The pedrail wheel is a type of wheel developed in the early 20th century for all-terrain locomotion. It was used in agricultural machinery and was considered as a possible technique for the development of the tank in World War I, but was ultimately replaced by the more robust continuous track mechanism.
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According to the 1913 Webster's Revised Unabridged Dictionary, a pedrail is:
"A device intended to replace the wheel of a self-propelled vehicle for use on rough roads and to approximate to the smoothness in running of a wheel on a metal track. The tread consists of a number of rubber shod feet which are connected by ball-and-socket joints to the ends of sliding spokes. Each spoke has attached to it a small roller which in its turn runs under a short pivoted rail controlled by a powerful set of springs. This arrangement permits the feet to accommodate themselves to obstacles even such as steps or stairs."— Webster's Revised Unabridged Dictionary, published 1913 by C. & G. Merriam Co.[1]
The pedrail wheel was invented in 1903 by the Londoner Bramah Joseph Diplock.[2][3] It consists in the adjunction of feet (Latin radical "ped") to the rail of a wheel, in order to improve traction and facilitate movement in uneven or muddy terrain.[4] Sophisticated pedrail wheels were designed, with individual suspension for each foot, which would facilitate the contact with uneven terrain.
Bramah Joseph Diplock also invented the pedrail locomotive which was featured in the 7 February 1904 New York Times.[5]
H. G. Wells, in his short story The Land Ironclads, published in The Strand Magazine in December 1903, described the use of large, armoured cross-country vehicles, armed with cannon and machine guns, and equipped with pedrail wheels, to break through a system of fortified trenches, disrupting the defence and clearing the way for an infantry advance:
"They were essentially long, narrow and very strong steel frameworks carrying the engines, and borne upon eight pairs of big pedrail wheels, each about ten feet in diameter, each a driving wheel and set upon long axles free to swivel round a common axis. This arrangement gave them the maximum of adaptability to the contours of the ground. They crawled level along the ground with one foot high upon a hillock and another deep in a depression, and they could hold themselves erect and steady sideways upon even a steep hillside."—H.G. Wells, The Land Ironclads §4, December 1903.[6]
In War and the Future, H.G. Wells acknowledged Mr. Diplock's pedrail as the origin for his idea of an all-terrain armoured vehicle:[7]
"The idea was suggested to me by the contrivances of a certain Mr. Diplock, whose "ped-rail" notion, the notion of a wheel that was something more than a wheel, a wheel that would take locomotives up hill-sides and across ploughed fields, was public property nearly twenty years ago"—War and the Future H.G. Wells.[8]
During World War I, as trench warfare immobilized the front, proposals were made for pedrail machines that could make their way to enemy lines: in June 1914, Major Glasfurd, who was fighting in France, proposed an idea for such a pedrail machine.[9]
The Royal Navy and the Landships Committee (established in February 1915),[10] largely at Winston Churchill's urging, agreed to sponsor experiments and tests of armoured tractors as a type of "land ship". In March 1915, Churchill ordered the building of 18 experimental landships: 12 using Diplock pedrails, and 6 using large wheels, but construction failed to move forward, as the wheels seemed impractical, and the pedrails met with industrial problems.[11] When these attempts had failed by July 1915, continuous tracks were tested, and they soon became the technique of choice for the development of the tank.[12] The pedrail was considered as too weak for an all-terrain armoured machine, and "caterpillar" continuous tracks were ultimately preferred.[13]
The system was also used as a way to prevent weapons from bogging down in muddy roads, by spreading the load, as in heavy guns such as the Big Bertha. It was also incorporated in early Four-wheel drive designs.