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 to help the locomotive negotiate curves and to support the front portion of the boiler.

Overview

Many leading bogies do not have simple rotational motion about a vertical pivot.

Bogies with a sliding motion controlled by springs was patented by William Adams in 1865.^[1] Other designs used swing links to take the weight of the bogie with a centering action. 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 wheels (a type that became known as the Jervis). In the Whyte system of describing locomotive wheel arrangements, his locomotive would be classified as a 4-2-0, that is to say, it had four leading wheels, two driving wheels, and no trailing wheels. In the UIC classification 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 condemned the practice in 1895, following an accident involving two 0-4-4s at Doublebois, Cornwall, on the Great Western Railway.^[2] Other designers, however, persisted with the practice and the famous 0-4-2 Gladstone class passenger expresses of the London, Brighton and South Coast Railway remained in trouble-free service until 1933.^[3] A single leading axle (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's 6-4-4-6 S1 duplex locomotive and 6-8-6 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

^ Simmons, Jack; Biddle, Gordon (1997). The Oxford Companion to British Railway History. Oxford: Oxford University Press. ISBN 0-19-211697-5.
^ Rolt, Lionel (1955). Red for Danger. London: Bodley Head. ISBN 0-7153-7292-0.
^ Gladstone at the National Railway Museum, York Archived 2006-10-15 at the Wayback Machine Accessed 22 December 2006.

[1] Simmons, Jack; Biddle, Gordon (1997). The Oxford Companion to British Railway History. Oxford: Oxford University Press. ISBN 0-19-211697-5.

[2] Rolt, Lionel (1955). Red for Danger. London: Bodley Head. ISBN 0-7153-7292-0.

[3] Gladstone at the National Railway Museum, York Archived 2006-10-15 at the Wayback Machine Accessed 22 December 2006.

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v t e Locomotive design
Cab positioning Short hood / Long hood	Cab forward Sharknose Steeplecab Cab unit Hood unit Cowl unit Boxcab Dual Control Stand
Wheel arrangement	AAR wheel arrangement UIC classification Swiss classification Whyte notation
Valve gear types	Allan Baker Bagnall–Price Baguley Bulleid Caprotti Gab Gooch Gresley Hackworth Joy Kuhn slide Lentz Southern Stephenson Walschaerts
Bogie types	AAR type A switcher truck Arnoux system Articulated bogie Bissel truck Blomberg B Cleminson system Grovers bogie Jacobs bogie Krauss-Helmholtz bogie Mason Bogie Pony truck Radial steering truck Scheffel bogie Schwartzkopff-Eckhardt II bogie
Other running gear elements	Adams axle Axlebox Beugniot lever Carrying wheel Coupled wheel Driving wheel Equalising beam Gölsdorf axle Journal box Klien-Lindner axle Leading wheel Luttermöller axle Radial axle Railway tire Road–rail vehicle Trailing wheel Train wheel Wheelset
Exhaust system types	Giesl Kylchap Kylpor Lemaître Lempor Lemprex
Common exhaust system elements	Blastpipe Smokebox Chimney

Overview

See also

References