Hitch - Weight Carrying Capacity

Campin LI wrote:
When no WDH is used, it seems to me that say 1000 lbs on the hitch pushes down on the receiver and the rear most bolts on the receiver are in tension to support that weight and the forward most bolts do less and maybe nothing because the forward most part of the receiver pushes up against the frame, assuming there is some kind of rotational force due to the hitch connection (On edit, forward most bolts probably have some tension as well). Then when WDH is used, the rear most bolts are in tension from the hitch weight as above and the forward most part of the receiver is pulling away from the frame so the forward most bolts are now also in tension.

The approximation equations for forces on the receiver rails are fairly simple.

Let:
a = longitudinal distance from centroid of front bolts to centroid of rear bolts
b = longitudinal distance from centroid of rear bolts to ball
TW = tongue weight
M = counterclockwise moment (viewed from driver's side) exerted by WD bars on ball mount

Then, the combined reaction on the front bolts equals (M-b*TW)/a

and, the combined reaction on the rear bolts equals {(a+b)*TW-M}/a

where a positive value of reaction means the receiver rail is pulling down away from the frame, and a negative value means the rail is pushing up against the frame.

With no WD (M=0), the front of the receiver pushes against the frame with a force of TW*b/a. The rear of the receiver pulls down with a force of TW*(a+b)/a.

If: a=24", b=12", TW=1000#, and M=0, we have upward force at front = 500# and downward force at rear = 1500#

If: a=24", b= 12", TW=1000#, and M = 60,000 lb-in, we have downward force at front = 2000# and upward force at rear = 1000#.

The actual load in a bolt would depend on number of bolts at front and rear and the preload in any given bolt.

Ron