Can you imagine a time when transmission spec’ing simply meant choosing between a 10-, 13-, or 15-speed in direct or overdrive? Life was simple once.

Transmission spec’ing is no longer a matter of fantasizing how fast you might be able to go if you found a long straight stretch of road and geared the truck accordingly. Actually, most of today’s driveline specs would have been the envy of the old bull haulers.

Two-six-four rears and a point-seven-three overdrive will move a truck to well over 100 mph if today’s engines ran as fast as the old ones did.

You don’t see flames shooting out the stack of an engine running 1,300 rpm. EPA really put the boots to the 100-mph truck.

The gear-fast, run-slow philosophy started taking hold in the 1990s as engine speed at cruise started trending downward. Gone are the days of the 1,800 to 2,000 rpm cruise speeds with 4.11:1 and 5.25:1 rear ends. To make up the difference at 1,300-to-1,500 rpm cruise speed, we use 3.34:1 and even 2.64:1 rear ends.

With all that evolution going on around it, the transmission still has to match engine speed with road speed in the most efficient manner possible under a hugely varying combination of circumstances and applications. Transmission spec’ing is no longer a task for the feint of heart.

DROP THE REVS

The focus on engine speed became really critical in 2007 when EPA’s emissions mandate pushed EGR rates to 30 percent and beyond. Fuel economy sweet spots became sweet dots; a few hundred rpm over or under the optimum speed will drain fuel tanks at alarming rates, yet many Canadian fleets are forced to contend with the dichotomy of spec’ing for U.S. Interstate highways and Ontario’s Highways 11 and 17.

The maintenance director at Bison Transport, Itamar Levine, struggles endlessly with this.

“We have this schizophrenic situation where we run half our miles between Toronto and Winnipeg on Hwy 11 and 17 at 90 km/h, and the other half in the U.S. on flat smooth roads at 100 km/h,” he notes.
 

Transmission accomplished

“I have to spec a truck that can do both, well. I don’t want drivers running a gear down in Ontario. Maybe we need to go back to the old two-speed rear ends we used to have on straight trucks?”

He offered no silver bullet, by the way, but did say that higher torque engines with broad peak torque bands are really helping make the trucks more drivable. And to stay on top of the drivers who don’t get it, Levine relies on ECM downloads
to show time in top gear and time one gear down.

“They have really helped us get the drivers to come around,” Levine says. “The engines can do it, but the drivers don’t always think so. We pushed them that way, and now they see that the engines do perform quite well at low rpm at 90 km/h.”

Before we had great wide swaths of torque, and gobs of it, the 13- and 18-speed transmission filled the gap. Splitting gears let drivers stay closer to peak torque longer. That’s not technically as necessary today, but Canadians still prefer their multi-speed transmissions to their simpler and cheaper cousins.

Mike Sharpe, territory manager for Roadranger Marketing in Canada says with our higher gross vehicle weights and demanding operating terrain, the multi-speeds fill a big gap.

“Unless mountains are a factor, the 10-speeds are a good choice up to 110,000 lb GVW,” he says. “Above that, the 18-speeds are the better option.”

LOOK MA, NO HANDS

Automated manuals have come into their own, and with fewer and fewer reasons to say no to them, they are now on the spec’ing short list at more and more fleets. Eaton’s UltraShift Plus has resolved the clutching issues associated with previous generations of their automated product line.

That transmission, along with the I-Shift from Volvo and Mack’s mDrive are bringing functionality to market that previously was thought impossible, especially with the integration of some of the emerging safety systems such as adaptive cruise control and electronic stability control.

The transmission is an integral part of the drivetrain, so the ability to control shifting along with braking and throttle application, allows those safety systems a high degree of permission to intervene, and greater functionality.

Transmission spec’ing was simpler once

Automateds continue to offer popular ratios and gear sets, but with the added degree of electronic oversight, even poor drivers can achieve safer and more fuel efficient operation with ultimately less potential for driveline damage.

“We see continual growth year over year in automateds, but I can’t say when they will surpass manuals,” says Eaton’s North American product planning manager, Shane Groner. “Growth in that market will not be linear, like two or three percent each year. With the UltraShift Plus, we’ve added so much more application flexibility that it’s finding a place in more and more markets. I think we’ll see the trend toward automateds steepen considerably in coming years.”

Additionally, communication between engine and transmission is improving. This allows for closer integration of the engine’s operating parameters with the transmission’s functionality. The result is optimized performance in a broader range of applications.

So, you can debate the virtues of direct versus overdrive, or 10 speeds versus 13, all you want. The equipment is becoming more sophisticated, and at the same time, it’s lessening the impact of those concerns in many cases.

EPA’s emissions regs have forced us to focus more on engine speed at cruise, and any transmission that’ll help us stick close to the sweet spot has to be an advantage. Until we see a constant-velocity transmission in trucking, or diesel-electric drivetrains, we’ll have somewhere between six and 18 decisions to make.

SIDEBAR

Direct vs. Overdrive:

Believe it or not, with everything else to consider in spec’ing a drivetrain, the debate over direct versus overdrive rages on.
That the debate is still a hot one after all these years tells us there’s merit in weighing the alternatives.

A direct-drive transmission will always be more efficient than an overdrive if you can gear it properly. In top gear, the power path through the transmission is, as the name implies, direct—no countershafts, no additional gear meshes, fewer bearings, etc. Torque goes right through the main shaft in and out of the transmission. In other words, there’s less parasitic loss.

Direct boxes are less complex and there are few moving parts. Weight differences are negligible, but fuel savings, while in the white-noise region, are quantifiable. And if you’ve got hundreds of trucks, it can make a difference.

Direct drives have two limitations: above 1,750 lb-ft, there can be some torque handing concerns.

Overdrive mutes the impact of the high torque output, protecting the rest of the driveline.

Direct drives can require a more robust driveline to accommodate higher torque output. And to keep engine speed down, you need to speed up the final drive ratio at the differential. Until recently, really tall rear ends weren’t common.

Today, Dana Spicer offers its DSP40 axle with a 3.07:1 ratio, and Meritor offers its 14X Amboid axle with a 2.64:1 ratio. Direct transmissions could come into vogue once again.

Bison’s Levine just spec’d 50 Volvos with 12-speed direct I-Shifts and Meritor’s 2.64:1 rears. He’s expecting better performance from them than his previous overdrive transmissions. EatonFuller’s 13- and 18-speeds are offered only in overdrive configuration.