system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the center axle on dump trucks used for? And what is it called? The only time I ever see them down is if they are loaded. But I’ve seen them up with a full load too.
gpt
Former dump  truck owner here. 

The center lift axle increases load capacity so more, and / or heavier material can be carried without overloading the tires or axles when driving 

It's especially the  front axle and tires at risk for overloading 

But these lift axles make steering around turns more difficult. So the lift axle is raised before the truck goes into a sharper turn. If the load is lighter,  the lift axle is kept up all the time for driving ease.