Power Unit for Triples -A C.O.E. 4 × 4 Tractor 690550
Two configurations of highway truck-trailer combinations being evaluated for hauling gcw loads over 100,000 lb are triples (27 ft trailers) and long doubles (40 or 45 ft trailers). If and when these units become legal, the triples will probably be more widely used because of their better maneuverability which would make it feasible to operate both on and off the thruways.
It is generally agreed that two driving axles would be required for the higher gcw loads in order to provide the needed gradeability and give acceptable axle component life and tire mileage. In the case of triples, the 4 × 4 tractor is the ideal power unit. Tests have shown that on snow-covered grades, the 4 × 4 has superior traction characteristics over the tandem 6 × 4 tractor having the same fifth wheel loading.
The axles for a 4 × 4 power train should provide for a 2 to 1 proportioning of the torque between the rear and front axles for equal tire tractive effort. In order to obtain optimum axle life and tire mileage, the front axle should be designed to operate as a full-time drive unit.