system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do dump trucks have an extra pair of wheels under them that don't touch the ground?
gpt
Used to work in a Freightliner plant.  They're called drop axles.  They're lowered when the truck (often a dump truck) is carrying a heavy load, to help distribute the weight across the frame.  They have inflatable airbag suspensions.