5th Wheel Kingpin Rating

With respect to horizontal rating, most over-the-road 5th wheels publish a maximum drawbar pull capacity of 150,000 lbs.  For a rating in excess of this you would have to purchase a specialty product with a very hefty price tag to boot. This quoted rating refers to the amount of horizontal force the 5th wheel can withstand, not the maximum allowable trailer weight.  Horizontal force is defined in the picture above.

To determine if the trailer weight of 250,000 pounds is acceptable, we can look at a couple practical examples to understand how much force the 5th wheel will encounter. 

Example 1:  The peak horizontal force the 5th wheel will see is limited by how much traction the tractor has at the ground.  The maximum force is exerted just before the drive tires spin under power.  So, if we calculate the maximum tractive force of the tractor, we’ll know how much force is transmitted through the 5th wheel.  

The formula is simple:

Multiply the vertical load on the drive axles by the coefficient of friction of the road surface.  Let us assume 78,000 pounds on drive axles and a worst-case coefficient of friction of 1.0.  Therefore, the maximum horizontal force is 78,000 lbs x 1.00 = 78,000 lbs.  This is well within the capacity of the 150,000 lbs. 

Example 2: Another example is to calculate the Total Grade Resistance of the trailer, or in other words the amount of force required to pull a trailer up a given grade on a given road surface.

The formula for total grade resistance or TGR = (Gross Weight) x (Grade% + Rolling Resistance%) / 100.  In this example, we will use 250,000 pounds for the gross trailer weight, 8% for the grade and 16% for the rolling resistance which is representative of rutted soft ground.  So, the TGR = 250,000 x (8% + 16%)/100 = 60,000 lbs.  This is also well within the drawbar pull rating of 150,000 lbs.

None of us want to encounter driver Joe’s situation above. Prime Mover scenarios aside, we can rest assured that most over-the-road 5th wheel offerings have the pulling capability for your particular application.