One of the biggest things happening with drive axles today is a continued trend toward faster ratios, i.e. smaller ratio numbers – and it’s no wonder. The engine manufacturers keep dropping the “sweet spot” – in some cases, even the peak torque rpm.

“There does appear to be a continued movement to faster axle ratios, especially with the Caterpillar engines,” says Leo Wenstrup, Dana Corp.’s senior product manager – drive axles for commercial vehicle systems. And Gary Ganaway, ArvinMeritor’s director of next-generation tandem axle, agrees. “We are seeing faster ratios, typically by one step size, for the 2007 engines,” Ganaway says. This is necessary to accommodate the lower rpms of new engines, he says.

Ganaway’s colleague, Joe Malkowski, program manager for next-generation tandem axle, says that ArvinMeritor’s OEM customers have not singled out any special requirements for Caterpillar engines. “In addition to the engine, the axle ratio is a function of desired cruising speed, tire size and transmission,” Malkowski says.

Other somewhat radical changes relate to the escalating cost of fuel. “The distribution of ratio usage does indicate a move to direct-drive transmissions,” Wenstrup says. Consider the ratio needed for an ideal level road cruise rpm for a 2007 engine with a typical .74:1 ratio overdrive transmission and 502 rpm tires. A 3.40:1 axle will give 1,263 rpm at 60 mph when running the overdrive. If running a direct-drive transmission with the same tire size, a 2.50:1 axle would give 1,255 rpm, while a 2.53:1 axle would give 1,270. Dana’s fastest axle is a 2.53:1 design, while Meritor offers one down to 2.50:1, with a 2.67:1 configuration on the way.

A direct-drive transmission carries the power from the clutch to the front driveshaft without the four gear meshes required when running in overdrive. Because all the transmission gears turn slower with the lower numerical axle ratio needed with direct drive, oil churning losses also are reduced. ArvinMeritor anticipates continued growth in the use of direct-drive transmissions, with some fleet studies showing as much as a 1.5 percent fuel economy improvement in some applications.

The advantage of overdrive is that it reduces the torque carried by the transmission output gearing, driveshaft and axle input gearing by the same percentage as the ratio difference (typically about .74:1), thus allowing the use of less beefy parts.

Another substantial – though perhaps not surprising – trend is the use of single drive axles. The trend at Dana has been not only to lower ratio numbers, Wenstrup says: “We are also seeing an increase in fast-ratio, heavy single axles, which supports the move to 6-by-2 configurations for linehaul applications.” Things are similar at ArvinMeritor: “We are seeing the highest demand for faster ratios in single drive axle applications,” Ganaway says.