Christie continued to submit designs to the Ordnance board, but none was deemed acceptable. A major reason was the poor cross-country performance, due to limited suspension capabilities. He turned his attention to this problem, and after five years of development (at a cost of $382,000) he produced the revolutionary prototype tank chassis M1928 (Model 1928) design. He proudly referred to it as the "Model 1940" because he considered it to be 12 years ahead of its time. The M1928 still retained large road wheels with no return rollers for the tracks from his earlier designs, so that the tracks could be removed for road travel, allowing for greater speed and range. What made this prototype revolutionary was its new "helicoil" suspension system, whereby each wheel had its own spring-loaded assembly. This reduced space in the interior of the tank, but (combined with a very light overall weight) allowed for unprecedented high-speed cross-country mobility, albeit at the cost of extremely thin armor. Another interesting feature of the M1928 and later Christie designs was sloped armor in front, which could better deflect projectiles. The sloped armor helped to compensate for its thinness. The Army purchased several of Christie's tank prototypes for testing purposes and Christie's patent, allowing them to produce prototypes based on his design.