Remember that most modern powertrains utilize some degree of downspeeding. Here’s a quick refresher on downspeeding details.
Axle ratio: This ratio is simply the number of teeth on the ring gear of the rear axle compared to the number of teeth on the pinion gear on the driveshaft. In an axle ratio of 2.16:1, teeth on the rear axle for every one tooth on the driveshaft. To make it all work, the transmission needs to be tuned to handle the engine speeds and an axle needs to be ready to accept it; faster ratios enable the engine to maintain the 200 HP needed at the wheel end despite a lower engine RPM.
Direct drive transmissions: Aggressive downspeeding with direct drive transmissions (ratios from 2.15-2.47/2.64) should be limited to long haul fleets that spend 80-90% in top two gears and have at least 30 miles in between stops. Direct-drive transmissions require the lowest numeric ratios.
Click here to read how today’s axles integrate into the latest powertrains.
Overdrive transmissions: Slightly faster ratios with overdrive transmissions are more versatile in applications such as line haul, regional and city delivery, with ratios ranging from 2.64 to 4.11. Overdrive transmissions utilizing small-step technology allow additional downspeeding in overdrive and efficiency in direct drive to keep the engine in peak range at various highway speeds.
Torque: For every 100 RPM drop in engine speed to save fuel, the torque goes up significantly to maintain full power. In previous conversations with Fleet Equipment, Dana mentioned that in its testing of high-torque axle applications, the company found that decreasing an engine’s RPM at cruise speed from 1,450 RPM for the typical engine to 1,125 RPM for a downsped engine increased torque loads in the driveline by 57%. These higher torques place added stress on the axle, driveshaft and inter-axle shaft, greatly reducing the life of these components, especially u-joints.
