A lift axle, or tag axle, as it is often called, is a heavy truck axle that is designed to be lifted off the road's surface when not needed. A lift axle can be mounted on the truck, the trailer or both and is commonly found on tanker-style trucks as well as trucks used to haul over-sized loads. Typically using an air bag system to lift and deploy the lift axle, the driver is able to regulate the amount of pressure exerted upon the axle by increasing the amount of air pressure that deploys the axle. While this type of axle does nothing in regard to propelling the truck down the highway, it is equipped with brakes and can aid in the stopping power of the vehicle's braking system.
The purpose of a lift axle is to place the vehicle's total weight over a broader area, thereby allowing the vehicle to meet weight-imposed restrictions on certain roads. When a truck passes over a set of roadside scales, the purpose is to weigh the vehicle and determine the amount of weight that is supported by each axle. The weight distribution can be manipulated by several methods. The position of the trailer's mounting point on the truck can be moved ahead or back to disperse the weight forward or backward, and the trailer axles can often be slid forward or back for the same results. By applying air pressure to the lift axle, the driver is able to add one more axle into the equation and relieve some of the weight being supported by the other axles.
By applying more air, the lift axle takes on a greater percentage of the load's weight. By decreasing the air pressure, the driver can decrease the weight-carrying capacity of the axle or eliminate it completely by raising it up and off of the road. Often, a driver will raise the axle to enable the vehicle to turn easier and not have to drag the lift axle sideways through a turn. This action applies both to the tag axle on the truck as well as the trailer's tag axle.
Many lift axle designs call for a single tire and wheel to be used on the axle and wheels to be used on the other axles. These tires can also be of a smaller diameter than the other tires. It is not uncommon for the lift axle to operate with a much smaller tire, however, the tire is often of a greater ply rating and able to support more weight than the larger truck tires.