driving w/ tandams all the way back, '53

Almost correct. Couple things to add though:
1. Kingpin distance is for "off tracking" and is not always figured at the center of the tandem axle group. There are 3 different measurement locations...check the state for the location they use.
a. Center of forward rear axle.
b. Center of both rear axles.
c. Center rear tandem axle.

CA is 40' from King Pin to center of rear axle.

 

I've never seen a limitation in GA, where did you see this?

 

What does GA enforce? I've never seen a max length listed for GA

 

My quote covered your points "b and c". What state uses point "a"

 

See what happens when you let your fingers do the waking before they wrap around the handle of a cup of coffee! NOW I SEE IT!
Now you want me to think?

That could constitute abuse......if I were 45 years younger!

been so long since I looked.....back when UT changed from 41' to unrestricted.....but you're right....only "b" and "c"!

 

My bad, I think it's La.

 

Very incorrect. Up until recently (within the last two weeks) I was dragging a 53' RGN. The only state that had a problem with it was CA. Might want to check facts prior to posting falsehoods.

 

I'm going to beg to differ on this one, Otter. While you do indeed have 2 lift points with the tandems forward, I would think the total effected area would be less. The turbulence from the drives and tandems effectively shield part of that area from the wind. By sliding the tandems back, you actually lose part of that shield, and increase the underside area affected.

The are other benefits of tandems back, though. Back does increase polar moment, meaning it takes more force to yaw the trailer; but it also shifts more of the wind load to the tractor. With the tandems back, the side loads are roughly split between the drives and the tandems. Slide that tandem 10' forward, and suddenly that changes. Now the tandem has the side load of half of the trailer in front of the tandems plus all the side load of that 10' behind the tandems. So the side load area that affects the tandem goes from 53/2 = 26 1/2, to 43/2+10 = 31 1/2.

And it gets worse. Because that 10' of trailer is now outside of the drive-tandem spread, the side force back there now has a significant effect on yaw. Take a pencil, put a finger near each end, and push in the center. The whole pencil tries to move linearly in the direction you are pushing. (Yes, the trailer still yaws, but because there is more traction from the heavier drives.) Now move one of your fingers an inch or so in from the tip and push on that tip. As Scooby would say, "Rut Roh!" Now instead of just pushing us sideways, the wind is also trying to rotate the trailer. (If you want to be absolutely correct, the wind always trying to rotate the trailer. It's just that stretched out, it pushes the nose to rotate one direction, and the trailer the other; and the structure of the trailer prevents this, leaving only a lateral force.) So by sliding the tandem forward, not only do you decrease its resistance to yaw, you also introduce a yaw force into the mix.

Yep, while those wings do increase the total side force on the trailer, they definitely do help keep the air from the underside, reducing lift.



And I fear my cover as just another carhauler is now officially blown.

 

I get that bridge laws and off tracking laws are actually two different definitions, but they're functionally the same thing: you slide the tandems up to come into compliance on both of them. Kingpin/tandem distance is fairly easy to measure and enforce, but how do you really figure out if you're over on bridge law?

 

No big deal, I had just never heard of a state using the front trailer axle, so I thought I would learn something! None of this applies to my 48' step deck, but you never know when I'll finally lay out the cash for a 53' step or get a 53' RGN