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

weight restrictions on the axles.

moving the axles moves the weight,forward or back.

 

Axle weight limit, bridge law, kingpin law

But mainly to balance weight. You don't want all the weight on the trailer or drives, but to spread them out evenly

 

Mostly to distribute weight.

If the load is lite, put them where ever. Maybe push them to the middle or front for easier turning.

 

Aside from weight issues, you have a tighter turning radius with them slid all the way up. Slid all the way back makes it easier to estimate where the back end and off side of the trailer is when backing. You will eventually find docks that need all of one and none of the other. In other words, some docks have tight side clearances, but plenty of room to do an alley dock. Slide them all the way back. Others have plenty of room at the sides, but little room in front of you for maneuvering. Slid forward can save you here. Your worst nightmare is tight side clearances combined with a lack of maneuvering room.

Finally, a lot of shippers prefer tandems slid all the way back. It makes it less likely the forklift will push the trailer away from the dock while loading and unloading.

 

Also Forklift has less bounce when tandems are all the way back, a note you never ever drive with tandems full back.

Regarding the hitting anything, that is where you come in. The Professional. Managing two turning circles, your tractor's and your trailer's that change as necessary. If I recall it's 50 to 70 feet down to zero or minus (When you are pivoting in place by force)

There is another reason not yet touched upon by the previous posters, Ride quality. A properly placed tandem in either the trailer and or the slidable 5th wheel (Which is not moved very often, keeping the front of trailer equal to or just behind the vertical front of the forward drive tires...) will improve your ride very much, If you were loaded certain heavy on one end of the trailer, it will try to beat you black and blue all the way down the road or handle like a huge anchor from God preventing you from doing anything and possibly even inducing understeer or oversteer etc.

The holes on the rails on which the tandems rest mean a certain weight usually 250, 500 or 750 pounds each. To slide forward towards drives is to gather weight to slide back from drives is to load the tractor with more.

There are additional operations possible with trailer when you are in a tight place in addiition to sliding tandems all the way up. You unhook and then approach it from the side very extreme gentleness for it's king pin with your 5th wheel. Once you capture it you can spin it out of there. Minding your ### end so you don't tear into something on the other side of it.

 

Like ppl said it's all about the weight each hole on the tandems is anywhere between 250 to 500 lbs depending how far they are spaced apart.You'll learn all this as you go and also ask other drivers for help.

 

To state another way what has been replied ... You're limited to 34k lbs on a dual axle set. Understanding that loads in trailers all vary (with weight distribution), then there needs to be a way to try and counter this to keep weight legal on trailer [and tractor]

Where the tandems are situated will alter the weight in the trailer, and thereby altering the weight on the tractor.