A quad axle vehicle has four axles at the rear, with a tandem drive axle unit and two air lift axles. It allows for a larger payload and can operate on soft ground. Aerial lift axles are raised when unloaded to save fuel and tires, and weight restrictions require a specific number of axles. Some cement trucks have a unique quad axle design with a rear steer axle for better maneuverability.
A quad axle vehicle has four axles at the rear of the vehicle, commonly a tandem drive axle unit along with two air lift axles used to meet weight restrictions as well as distribute the loaded weight of the vehicle. in a larger area. More common on dump trucks and other heavy trucks, the quad axle arrangement allows the truck to operate on soft ground without sinking and leaving large ruts. In the typical quad axle layout, the tandem driveshafts are commonly positioned at the rear of the chassis, with the two aerial lift shafts positioned forward. Some vehicles, such as large cement mixer trucks, use a four-axle configuration that places the drive unit in tandem between two aerial lift axles.
When unloaded or lightly loaded, a four-axle truck will work with the aerial lift axles in the raised position. This is done to save fuel and to save tires. When a quad axle truck makes a sharp turn with the aerial lift axles in the lowered position, the first two axles often drag and slide on the ground. By raising the aerial lift axles, the truck can negotiate the turn with less resistance and without creating flat spots on the aerial lift axle tires caused by sideslip on the road.
Weight restrictions on many highways require a specific number of axles to support a specific amount of weight in order to legally operate on the highway. This is sometimes a seasonal restriction, however many roads and highways maintain the restrictions year-round. When using a four-axle vehicle, the extra axles allow the truck to carry a larger payload. While tandem drive axles are usually made up of four sets of wheels and dual tires, it is not uncommon for the two additional aerial lift axles to be powered by individual tires on each end of the axle.
In some cement truck applications, the air lift axles in the quad axle design are positioned one forward and one behind the tandem drive axles. A unique arrangement in this style of quad axle design is that the rearmost axle can turn the tires in a steer axle design. This allows the truck to be much more maneuverable when backing up on a job site to dump a load of concrete. Even in a front dump application, the rear axle is often a steer axle to avoid skidding the rearmost axle across a yard and damaging grass.
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