A carrying wheel on a

steam locomotive A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, oil or, rarely, wood) to heat water in the loco ...

is a wheel that is not driven; i.e., it is uncoupled and can run freely, unlike a coupled or

driving wheel On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons (or turbine, in the case of a steam turbine locomotive). On a conventional, non-articulated locomotive, the driving wheels are all coupled ...

. It is also described as a running wheelWörterbuch der Industriellen Technik, Dr.-Ing. Richard Ernst, Oscar Brandstetter Verlag, Wiesbaden, 5. Auflage, 1989, . and their axle may be called a carrying axle. A carrying wheel is referred to as

leading wheel The leading wheel or leading axle or pilot wheel of a steam locomotive is an unpowered wheel or axle located in front of the driving wheels. The axle or axles of the leading wheels are normally located on a leading truck. Leading wheels are used ...

if it is at the front, or a

trailing wheel On a steam locomotive, a trailing wheel or trailing axle is generally an unpowered wheel or axle ( wheelset) located behind the driving wheels. The axle of the trailing wheels is usually located in a trailing truck. On some large locomotives, ...

if it is at the rear of the locomotive.

Weight distribution

In particular reference to steam engines, the carrying wheels have a very important purpose of allowing the engine's

weight distribution Weight distribution is the apportioning of weight within a vehicle, especially cars, airplanes, and trains. Typically, it is written in the form ''x''/''y'', where ''x'' is the percentage of weight in the front, and ''y'' is the percentage in ...

to be altered. For example in the use of leading wheels it would allow the boiler to be located further forward of the driving wheels, the weight of which counters the leverage imposed by the drawbar and the load of the pulled wagons/cars about the fulcrum of the rearmost driving wheel. Similarly the trailing wheels can move the fulcrum to the rearmost trailing wheel. Such change can dramatically improve the operating speeds of engines and their

tractive effort As used in mechanical engineering, the term tractive force can either refer to the total traction a vehicle exerts on a surface, or the amount of the total traction that is parallel to the direction of motion. In railway engineering, the term t ...

References

Steam locomotive technologies {{locomotive-stub