section (two if boxed) and two shorter horizontal flanges. The height of the vertical section provides opposition to vertical flex when weight is applied to the top of the frame (beam resistance). Though typically flat the whole length on heavy-duty trucks, the rails may sometimes be tapered or arched for clearance around the engine or over the axles. The holes in rails are used either for mounting vehicle components and running wires and hoses or measuring and adjusting the orientation of the rails at the factory or repair shop. The frame is usually made of steel, but can be made (whole or in part) of aluminum for a lighter weight. A tow bar may be found attached at one or both ends, but heavy tractors almost always make use of a fifth wheel hitch. Body types Box trucks ("tilts" in the UK) have walls and a roof, making an enclosed load space. The rear has doors for unloading; a side door is sometimes fitted. Chassis cab trucks have a fully-enclosed cab at the front, with bare chassis frame-rails behind, suitable for subsequent permanent attachment of a specialized payload, like a fire-truck or ambulance body. Concrete mixers have a rotating drum on an inclined axis, rotating in one direction to mix, and in the other to discharge the concrete down chutes. Because of the weight and power requirements of the drum body and rough construction sites, mixers have to be very heavy duty. Dump trucks ("tippers" in the UK) transport loose material such as sand, gravel, or dirt for construction. A typical dump truck has an open-box bed, which is hinged at the rear and lifts at the front, allowing the material in the bed to be unloaded ("dumped") on the ground behind the truck. Flatbed trucks have an entirely flat, level platform body. This allows for quick and easy loading but has no protection for the load. Hanging or removable sides are sometimes fitted. Refrigerator trucks have insulated panels as walls and a roof and floor, used for transporting fresh and frozen cargo such as ice cream, food, vegetables, and prescription drugs. They are mostly equipped with double-wing rear doors, but a side door is sometimes fitted. Refuse trucks have a specialized body for collecting and, often, compacting trash collected from municipal, commercial, and industrial sites. This application has the widest use of the cab-over configuration in North America, to