Trailers are unpowered vehicles used for hauling goods. They are pulled by powered vehicles such as cars and trucks, and to make this towing process possible, wheels must be mounted to the trailer unit. The wheels are mounted to a trailer axle, which is a connecting shaft that runs from a wheel on the left side of the trailer to a wheel on the right side of the trailer. The specific function and design of a trailer axle can vary depending on the type of trailer as well as the added features of the vehicle.
Many unpowered trailers feature one or more trailer axle that does little more than support the weight of the trailer and the contents within, and allow the wheels to spin freely. Such an axle, known as a dead axle, features a housing, usually made of steel, with a rotating shaft contained within. This shaft is usually set with bearings that promote stability and smooth turning of the wheels, which will be mounted to either end of the shaft. The entire trailer axle unit is mounted to the chassis of the trailer, and sometimes suspension components such as leaf springs are connected to the axle as well to enhance trailer control and stability.
Sometimes the trailer may feature more than one trailer axle. A double axle trailer, for example, will feature two axles to which four or more wheels can be attached. This design helps distribute heavier loads and stabilize longer trailers, and it is usually reserved for larger units designed to haul exceptionally heavy objects. Even larger trailers may feature three or more axles, and box trailers hauled by tractor trailer trucks usually feature several axles that are much larger for added strength and durability. Each axle must be rated with a weight capacity so a buyer knows which axle will best suit his or her needs. If the weight of the trailer and the items being towed exceed the weight capacity of the axle, failure of the trailer axle may occur, leading to instability of the trailer and towing vehicle.
Larger trailers often have axles that feature braking systems as well. These brakes are activated when the brakes of the towing vehicle are activated. Larger trailers hauling heavier loads are often required by law in some regions to feature such braking systems to ensure the safety and stability of the trailer and towing unit during emergency stopping.