On many large (and not-so-large) trucks I see an additional set of tires that don’t make contact with the road, but appear to be hitched up several inches from the ground. What’s up with that?
I see them all the time on the big earth moving dump trucks…they are a hydraulically lowered extra axle engaged on demand, and disengaged unloaded to reduce tire wear and rolling drag.
They are called lift axles. Weight limits are in pounds/axle as well as total weight, so an additional axle can allow more weight to be carried. They are lifted when the truck is lightly loaded or empty for the reasons Enola Straight gave, as well as increasing the traction on the remaining tires when the truck is empty, and so increasing braking ability.
Cool. So, like if the driver is going through a tollbooth, he can lift the axle off the ground and save 50 cents?
And in really windy weather, the driver can hit a button to extend the axle length so that it acts like an outrigger to keep the entire rig remains stable. However, it really messes up vehicles that are parked as well as those in the oncoming traffic lane.
