Lift axle

Why it matters

Road weight limits depend on how many axles carry the load and how far apart they are. A lift axle adds one when you need it and gets out of the way when you don't, saving tires and making the truck easier to turn empty.

The catch: an axle only counts when it's down and carrying weight. A tri-axle dump truck with its pusher up is, on the scale, a tandem.

Lift up: 3 axles, 20 ft first to last

51,000 lb Interstate gross

Lift down: 4 axles, 22 ft

56,500 lb Interstate gross

Legal weight the lift axle adds

5,500 lb

Rule

Bridge Formula, 23 CFR 658.17

EXAMPLE spacings. Run your own axle positions in the Bridge Formula calculator; states set limits on other roads.

Setups with more lift axles, quads and quints, are compared in tri-axle vs quad vs quint.

The lift axle switch

Most lift axles are air-operated. A switch or valve in the cab raises and lowers the axle, and a pressure regulator sets how much air goes to its suspension bags, which decides how much of the load the axle takes. Too little air and the lift axle barely touches the road while the drives carry the weight; too much and it lifts load off the drives.

Set it the same way every time, then check the result on a scale ticket axle by axle. States have their own rules about lift axle controls and use, so check yours before you change a setup.

Lift axles in practice

• Lower it before you load. Weight on the drives alone can overload them.
• Raise it for tight turns where the maker allows; a lift axle dragged around a turn scrubs tires. More on that in dump truck tires.
• Check it on the pre-trip: air bags, lines, and that it goes up and down.
• It doesn't change your GVWR. The door sticker rating stays what the maker set; see GVWR.

Lift axles show up in hauling aggregate, where every legal ton counts, and as an alternative to a pup trailer. Check any setup in the Bridge Formula calculator.

TruckSprint no longer dispatches dump trucks. For road freight, our desk dispatches equipment like the 26 ft box truck.