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.
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.
The Whyte notation is a classification method for steam locomotives, and some internal combustion locomotives and electric locomotives, by wheel arrangement. It was devised by Frederick Methvan Whyte, and came into use in the early twentieth century following a December 1900 editorial in American Engineer and Railroad Journal.
In rail transport, a wheel arrangement or wheel configuration is a system of classifying the way in which wheels are distributed under a locomotive. Several notations exist to describe the wheel assemblies of a locomotive by type, position, and connections, with the adopted notations varying by country. Within a given country, different notations may also be employed for different kinds of locomotives, such as steam, electric, and diesel powered.
The UIC classification of locomotive axle arrangements, sometimes known as the German classification or German system, describes the wheel arrangement of locomotives, multiple units and trams. It is used in much of the world, notable exceptions being the United Kingdom, which uses a slightly simplified form of UIC, and in North America, where the AAR wheel arrangement system is used to describe diesel and electric locomotives. In North America, the Whyte notation is only used for steam locomotives.
Under the Whyte notation for the classification of steam locomotives, 4-2-0 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 Jervis type, the name of the original designer.
In the Whyte notation, a 6-2-0 is a railroad steam locomotive that has an unpowered three-axle leading truck followed by a single powered driving axle. This wheel arrangement is associated with the Crampton locomotive type, and in the USA the single class were sometimes referred to as Cramptons.
Under the Whyte notation for the classification of steam locomotives, 0-10-2 represents the wheel arrangement of no leading wheels, ten powered and coupled driving wheels on five axles, and two trailing wheels on one axle.
Under the Whyte notation for the classification of steam locomotives by wheel arrangement, 2-4-4 is a steam locomotive with two unpowered leading wheels followed by four powered driving wheels and four unpowered trailing wheels. This configuration was only used for tank locomotives; no tender locomotives with this wheel arrangement were made.
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, a booster engine was mounted on the trailing truck to provide extra tractive effort when starting a heavy train and at low speeds on gradients.
Under the Whyte notation for the classification of steam locomotives, 4-2-4 represents the wheel arrangement of four leading wheels on two axles, two powered driving wheels on one axle, and four trailing wheels on two axles.
Under the Whyte notation for the classification of steam locomotives, 2-12-0 represents the wheel arrangement of two leading wheels on one axle, twelve powered and coupled driving wheels on six axles, and no trailing wheels.
B-B and Bo-Bo are the Association of American Railroads (AAR) and British classifications of wheel arrangement for railway locomotives with four axles in two individual bogies. They are equivalent to the B′B′ and Bo′Bo′ classifications in the UIC system. The arrangement of two, two-axled, bogies is a common wheel arrangement for modern electric and diesel locomotives.
In Whyte notation, a 4-6-6-2 is a steam locomotive with four leading wheels in an unpowered bogie at the front of the locomotive followed by two sets of driving wheels with six wheels each, followed by two unpowered trailing wheels at the rear of the locomotive.
Under the Whyte notation for the classification of steam locomotives, 0-8-4 represents the wheel arrangement of no leading wheels, eight powered and coupled driving wheels on four axles, and four trailing wheels on two axles.
In the Whyte notation for describing steam locomotive wheel arrangement, a 2-6-6 is a locomotive with a two-wheeled leading truck, six driving wheels, and a six-wheeled trailing truck. All the locomotives produced of this arrangement have been tank locomotives, and the vast majority in the United States. It was a popular arrangement for the larger Mason Bogies, as well as some of the largest suburban tank locomotives.
Co-Bo or Co′Bo′ is a wheel arrangement in the UIC classification system for railway locomotives. It features two uncoupled bogies. The "Co" bogie has three driven axles and the "Bo" bogie has two.
An 0-8-6, in the Whyte notation for the classification of steam locomotives by wheel arrangement, is a locomotive with no leading wheels, eight driving wheels fixed in a rigid frame, and six trailing wheels. All locomotives using this wheel arrangement were tank locomotives.
A 4-4-6, in the Whyte notation for the classification of steam locomotives by wheel arrangement, is a locomotive with:
An 0-8-6-0, in the Whyte notation for the classification of steam locomotives by wheel arrangement, is an articulated locomotive with no leading wheels, eight driving wheels fixed in a rigid frame, six driving wheels and no trailing wheels. In the UIC system, this would be described as a DC't arrangement.
Under the Whyte notation for the classification of steam locomotives, 2-2-2-0 usually represents the wheel arrangement of two leading wheels on one axle, four powered but uncoupled driving wheels on two axles, and no trailing wheels, but can also be used to represent two sets of leading wheels two driving wheels, and no trailing wheels. Some authorities place brackets around the duplicated but uncoupled wheels, creating a notation 2-(2-2)-0, or (2-2)-2-0, as a means of differentiating between them. Others simply refer to the locomotives 2-2-2-0.
Under the Whyte notation for the classification of steam locomotives, 2-2-2-2 could represent either the wheel arrangement of two leading wheels, four powered but uncoupled driving wheels, and two trailing wheels; or of two independent leading axles, two driving wheels, and two trailing wheels.