A tag axle is typically found on a semi tractor. It is the axle behind the drive axle and is usually able to be elevated from making contact with the roadway when not needed. It can also be found on semi trailers. These are also able to be lifted when not in use. This allows the tires to avoid unnecessary wear and also saves fuel. The axle is lowered when it becomes necessary to help spread the vehicle's weight over a greater area. It is also possible to find a tag axle on farm equipment, as well as certain heavy construction machinery.
Most tag axle systems are air operated. The driver or operator is able to control the axle by moving a switch inside the driver's compartment of the vehicle. By adding air to the axle, the driver can take weight off of the other axles on the vehicle. Conversely, removing air will place the weight on the other axles as the tag axle rises off of the road's surface. This helps enable drivers to obey certain seasonal vehicle weight restrictions or on certain weight-limited roads.
In foul weather, the operator may choose to lift the tag axle in an attempt to place more weight on the drive axle. This will create better traction and help the driver control the vehicle. The driver may also choose to lift a tag axle when maneuvering into a tight spot. This will allow the vehicle to turn easier rather than dragging and sliding an extra axle around on a parking lot. This also saves on tire wear.
When being weighed on a scale, the operator may be advised to reposition his or her axle to meet weight restrictions. Many roads have restrictions that require a vehicle to have a certain amount of weight distributed between all of its axles. By adjusting the pressure that the tag axle pushes against the road, the driver can vary the weight that each axle displaces upon the road's surface. This allows the vehicle to meet the required restriction and legally travel upon the road.
Certain vehicles have many such axles. Vehicles used to move over-size loads may have many air adjustable axles. This allows the trucks to distribute the weight of the over-limit cargo evenly over a greater distance and greater number of axles. It also allows the vehicle to turn much easier when it is not carrying a load. The driver can choose to use only the number of axles that are needed and avoid consuming excess fuel when the vehicle is empty.