The Wheel-cum-track Machine The Wheel-cum-track Machine As an idea for saving track wear it was clever; use wheels when you can, and tracks when you have to. But as a useful combat vehicle it was a total disaster. Praying Mantis An amazing design from World War II, based on a Bren Gun Carrier. Two men occupy the crew compartment. They can lower it down to hide or raise it up to shoot. A good idea which did not work. Praying Mantis Chieftain 900 Chieftain 900 For every successful production tank there are always many experiments that fall by the wayside. This was an attempt to improve the British Chieftain with Chobham armour and a new engine for the export market; a project which did not succeed.