Meritor offers tips on how to downspeed safely

As more fleets spec’ downsped powertrains in the pursuit of greater fuel economy, not all are cognizant of the effect engine downspeeding can have on related drivetrain components.

That was the warning from Bob Ostrander, chief engineer with Meritor, who conducted a Webinar on downspeeding this week.

While Ostrander recognized downspeeding can improve fuel economy by about 1% for every 100 rpm slower the engine turns at cruise speed, he also said it has the potential to place more torque on drivertrain components. A traditional pre-EPA10 linehaul spec’ would typically feature an overdrive transmission and axle ratios in the “mid-3s”, with all drivetrain components rated at about 1,850 lb.-ft. of torque.

“As we get into a downsped spec’ with today’s engines, many of these are coupled with direct drive transmissions and we’re seeing rear axle ratios of 2.47 and faster – Meritor just recently released a 2.28 ratio – and as a result, you then have to be cognizant you’re going to need higher torque-rated drivetrain components to handle the torque coming through the system,” Ostrander warned. “Keep in mind, if you don’t have the rear axle doing torque multiplications, you’re going to have to make up for it someplace else.”

One of the risks involving downspeeding, Ostrander pointed out, is deploying it into unsuitable applications such as city delivery routes with frequent starts and stops. This becomes challenging when a single truck performs both linehaul and regional haul work.

But there are alternatives to downspeeding in these applications, which provide similar benefits, Ostrander explained, including pairing slower rear axle ratios with a close-coupled overdrive transmission.

“You will be able to accomplish the fuel savings you’re looking for with a slower axle ratio coupled with an overdrive transmission and you can get most of what you are trying to seek without running the risk of subjecting the truck and drivetrain components to very high torques and the issues that may come along with it, including the reduced life and performance of your products,” Ostrander explained.

For linehaul applications where downspeeding is well suited, Ostrander warned about the risks associated with the rapid torque rise produced by newer, downsped engines. He said today’s downsped engines can go from idle to peak torque in as little as half a second. During this torque spike, Ostrander said actual peak loads are sometimes far greater than the pre-determined output torque and can momentarily overshoot the approved torque loads until the system dampens it out.

“That’s where you can end up with some issues,” he said. “It doesn’t take long to end up with fracturing issues. These quick, momentary transient conditions can lead to unintended consequences such as fractured components, from clutches to rear axles.”

Ostrander suggested fleets talk to their engine OEMs to discuss strategies for mitigating these peak torque transient conditions.

He also noted ceramic clutches in more widespread use today have a faster response time, making it easy for an inexperienced driver to accidentally slip off the clutch, which can send a shock load through the system. Even low rolling resistance tires can play a role, Ostrander said, since slip torque doesn’t always occur, again placing more strain on components.

One option is to upsize the driveline to something like, in Meritor’s portfolio, the RPL35, which is designed to withstand higher torque loads. While a traditional powertrain, with a 1,550 lb.-ft.-rated engine and 3.55 rear axle ratios and overdrive transmission will see torque into the driveshaft of no more than 16,000 lb.-ft., a downsped powertrain with 2.47 rear axle ratios and a 1,750 lb.-ft.-rated engine could produce torque loads closer to 21,000 lb.-ft.

“So we’re talking about a 5,000 lb.-ft. difference and we have to understand and account for that as we’re sizing and spec’ing our trucks,” Ostrander pointed out. The RPL35 is rated to 21,500 lb.-ft.

With drivetrain torques as much as 44% higher than in a traditional linehaul spec’, Ostrander said fleets employing downspeeding that haven’t accounted for this have seen issues such as broken driveshaft u-joints, fractured rear axle pinion stems and damaged transmission gears, synchronizers and clutches.

Another step fleets can take is to torque-limit the engine. This can limit the maximum amount of torque produced by the engine until the truck reaches cruising speed, eliminating the torque spikes Ostrander alluded to.

“You really need to talk to your engine folks and ask, what controls do they have in place to clip that peak overshoot, to try to bring that down?” Ostrander said. “The torque values coming off the cam bus to the engine are not necessarily the real torque driven through the system momentarily. Talk to your engine people about what strategies they have to clip that torque.”

It’s also a good idea to reduce engine speed at clutch engagement to about 700-900 rpm, Ostrander said, especially if ceramic clutches are used.

The bottom line, Ostrander said, is that when employing downspeeding, fleets need to consider the effect on all related components, and speak with their suppliers to ensure the entire drivertrain is spec’d accordingly.

“Talk to your truck OEM, your engine supplier. Make sure you have the right controls in place so you can optimize for downspeeding but mitigate these peak transient torques,” Ostrander advised. “You have to take a look at everything from the engine all the way back.”

For trucks that have already been deployed with downsped powertrains and traditional components, Ostrander suggested speaking with engine reps to see if they can re-program the engine to reduce torque spikes. He also warned against simply spec’ing larger drivetrain components as the sole approach to addressing these issues.

“Don’t simply say you’re going to put bigger drivetrain components in there because if you don’t put in proper mitigation controls for peak torque, you’re simply going to move that problem someplace else,” Ostrander said. “You may end up moving that problem into a very expensive component and I know of cases where that has happened. Take a look at the whole truck and if you’re going to put in larger-sized components, let’s make sure the entire vehicle is properly designed to handle that.”

Meritor has produced a white paper on the effect downspeeding has on drivetrain components, available for download here.

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James Menzies is editor of Today's Trucking. He has been covering the Canadian trucking industry for more than 20 years and holds a CDL. Reach him at or follow him on Twitter at @JamesMenzies.

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