Closing the gap

Trailer gap is measured from the nose of the trailer to the back panel of the sleeper. Sleeper extenders are not a consideration for the dimension. If you can decrease this measurement, aerodynamics is improved, and 42 inches is the target value. Especially if operating above 55 MPH where fuel economy is exponentially affected. When pulling a reefer, the added dimension of the refrigeration unit does not affect this value. Today’s reefers fit within the radius created by the corner of the trailer in a turn.

It is imperative to get all the trailer dimensions, most important the kingpin location along with the landing gear. A 36” kingpin setting is the most common for box trailers, however there is no standard with many vocational trailers varying +/-. You want to design for the worst-case scenario (largest value if the customer has multiple trailers).

There is no standard landing gear location, this value is required to ensure the outer corner of the mudflaps (or if no mudflap outboard edge of the tires) does not contact the landing gear structure at the centerline of the trailer in a turn. Most sales tools do not calculate this dimension, so you’ll either have to do it long hand, or use a Z-factor calculator. The important thing is to gather the information.

5th wheel position, calculated to properly load the steer and drive axles when fully loaded, must also consider these dimensions. As you close the trailer gap you are shifting weight forward to the steer axle. If the landing gear wont clear then you need to move the 5th wheel rearward, unloading the steer axle. Many times, you may have to determine what is important, as it could be a tradeoff.