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Undercarriage

Beneath the carriage body is the undergear or undercarriage (or simply carriage), consisting of the running gear and chassis. The wheels and axles, in distinction from the body, are the running gear. The wheels revolve upon bearings or a spindle at the ends of a bar or beam called an axle or axletree. Most carriages have either one or two axles. On a four-wheeled vehicle, the forward part of the running gear, or forecarriage, is arranged to permit the front axle to turn independently of the fixed rear axle. In some carriages a dropped axle, bent twice at a right angle near the ends, allows for a low body with large wheels. A guard called a dirtboard keeps dirt from the axle arm.

Several structural members form parts of the chassis supporting the carriage body. The fore axletree and the splinter bar above it (supporting the springs) are united by a piece of wood or metal called a futchel, which forms a socket for the pole that extends from the front axle. For strength and support, a rod called the backstay may extend from either end of the rear axle to the reach, the pole or rod joining the hind axle to the forward bolster above the front axle.

A skid called a drag, dragshoe, shoe or skidpan retards the motion of the wheels. A London patent of 1841 describes one such apparatus: "An iron-shod beam, slightly longer than the radius of the wheel, is hinged under the axle so that when it is released to strike the ground the forward momentum of the vehicle wedges it against the axle". The original feature of this modification was that instead of the usual practice of having to stop the carriage to retract the beam and so lose useful momentum the chain holding it in place is released (from the driver's position) so that it is allowed to rotate further in its backwards direction, releasing the axle. A system of "pendant-levers" and straps then allows the beam to return to its first position and be ready for further use.

A catch or block called a trigger may be used to hold a wheel on an incline.

A horizontal wheel or segment of a wheel called a fifth wheel sometimes forms an extended support to prevent the carriage from tipping; it consists of two parts rotating on each other about the kingbolt or perchbolt above the fore axle and beneath the body. A block of wood called a headblock might be placed between the fifth wheel and the forward spring.