{{Short description|Unpowered wheel at the front of a locomotive}}
[[File:462leading flip2.jpg|thumb|300px|The leading wheels (boxed) on [[Pennsylvania Railroad 1737]]]]
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 [[Bogie|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 (locomotive engineer)|William Adams]] in 1865.<ref>{{Cite book | last=Simmons | first=Jack |author2=Biddle, Gordon | title=The Oxford Companion to British Railway History | publisher=Oxford University Press | date=1997 | location=Oxford | isbn=0-19-211697-5}}</ref> 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 wheel]]s (a type that became known as the ''Jervis''). In the [[Whyte notation|Whyte system]] of describing [[wheel arrangement|locomotive wheel arrangements]], his locomotive would be classified as a [[4-2-0]], that is to say, it had four leading wheels, two [[driving wheel]]s, and no [[trailing wheel]]s. In the [[UIC classification of locomotive axle arrangements|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 [[Her Majesty's Railway Inspectorate|Railway Inspectorate]] condemned the practice in 1895, following an accident involving two [[0-4-4T|0-4-4]]s at [[Disused railway stations on the Cornish Main Line#Doublebois|Doublebois]], Cornwall, on the [[Great Western Railway]].<ref>{{Cite book | last=Rolt | first=Lionel | author-link=L. T. C. Rolt | title=Red for Danger | publisher=[[The Bodley Head|Bodley Head]] | date=1955 | location=London | isbn=0-7153-7292-0}}</ref> Other designers, however, persisted with the practice and the famous [[0-4-2]] [[LB&SCR B1 class|Gladstone class]] passenger expresses of the [[London, Brighton and South Coast Railway]] remained in trouble-free service until 1933.<ref>[http://www.lbscr.demon.co.uk/photos/Gladstone-214.html ''Gladstone'' at the National Railway Museum, York] {{webarchive|url=https://web.archive.org/web/20061015001942/http://www.lbscr.demon.co.uk/photos/Gladstone-214.html |date=2006-10-15 }} Accessed 22 December 2006.</ref> 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 locomotive|Crampton]] type and the [[Pennsylvania Railroad]]'s [[6-4-4-6]] [[Pennsylvania Railroad class S1|S1]] [[duplex locomotive]] and [[6-8-6]] [[Pennsylvania Railroad class S2|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]] * [[Adams axle]] * [[UIC classification of locomotive axle arrangements]]
==References== {{Reflist}}
{{Locomotive running gear}}
[[Category:Train wheels]]