Trev's Tale - the pre-quel

Modelling tracked vehicles during the Chobham years.

Exploring Chobham Common

As mentioned in the Tank Museum volunteer years blog, I used Chobham Common as the ideal area to operate and develop model tracked vehicles. The common is about 1 mile square, being bisected by the M3 motorway. It is mainly sandy heath land with some low wet areas and criss-crossed with wide sandy firebreak tracks separating large areas of heather, gorse and stands of young pine trees. The tracks are cut with rain eroded gullies where they run downhill, these are between 1 -4 feet wide, up to 3 feet deep and of a Vee cross section. Lower levels of the heath often have standing water over a viscous clay stratum, fed by drainage ditches.

This area was an ideal terrain to run tracked models. The first efforts were creating a simple test bed (see earlier Photograph) which worked well but lacked the ability to traverse much of the area. Thus, it was decided to carry out a terrain study to determine exactly was needed to produce a vehicle capable of crossing at least 90% of the traffic-able area of the common. As no reversing gear was available, (a feature beyond my ability to produce at that time), the model would need a considerable cross-country performance for its size.

Over a three-year period measurements of a range of critical obstacles were made throughout each season, and various test beds developed to negotiate them. Slowly information was gained to fully understand the design requirements for production of a successful model.

Mapping the ground

A map was drawn to a scale of 25 inches to 1 mile, scaled form the 25000 :1 OS map of the area, and the various topographical obstacles to model tracked vehicle movement plotted. Various measuring devices were employed, measuring rods of 1.5” battens marked of in feet and inches, 4 & 6 feet long ( any gully more than 6 feet wide could be crossed anyway ). A ground penetrometer, effectively telescoping tubes with an internal coil spring, a calibrated load scale and a foot, 5inches square. When pushed into soft ground, it registered a load value which gave a pressure in pounds per square inch, this enabled track mean ground pressure to be determined. A simple grousered⁕ sled 5”x 12” was loaded with water containers to a known weight and towed across soft ground using a spring weight scale to determine track slip, on mud, wet clay and loose sand. Most features were photographed for analysis back at home. Finally, my wellington boots were marked out in inches from the sole upwards to sample the depths of the many large puddles that formed during the winter months

Map to follow!

From testbed to model tank

Having produced several testbeds and made an in-depth study of the data collected, it was determined that a model with the following dimensions was needed

• 48 inches long,

• a track width of 5 inches,

• ground clearance of 4 inches,

• a track ground length to track centre distance ratio of 1.5:1 and

• a 1.5 bhp Briggs and Stratton 4-stroke petrol engine would be a suitable starting point for the design.

An all-metal model was constructed conforming to this specification, using a simple loose belt/pulley setup to each track. Steering was affected by jockey wheel pulleys reducing tension on either side. For several years this test bed proved successful, being able to cross most of the terrain it was designed to traverse.

The next steps

The next stage required the design and construction of a true regenerative steering system based upon full size designs, information about which was gained during the Bovington years.

⁕ Grousers are devices intended to increase the traction of continuous tracks, especially in loose material such as soil or snow. This is done by increasing contact with the ground with protrusions, like conventional tyre treads, and comparable to athletes' cleated shoes.