In 1921 Johnson designed a slightly smaller vehicle closely based on the Medium D, designed to provide machine-gun support for infantry attacks. An 8 ton amphibious light tank known as the “Light Infantry Tank”, was similar in configuration to the Medium D but had some improvements in the track and suspension. It was powered by an American 100hp Hall-Scott four-cylinder water-cooled aero engine driving through a four-speed gearbox to Rackham steering clutches, and it ran on Johnson’s other new invention, Snake Track. Snake Track was a revolutionary approach to track design; although it was not adopted widely or used for very long, it was a clever alternative to the wire rope system Johnson had been using previously, and was much stronger and more heavily built. With Snake Track each shoe took the form of an oval plate of pressed steel, welded to a segment of tubular backbone. Each segment was joined to its neighbours by a simple ball joint so that each link was free to swivel to any angle, while the complete track not only went round the suspension in the usual way, but was able to form a curve to make steering easier. Its one drawback was that it relied on grease to ease the action, and grease had a tendency to collect grit and sand which together made an excellent grinding paste for wearing away metal. The Light Infantry Tank managed a top speed of 30mph, which was unheard-of for a tank in those days and the vehicle was very manoeuverable. The vehicle was also amphibious, the tracks supplying the propulsive power in water and it demonstrated its amphibious qualities on the Fleet Pond in Hampshire. Even so it didn’t last very long, being sent back to the Tank Design and Experiment Department in June 1922 and ending its days in the original open-air Tank Museum at Bovington before being scrapped. All that remains of it now is a section of its suspension and a few links of Snake Track.
Light Infantry D. T. & E Tank – rope spring track – experimental. Only a single tank was produced in 1921.
Philip Johnson’s suspension.
This vehicle was fitted with the Jonson Snake Track. This track was completely flexible in all directions; each track plate was fixed to a hollow tubular backbone which was joined to the next link by a spherical joint protected by mud seals. The track no longer used a cable but was composed of ball jointed segments with the track shoes welded on the strut between the ball joints. The cable suspension also had a cam tensioning device to stop the cable going slack at the rear end of the tank.
Philip Johnson’s Snake Track.
No light D has survived.