The engine, transmission suppliers and OEMs work together to determine the ideal operating engine RPM for a desired road speed. This information is passed on to rear axle suppliers, like Meritor, to design and develop an axle ratio that best fits the engine and OEM’s needs, said Karl Mayer, Meritor director of product strategy, axles. For example, Meritor currently offers several rear axle ratios from the low threes down to 2.47 for downspeeding. A super-fast 2.28 ratio will be available for orders later this summer, with even faster ratios on the horizon.
Mayer clarified that Super-fast axle ratios (such as 2.31 or 2.28) must only be used with Linehaul or Long haul application (greater than 60,000 miles per year and 30 miles or greater between stops). Super-fast axle ratios should not be used in applications where the truck is used for city delivery or pick-up and delivery (P&D) type service.
The additional torque with faster axle ratios to support downsped engines generally affects the driveshaft, axles and suspension.
“The suspension and attaching hardware must react to the higher loads at vehicle launch. The driveshaft and U-joints transmit the high torque directly into the axle,” said Steve Slesinski, director, global product planning, Dana Commercial Vehicle Driveline Technologies. “The axle input shaft, bearings and gearing must be designed appropriately to handle the higher torque. Also, the continuously higher torque that is applied at highway speed must also be considered so that U-joints, shafts and axles have acceptable long-term life results. No effect at the wheel-end is anticipated, since the torque at the tire remains unchanged with lower cruise engine RPMs.”
When it comes to spec’ing the proper ratio and driveline components that are up to the increased-torque tasks, that is ultimately determined by the OEM considering various factors including engine torque, tire size, axle ratio, cruise speed, transmission gearing and application duty-cycle, Slesinski stated. “In general, at approximately 2.64 rear axle ratio, the more robust SPL350 main driveline should be considered,” he explained. “Successful long-term results with ratios at 2.64 and above have been proven with the SPL250 main driveline.”
Dana’s AdvanTEK 40 tandem drive axle was developed with the fastest ratios in mind. Because a 57% increase in torque occurs when moving from a 3.55 ratio to a 2.26 ratio, the axle structure, gears, bearings, shafts, inter-axle differential and power divider components have been fortified to meet performance and life expectations.
You’ve also likely read a lot about 6×2 configurations. According to Detroit, the main reason for choosing a 6×2 over a 6×4 is the available ratios, and the desire to improve fuel economy, because there is an increase in efficiency with a 6×2 configuration. However, 6×2 configurations are not required for downspeeding, but are growing in popularity as applications permit its use.
