What Is Lift Axle in Trucking?
Also called: Liftable Axle, Pusher Axle, Auxiliary Axle, Booster Axle
A lift axle is an extra, non-driven axle that can be raised off the road when the truck does not need it and lowered to carry more weight. Federal rules say its brakes must work whenever it is lowered and its tires touch the road. Each State decides whether it counts under the federal bridge formula.
Last reviewed September 23, 2026 · Facts checked November 3, 2026
Key Points
- Peterbilt says adjustable auxiliary axles, commonly known as pusher or tag axles, add to the load a truck can carry when they are down (2024 Model 579 Operator's Manual)
- Peterbilt lists three kinds: liftable and steerable, liftable and non-steerable, and non-liftable. Some need lift calibration
- Brakes on lift axles need not work while the axle is raised. They must be able to be applied whenever it is lowered and its tires touch the road (49 CFR 393.48(c)(6))
- FHWA says each State may decide whether variable load suspension, booster, cheater, dummy or tag axles count under the federal bridge formula. Some States require a minimum weight on the axle, or the control outside the cab
- Never go faster than 5 mph with a load and the auxiliary axle raised. Lower the axle as soon as you receive a load (Peterbilt)
- Axle creep ratings are weight and speed limits for a loaded truck with the axle up. The axle maker assigns them, and they must not be exceeded (Peterbilt)
- Never run more air pressure in the lift axles than the load needs. Too much can cut traction and stability at the steer and drive axles and lengthen braking distance (Peterbilt)
- More air pressure in the lift axle pushes it down harder and takes load and traction off the drive axles. Less pressure moves weight onto the drive axles (Peterbilt)
- The air brake standard, FMVSS 121, tests a truck with a liftable axle at GVWR with the axle down, and at unloaded weight with the axle up (49 CFR 571.121)
Tag axle vs. pusher axle vs. lift axle
| Tag Axle | Pusher Axle | Lift Axle | |
|---|---|---|---|
| Where it sits | Behind the drive axle | Ahead of the drive axle | Describes what it can do, not where it sits |
| Driven? | No | No | No |
| Can it be raised? | Some can. Most North American 6x2 fleets use one that cannot (Hendrickson) | Some can | Yes |
| What it adds | Load capacity when it is down | Load capacity when it is down | Load capacity when it is down |
| Counts under the bridge formula? | Each State decides (FHWA) | Each State decides (FHWA) | Each State decides (FHWA) |
What is a lift axle on a truck?
A lift axle is an extra axle that can be raised off the road and lowered again. It is not driven, so no engine power goes to its wheels. Peterbilt describes three kinds. Some are liftable and steerable. Some are liftable and non-steerable. Some do not lift at all. When the axle is down, it adds to the load the truck can carry. A lift axle is also called a liftable axle, an auxiliary axle, or a pusher or tag axle.
Why do trucks have lift axles?
To carry more weight when the load needs it, and to ride on fewer axles when it does not. Peterbilt says that without the extra axle, excessive weight can shorten the life of the frame rails, axles, suspension and brakes. With the axle raised, Hendrickson says traction on the drive axle goes up and tire wear goes down. Hendrickson also claims up to 2 percent better fuel economy from lifting the non-driven axle, based on tests it commissioned. That is a maker claim, not an independent figure.
How does a lift axle work?
Most lift axles ride on air suspension. A dash switch or valve raises and lowers the axle. On Peterbilt trucks with liftable auxiliary axles, a knob can also set the air pressure in the axle's suspension. More pressure pushes the axle down harder. That raises the load on the lift axle and lowers the load and traction on the drive axles. Less pressure moves weight back to the drive axles. The truck can also override the switch to protect the axle system. Some systems are fully automatic. Hendrickson's OPTIMAAX reads the air pressure in the drive axle suspension, turns it into weight, and lifts or lowers the axle on its own. It lowers the axle when the ignition is off or the parking brake is set.
When should you raise or lower a lift axle?
Peterbilt says to raise liftable auxiliary axles only to improve maneuverability off road, or when the vehicle is unloaded. Lower the axle as soon as you receive a load. With a load and the axle raised, never go faster than 5 mph. Do not drive or park an empty truck, or unload it, with the axle down if the axle has no park brakes. That can cause loss of control or roll-away. Raise it or dump its air first. A self-steering lift axle raises by itself when the park brake is applied or the truck is put in reverse.
Do lift axles have brakes?
Yes. A commercial motor vehicle must have brakes acting on all wheels (49 CFR 393.42(a)). Lift axles have a special rule. Their brakes need not be capable of working while the axle is raised. They must be capable of being applied whenever the axle is lowered and its tires touch the road (49 CFR 393.48(c)(6)). Peterbilt notes that some lift axles have no park brakes.
Do lift axles count toward weight limits and the bridge formula?
That is up to each State. FHWA says each State may decide for itself whether such axles count under the federal bridge formula on the Interstate. Some States require them to carry a minimum amount of weight, or the control to be outside the cab, to count as an axle. We found no federal rule that says more. Peterbilt tells owners to contact their local commercial weight enforcement office for limits in their area, because exceeding weight limits may result in citations. See the bridge formula and tandem axle pages for the federal limits.
What is the difference between a lift axle, a tag axle and a pusher axle?
Tag and pusher describe where the axle sits. Lift describes what it can do. A tag axle is behind the drive axle. A pusher axle is ahead of it. Either one can be liftable or not. Hendrickson says most North American 6x2 fleets use a tag axle that does not lift. The table shows the main points.
Related Free Tools
Weight & Axle Load Calculator
Check federal axle weight limits and bridge formula compliance before you scale.
Scale / Weigh Station Guide
PrePass eligibility, what to do at a scale, violations, and a state-by-state guide.
Pre-Trip / Post-Trip DVIR Checklist
A 49 CFR 396.11 compliant Driver Vehicle Inspection Report with defect tracking.
Frequently Asked Questions
What is a lift axle on a truck?+
An extra axle that is not driven and can be raised off the road or lowered to carry more weight. Peterbilt calls adjustable auxiliary axles, commonly known as pusher or tag axles, ones that add to the load a truck can carry when they are down.
Why do trucks have lift axles?+
To carry more weight when the load needs it, and to ride on fewer axles when it does not. Hendrickson says raising the axle can add traction on the drive axle and reduce tire wear.
How does a lift axle work?+
It rides on air suspension. A dash switch or valve raises and lowers it. Air pressure sets how much load it carries. More pressure takes load and traction off the drive axles. Some systems, such as Hendrickson's OPTIMAAX, lift and lower the axle automatically.
Do lift axles have brakes?+
Yes. Their brakes need not work while the axle is raised. They must be able to be applied whenever the axle is lowered and its tires touch the road (49 CFR 393.48(c)(6)).
Are lift axles powered?+
No. The lift axle is a non-driven axle. Hendrickson describes the non-driven axle as having no internal gearing to power its wheels.
How fast can you drive with the lift axle raised?+
Peterbilt says to never go faster than 5 mph when driving with a load and the auxiliary axle raised. Lower the axle as soon as you receive a load.
What psi should a lift axle be at?+
There is no single number. Peterbilt says to weigh the axles on a scale, adjust the suspension air pressure to get the axle load you want, and write down the pressure-to-load setting. Never use more pressure than the load needs. Repeat the steps if you change tire size.
Are lift axles legal?+
Whether a lift axle counts under the federal bridge formula is up to each State (FHWA). Some States require a minimum weight on the axle or the control outside the cab. Ask the State weight enforcement office where you drive.
What is a self-steering lift axle?+
A liftable axle that also steers. Peterbilt calls it a self-steering lift axle and says it raises by itself when the park brake is applied or the truck is put in reverse.
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