A tri-axle vehicle refers to the number of driving axles the vehicle possesses, not including the steering axle. This type of axle configuration is commonly associated with large trucks and heavy equipment.
The most common tri-axle arrangement consists of a tandem drive axle assembly with an air-lift third axle to allow for greater load weights to be transported on the roads. In this configuration, the lift axle can be either in front of or behind the tandem drive axles. Dump trucks, tow trucks and trucks that specialize in hauling heavy loads commonly use a tri-axle design. Other vehicles operating with three axles in the rear are wheeled-cranes, fire trucks and cement mixing trucks.
Weight restrictions placed upon public highways and roads call for a truck to displace only a specified amount of weight upon each of the truck's axles. This results in the addition of an extra axle on trucks expecting to be in excess of the restriction requirements. By using an air-lift axle in the tri-axle configuration, the driver can lift the extra tires off of the roadway when the truck is empty. This saves not only on tire wear, but also on fuel consumption. The lifting of the extra axle also allows the driver to negotiate tight turns easier than if required to slide the extra tires against the road while turning the truck.
While this is the reason for a heavy hauler to use a tri-axle design, there are other reasons to utilize this configuration. Trucks such as dump trucks that are regularly required to cross lawns and soft ground to deliver a load use a tri-axle to displace the weight over a greater area to prevent the sinking of the truck into the ground. Often, these trucks will be driven to the delivery site with the third axle in the raised position and lowered only when about to cross the soft ground.
The tri-axle configuration is also used on certain heavy capacity trailers. Much of the heavy equipment used on a construction and excavation site is unable to operate on public roads. This type of equipment requires a very high-capacity trailer to tow it to the work site. The tri-axle trailer functions on the same principal as the three-axle design on a truck by displacing the weight of the load out over more axles, thereby complying with posted weight restrictions on the roadways. The tri-axle design also allows the loaded trailers to be moved across soft ground.