when measuring downward deformation in the gooseneck tunnel of a dry van trailer. This area is structurally significant because it helps ensure consistency in measurements. The deformation can influence how the trailer maintains its structural integrity under load and how it interacts with the chassis and other components.

Choosing a reference point like the top face of the bottom side rail allows for a more straightforward assessment of the deformation—it provides a stable and easily identifiable point for evaluation. The other reference points are not as ideal because they may not represent the overall structural alignment and performance of the gooseneck tunnel during loading conditions as effectively as the top face of the bottom side rail does.