A quad axle vehicle has four axles at the rear of the vehicle, commonly a tandem drive axle unit coupled with two air lift axles used to meet weight restrictions as well as to distribute the vehicle's loaded weight over a greater area. Most common on dump trucks and other heavy trucks, the quad axle arrangement allows the truck to operate on soft ground without sinking in and leaving large ruts. In the typical quad axle design, the tandem drive axles commonly are placed in the rear of the chassis, with the two air-lift axles positioned ahead. Some vehicles, such as large cement mixing trucks, use a four-axle configuration which places the tandem drive unit between two air-lift axles.
When unloaded or lightly loaded, a quad axle truck will operate with the air-lift axles in the raised position. This is done to save fuel as well as to save tires. When a quad axle truck makes a sharp turn with the air-lift axles in the lowered position, the first two axles are commonly dragged and skidded across the ground. By raising the air-lift axles, the truck is able to negotiate the turn with less drag and without creating flat spots on the air-lift axle's tires caused from skidding them sideways over the roadway.
Weight restrictions on many roads require a specific number of axles to support a specific amount of weight in order to operate legally on the roadway. This is sometimes a seasonal restriction, however, many roads and highways maintain the restrictions year-round. By using a quad axle vehicle, the extra axles allow the truck to carry a larger payload. While the tandem drive axles are most commonly made up of four sets of dual wheels and tires, it is not uncommon for the additional two air-lift axles to operate with single tires on each end of the axle.
In some cement truck applications, the air-lift axles in the quad axle design are placed one forward and one behind the tandem drive axles. A unique arrangement in this style of quad axle design is that the most rearward axle is able to turn the tires in a steering axle design. This allows the truck to be much more maneuverable when backing into a job site to empty a load of concrete. Even in a front discharge application, the rear axle is often a steerable axle in order to avoid skidding the rearmost axle across a yard and damaging the lawn.
