Chassis and Wheels

• The foundation of a cargo trailer is the metal framework underneath. The frame of most cargo trailers is made from pieces of square tubular steel that are welded together for strength. The frame is painted or powder-coated for corrosion resistance. Wheel hubs and drum brake assemblies are attached to each side of the axle. The brakes are activated electrically from a brake controller that is mounted in the tow vehicle. Tires that are designed for trailer use only are mounted onto steel or alloy rims that bolt to the wheel hubs.
  
  

Axle

• The axle on a cargo trailer may actually determine the amount of weight that may be loaded into the trailer. The axle is made from a steel tube that has a certain weight rating. The total weight of the trailer and its contents cannot exceed the axle weight rating. The axle is bolted to leaf springs that are attached to the underside of the steel frame.
  
  

Box

• The cargo box is usually made from a wood or metal framework that is skinned on the outside with sheets of aluminum or steel riveted in place. The interior walls and ceiling of the box may be bare or covered with thin plywood. Floors are usually constructed of ¾-inch plywood that is attached directly to the steel frame. The rear cargo door may be hinged from the sides or from the bottom to make a loading ramp when opened. Some models may have a smaller side door.
  
  

Hitch

• The hitch attaches the trailer to the tow vehicle. The hitch has a hitch coupler that has a sliding mechanism that locks over the tow vehicle's hitch ball. A front of the trailer can be raised and lowered with a tongue jack that is operated by a crank.
  
  

Lighting

• Running lights, turn signals and brake lights are required on most trailers. The wiring for all of the lights is usually run on the underside of the floor and attached to the frame. The bundled wiring terminates at a connector at the front of the trailer that attaches to a receptacle on the tow vehicle to provide power to the lights.