1. The Steering of Tracked Vehicles by Articulation;Nuttall,1960
2. Methods of Evaluation of Off-Road Locomotion;Bekker;Operations Research Office, TM ORO-T-247,1953
3. High Speed Tractor Steering Transmission;Christensen;Society of Automotive Engineers,1960
4. The Evolution of the Chain-Track Tractor;Christensen;The Engineer, (Seven parts),1917
5. Development of the Agricultural Tractor in the United States;Gray;(USDA, 1954) American Society of Agricultural Engineers Reprint,1956