Kingpin setting vs axle weight

It appears to serve the same function as sliding trailer tandems on a trailer with fixed tandems in that it moves weight between the drives and trailer axles, but it also changes the overall length of the truck in the way a sliding 5th wheel does.

 

It would help in states where they really restrict you trailer wheelbase like California. If your trailer tandems were over weight a bit, but due to the rules you couldn't slide them back any, then sliding back the king pin could help you out some.

 

Picture your axles or just any axle as the fulcrum on a teeter totter. You add a weight. The closer you move it to the fulcrum, the heavier that point is. That's all your doing. The extra 18" on a 3' pin puts the weight of the trailer another 18" deeper into your tractor.
They could probably make it work just fine with an 18" pin, but it wouldn't be anywhere near as aerodynamic. 36" pins aren't that old, I've been in this game 11 years, and when I started a lot of the vans & reefers had 30" pins. They came up with 36" pin in conjunction with the new style tractors ( Century class era ) to close the gap between the back of the tractor and the front of the trailer.

Did this completely confuse now?

Put a concrete block on a 2x4. Put your foot under it. Slide the block across the 2x4.