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Drivetrain Spec'ing

by Jim Park

There’s no such thing as an ideal spec. Each truck’s performance is the result of a combination of known factors, variable conditions, and compromises its owner makes to move freight efficiently. Before you start, you need to nail down the known factors, factor in the variables, and then choose between several options to get the best-case-most-of-the-time scenario.

Known factors might include the type of route you plan to run, with Canadian or American weights, or both; what type of road you’ll be driving on, such as four-lane or two-lane highway, or what percentage of either; and flat, hilly, or mountainous terrain.

Among the variables, consider the likelihood of switching applications dramatically during the life of the truck; how much of a payment you can afford to carry; and how much you budget for operating costs.

Derek Varley, fleet manager with Mackie Moving Systems in Oshawa, Ont., suggests starting with some decidedly non-technical numbers: fuel costs. He suggests a revenue-and-cost exercise, starting with how much fuel you’ll burn over the life of the truck. For example, running 120,000 miles annually at 6.0 mpg will see 20,000 gallons of fuel burned each year. At eight miles per gallon, you’ll burn 15,000 gallons annually. Over five years, that’s 100,000 and 75,000 gallons respectively.

“When you carry that over five years, you can’t help but notice the difference,” Varley says. At today’s prices, you’ll be spending between $45,000 and $60,000 a year on fuel.

Engine & Road Speed
If you determine, for example, that you want to haul B-trains in Atlantic Canada, and you want to run at 100 km/h, each engine manufacturer can provide optimum operating ranges for their product under those conditions. If it’s determined that you need to run at 1600 rpm for best fuel economy at 100 km/h with a B-train, you’ll have to spec a drivetrain that will allow the engine to run at that speed.

You also have to consider startability (the ability to start the vehicle rolling on a certain grade when loaded), and gradeability (the truck’s ability to climb a hill). Your sales rep will have a computer program to determine what first-gear ratio provides adequate startability for your application. For example, the FreedomLine’s 14.21 low-gear ratio may be adequate, but Mack’s 18-speed offers a ratio of 16.42. Everything else being equal, it may offer better startability, but its final drive of 0.71 might make it too fast for the rolling terrain of New Brunswick. Gradeability might suffer. As you can see, it’s all about compromise.

John Bertwhistle, a district sales manager for ArvinMeritor, uses a formula to determine startability. You multiply the transmission first-gear ratio by the rear-axle ratio. The ideal number will be around 50. For example, the FreedomLine’s first-gear ratio is 12.33, times, a drive axle ratio of 3.90: 1 will give you a startability rating of 48. Not bad.

“There aren't many trucks these days that aren't geared properly,” says Bertwhistle. “Today’s engines have a lot more range than those of 10 or 15 years ago. Then, it was critical to get it right, and often they didn’t.”

In a less confusing example, let’s say you plan to haul to the U.S. (80,000 lb gross), mostly the Midwest and Texas out of Winnipeg: light loads, flat terrain, and nearly all Interstate highway.

Dave McKenna, Mack Truck’s product marketing manager, tells us that to keep the truck moving at 65 mph, fully loaded, requires 297 hp from the engine. “It’s a mathematical formula,” he says, “In real life, given headwinds and everything else, it could be slightly more.”

So, you start looking at something in the 425-450-hp range – that gives you a little bit of a margin. Your favorite engine brand in that horsepower range is set up to give best fuel economy at 1325 rpm, and your preferred road speed is 65 mph.

Now, you need to gear the truck so that it runs that way. There’s no point in gearing to go 75 if you don’t plan to go that fast because if the speed limit dictates that you run at 65, you’ll be running one gear back and fuel economy will suffer. It pays to be realistic at spec’ing time.

“There’s more than one way to get that engine speed at 65 mph,” offers Mike Sharpe, territory manager for Eaton Roadranger. “You need to look at the transmission, axles, and tires.”

You decide that you like brand X tires, and those best suited for your application are rated at 476 revolutions per mile. You can now determine the transmission/rear-axle combination.

Sharpe says overdrive transmissions are more popular today because they promote lower engine speeds and have better torque capacity. You can debate that one, but all the popular overdrive models offer a final-drive ratio of somewhere between 0.72 and 0.81. So, now it’s a choice of eight to 18 speeds.

A Quick Q&A
Do you need an 18-speed for that Winnipeg-Texas haul? Probably not. It’s more transmission than the application demands. The engine probably won’t ever see service pulling B-trains, so second life isn’t an issue. Eighteens are heavier and more expensive than lesser transmissions, and won’t add any savings to the job you’re doing.

Will a 13-speed work? Yes, and well. It offers closer steps between gears for optimizing engine speed (and fuel economy), but they’re more expensive than an 8- or 10-speed. It will add value at resale time, but there are more moving parts, and therefore they’re easier to damage, especially in the hands of a poor driver.
How about an 8- or 10-speed? They’ll work too. They’re lighter, less expensive, and easier to operate. There’s less to go wrong, but fuel economy could suffer slightly because of the wider gear steps. They’re also less popular at resale time, which could diminish the value of your trade-in.

In this case, given the terrain and the loads, the 10-speed might be the best choice from a mechanical perspective. But personal preference and resale value have to be considered. Says Sharpe, “All things being equal, most owner-ops would choose the 13-speed, whereas a lot of fleets would opt for the 10.”

What remains is to match your preferred road speed to the engine maker’s recommended engine speed for optimal fuel economy.

There are loads of variables involved in spec’ing a powertrain, and cost of operation should probably be first among them. If, as Varley suggests, there could be as much as two miles per gallon on the table if the wrong choice is made, that would be enough to put most owner-ops out of business. Spec smart, live longer.