It seems as if most everything is a trade-off. In the case of downspeeding, torque is both a positive and a potential source of concern. But downspeeding is here to stay, so when it’s time to spec your next truck, a clear understanding of the torque-downspeeding relationship will help you manage fleet costs by making the most productive and efficient choices.
Downspeeding is a dream on the highway
Fewer rpms means lower fuel consumption. If your truck will mainly cruise at highway speeds, you can save substantially. But of course it’s not that simple. It takes horsepower to keep your truck moving forward, and torque to move it uphill. The less you have to shift – especially downshift – the more efficiently your truck can function.
Today’s engines are designed to produce peak torque at lower rpm, typically in the 1,000 to 1,400 rpm range. That gives you a few hundred rpm of “breathing room” before you have to downshift, if you’re loaded and driving at 65 mph (just slightly above peak torque). Given the modest terrain changes of most highways, you may not have to shift at all. Maximum efficiency. Major fuel savings.
However . . .
When your engine has to operate at low rpm and in a low gear – at launch, when hooking up a trailer, under heavy load, on steep grades, any time wheel slip could occur — downspeeding requires a lot of torque. A lot. In lower gears, the transmission multiplies torque, to the point where it can damage your driveline. That’s because the transmission is multiplying the shock transmitted through it by every cylinder firing. Every component along the way is subjected to tremendous force – torque spikes that can reach almost 25,000 lb-ft in low gears.
According to Meritor, “the time it takes an engine to go from idle at 700 rpm to peak torque at 1,000 rpm is now measured in tenths of a second, whereas it may have taken a second or two in earlier, pre-2010 engines.”
And there’s more. The rear axle produces a multiplier effect, too. Torque increases as the driveshaft’s rotational speed decreases. So torque experienced by the driveline is actually higher than the engine’s published torque rating. With all that, it’s easy to see why downspeeding can cause faster component wear and tear, not to mention total drivetrain failures.
So it’s a trade-off. You have to manage fleet costs to protect your bottom line. The fuel economies downspeeding offers can be significant. But you run the risk of higher maintenance and repair costs and related down time, all of which erode profitability.
What to do?
Spec’ing your next truck requires some careful thinking and priority-setting. Downspeeding isn’t the automatic choice, because these powertrains are not the right option for every application. If your intended truck will be an 80,000-pound gross combination weight vehicle and will spend most of its time cruising the Interstates without much terrain change, it is in the sweet spot. Downspeeding will bring efficiencies as advertised.
But many companies like to manage fleet costs by using trucks for more than one purpose. And here it can get tricky. If your truck will also see duty in P&D operations, for instance, spec’ing a downsped powertrain will not generate the fuel savings you hope for. And you will be at risk for serious drivetrain damage.
So you have to assess your application(s) with an eye toward the trade-offs – adequate torque for startability and gradeability versus ability to negotiate hilly terrain under heavy load without causing damage to the drivetrain.
One option is to spec a calibration that limits the amount of torque that passes through the engine under certain circumstances or at particular speeds when the truck is in a low gear. You can also spec your driveline differently – choosing among axles and drive shafts specifically designed to optimize direct drive, downsped powertrain function. Newer axles offer higher torque-bearing capacity.
Perhaps the best plan is to talk with your local dealer. Here at Tracey Road Equipment, we understand your challenges as well as your options, so we can give you custom-tailored advice to spec your new truck and manage fleet costs.