The leading wheel or leading axle or pilot wheel of 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 an unpowered

wheel A wheel is a circular component that is intended to rotate on an axle bearing. The wheel is one of the key components of the wheel and axle which is one of the six simple machines. Wheels, in conjunction with axles, allow heavy objects to be ...

axle An axle or axletree is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In the former case, beari ...

located in front of the driving wheels. The axle or axles of the leading wheels are normally located on a leading

truck A truck or lorry is a motor vehicle designed to transport cargo, carry specialized payloads, or perform other utilitarian work. Trucks vary greatly in size, power, and configuration, but the vast majority feature body-on-frame constructi ...

. Leading wheels are used to help the locomotive negotiate curves and to support the front portion of the boiler.

Overview

Importantly, the leading bogie does not have simple rotational motion about a vertical pivot, as might first be thought. It must also be free to slip sideways to a small extent, otherwise the locomotive is unable to follow curves accurately, and some kind of springing mechanism is normally included to control that movement and provide a tendency to return to centre. A sliding bogie of that type was patented by William Adams in 1865. The first use of leading wheels is commonly attributed to John B. Jervis, who employed them in his 1832 design for a locomotive with four leading wheels and two

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 to ...

s (a type that became known as the ''Jervis''). In the

Whyte Whyte may refer to: *Whyte (surname), a family name * Whyte, West Virginia * Whyte Chemicals, a manufacturer and distributor *Whyte notation Whyte notation is a classification method for steam locomotives, and some internal combustion locom ...

system of describing locomotive wheel arrangements, his locomotive would be classified as a

4-2-0 Under the Whyte notation for the classification of steam locomotives, represents the wheel arrangement of four leading wheels on two axles, two powered driving wheels on one axle and no trailing wheels. This type of locomotive is often called a ...

, that is to say, it had four leading wheels, two

s, and no

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, ...

s. In the

UIC classification The UIC classification of locomotive axle arrangements, sometimes known as the German classification''The Railway Data File''. Leicester: Silverdale, 2000. p. 52. . or German system,Kalla-Bishop P.M. & Greggio, Luciano, ''Steam Locomotives'', Cre ...

system, which counts axles rather than wheels and uses letters to denote powered axles, the ''Jervis'' would be classified 2A. Locomotives without leading trucks are generally regarded as unsuitable for high speed use. The British

Railway Inspectorate Established in 1840, His Majesty's Railway Inspectorate (HMRI) is the organisation responsible for overseeing safety on Britain's railways and tramways. It was previously a separate non-departmental public body, but from 1990 to April 2006 it ...

condemned the practice in 1895, following an accident involving two

0-4-4 Under the Whyte notation for the classification of steam locomotives, 0-4-4 represents the wheel arrangement of no leading wheels, four powered and coupled driving wheels on two axles, and four trailing wheels on two axles. This type was only ...

s at

Doublebois Doublebois ( kw, Dewgoes) (pronounced ), is a village in south Cornwall, England, United Kingdom. It is in the civil parish of Dobwalls approximately three miles (5 km) west of Liskeard.Ordnance Survey: Landranger map sheet 201 ''Plymouth ...

, Cornwall, on the

Great Western Railway The Great Western Railway (GWR) was a British railway company that linked London London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 mill ...

. Other designers, however, persisted with the practice and the famous

0-4-2 Under the Whyte notation for the classification of steam locomotives, represents the wheel arrangement with no leading wheels, four powered and coupled driving wheels on two axles and two trailing wheels on one axle. While the first locomot ...

Gladstone class passenger expresses of the

London, Brighton and South Coast Railway The London, Brighton and South Coast Railway (LB&SCR; known also as the Brighton line, the Brighton Railway or the Brighton) was a railway company in the United Kingdom from 1846 to 1922. Its territory formed a rough triangle, with London at its ...

remained in trouble-free service until 1933.''Gladstone'' at the National Railway Museum, York
Accessed 22 December 2006. A single leading axle (known as a

pony truck A Bissell or Bissel truck (also Bissel bogie or Pony truck) is a single-axle bogie which pivots towards the centre of a steam locomotive to enable it to negotiate curves more easily. Invented in 1857 by and usually then known as a ''pony truck'' ...

) increases stability somewhat, while a four-wheel leading truck is almost essential for high-speed operation. The highest number of leading wheels on a single locomotive is six, as seen on the 6-2-0 Crampton type and the

Pennsylvania Railroad The Pennsylvania Railroad (reporting mark PRR), legal name The Pennsylvania Railroad Company also known as the "Pennsy", was an American Class I railroad that was established in 1846 and headquartered in Philadelphia, Pennsylvania. It was named ...

6-4-4-6 A 6-4-4-6 steam locomotive, in the Whyte notation for describing locomotive wheel arrangements, is one with six leading wheels, two sets of four driving wheels, and six trailing wheels. Other equivalent classifications are: * UIC classification ...

duplex locomotive A duplex locomotive is a steam locomotive that divides the driving force on its wheels by using two pairs of cylinders rigidly mounted to a single locomotive frame; it is not an articulated locomotive. The concept was first used in France in 1 ...

and

6-8-6 Under the Whyte notation for the classification of steam locomotives by wheel arrangement, represents the arrangement of six unpowered leading wheels arranged into a three-axle leading truck, eight powered driving wheels, and six unpowered trai ...

S2 steam turbine. Six-wheel leading trucks were not very popular. The Cramptons were built in the 1840s, but it was not until 1939 that the PRR used one on the S1.

References

{{Locomotive running gear Train wheels

Overview

See also

References