# Leading wheel

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Unpowered wheel at the front of a locomotive

The leading wheels (boxed) on [Pennsylvania Railroad 1737](/source/Pennsylvania_Railroad_1737)

The **leading wheel**, or **leading axle** or **pilot wheel**, of a [steam locomotive](/source/Steam_locomotive) is an unpowered [wheel](/source/Wheel) or [axle](/source/Axle) located in front of the driving wheels. The axle or axles of the leading wheels are normally located on a leading [truck](/source/Bogie). 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](/source/William_Adams_(locomotive_engineer)) 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](/source/John_B._Jervis), who employed them in his 1832 design for a locomotive with four leading wheels and two [driving wheels](/source/Driving_wheel) (a type that became known as the *Jervis*). In the [Whyte system](/source/Whyte_notation) of describing [locomotive wheel arrangements](/source/Wheel_arrangement), his locomotive would be classified as a [4-2-0](/source/4-2-0), that is to say, it had four leading wheels, two [driving wheels](/source/Driving_wheel), and no [trailing wheels](/source/Trailing_wheel). In the [UIC classification](/source/UIC_classification_of_locomotive_axle_arrangements) 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](/source/Her_Majesty's_Railway_Inspectorate) condemned the practice in 1895, following an accident involving two [0-4-4s](/source/0-4-4T) at [Doublebois](/source/Disused_railway_stations_on_the_Cornish_Main_Line#Doublebois), Cornwall, on the [Great Western Railway](/source/Great_Western_Railway).[2] Other designers, however, persisted with the practice and the famous [0-4-2](/source/0-4-2) [Gladstone class](/source/LB%26SCR_B1_class) passenger expresses of the [London, Brighton and South Coast Railway](/source/London%2C_Brighton_and_South_Coast_Railway) remained in trouble-free service until 1933.[3] A single leading axle (known as a [pony truck](/source/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](/source/6-2-0) [Crampton](/source/Crampton_locomotive) type and the [Pennsylvania Railroad](/source/Pennsylvania_Railroad)'s [6-4-4-6](/source/6-4-4-6) [S1](/source/Pennsylvania_Railroad_class_S1) [duplex locomotive](/source/Duplex_locomotive) and [6-8-6](/source/6-8-6) [S2](/source/Pennsylvania_Railroad_class_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.

## See also

- [AAR wheel arrangement](/source/AAR_wheel_arrangement)

- [Adams axle](/source/Adams_axle)

- [UIC classification of locomotive axle arrangements](/source/UIC_classification_of_locomotive_axle_arrangements)

## References

1. **[^](#cite_ref-1)** Simmons, Jack; Biddle, Gordon (1997). *The Oxford Companion to British Railway History*. Oxford: Oxford University Press. [ISBN](/source/ISBN_(identifier)) [0-19-211697-5](https://en.wikipedia.org/wiki/Special:BookSources/0-19-211697-5).

1. **[^](#cite_ref-2)** [Rolt, Lionel](/source/L._T._C._Rolt) (1955). *Red for Danger*. London: [Bodley Head](/source/The_Bodley_Head). [ISBN](/source/ISBN_(identifier)) [0-7153-7292-0](https://en.wikipedia.org/wiki/Special:BookSources/0-7153-7292-0). {{[cite book](https://en.wikipedia.org/wiki/Template:Cite_book)}}: ISBN / Date incompatibility ([help](https://en.wikipedia.org/wiki/Help:CS1_errors#invalid_isbn_date))

1. **[^](#cite_ref-3)** [*Gladstone* at the National Railway Museum, York](http://www.lbscr.demon.co.uk/photos/Gladstone-214.html) [Archived](https://web.archive.org/web/20061015001942/http://www.lbscr.demon.co.uk/photos/Gladstone-214.html) 2006-10-15 at the [Wayback Machine](/source/Wayback_Machine) Accessed 22 December 2006.

v t e Locomotive design Cab positioning Short hood / Long hood Cab forward Sharknose Steeplecab Cab unit Hood unit Cowl unit Boxcab Comfort cab 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

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Adapted from the Wikipedia article [Leading wheel](https://en.wikipedia.org/wiki/Leading_wheel) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Leading_wheel?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.