Why/when are a trailer's tandem wheels adjusted?

A handful of flatbeds do. No tankers do because of the nature of fluids to always "self evenly distribute"

 

You can get a trailer with "spread axles" that will allow you to avoid needing to move the tandems. Alot of the flat bed trailers feature this. I have seen it on vans so it isnt just skateboarders.

Basically figure the tandems like a seesaw. You dont want alot of weight one way or the other. Some drivers like to have more weight on their steers. Some companies (Western Express is one) that require their drivers that no matter the load to run their trailers on hole #5 from the front.

 

Most spread axle vans and reefers as well as some flat decks and steps that have spreads are also movable sliders.

 

It is for people who don't know how to load the trailer in such a way as to remain legal on axle weight. If trailer is loaded incorrectly you can fix your mistake in a state without bridge laws.

 

Man the state of trucking superstars lately

 

Some of you have never dealt with loads involving 3 and 4 pickups of a wide variety of products (floor loads no less) and where nobody has much to say regarding what may or may not work because they don't have the ability to time travel back and forth.

 

Gotta be psychic sometimes. Produce loads with multiple pickups will test your psychic abilities. They load it my way or it doesn't go on the trailer.
2000 pound oranges or lemons go single double single double sideways.
800 pound strawberries go double sideways. Lettuce always goes double sideways. Tandem always stays within the 41 foot mark or less.

 

Some things about tandems:

Moving them to the rear shifts weight from the rear axles to the front. Vice versa when you move Tandems to the front. My truck driving class had a bit of trouble realizing that even though you're moving the axles towards the front, you're also putting more weight to the rear doing so.

 

Some flatbeds do. Very few of them at my company do. They are almost always spread axle.

 

When moving trailer tandems, you are not really "shifting weight" per se. You are increasing or decreasing the amount of weight behind the trailer axle "pivot point", which works to "lift weight off of" the drive axles as the trailer pivot point is moved further forward