Drivetrains 101

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Everyone knows that designing a drivetrain is a different task than it was in the glory days before low-emissions engines arrived on the scene. But there are some basic essentials that haven’t changed.

We’ll have a look at them here, and then we’ll examine some more specific issues.

First, what will the truck do? Local or long haul, it has specific duties in a definable operating environment, demanding a particular spec. Has anything changed since the last time you spec’d a truck? Is it likely to change in the near future? Bear both in mind and be prepared to accommodate possible future work.

And second, do your homework. You can rely on your dealer’s salesman to use his computer spec’ing program effectively, and the factory engineers approving the spec will catch any gross errors. But you’d do well to ensure that you’re absolutely up to date with the latest technologies — engines, transmissions, tires, you name it –because they’re changing all the time. Ideally, you’ll meet the sales guy knowing at least as much or more than he does about the components available to you — or soon to be.

More than ever, you must stay on top of the potential efficiencies available to you because you need to be at your competitive best, and the latest technologies can help you get there.

That said, don’t forget the basics of serviceability, durability, safety, and of course cost.

When calculating gradeability, make sure to include the steepest
grade the truck will comfortably climb at highway speed.

Engines: Do you want more engine than you really need? Can you define adequate power? Start by deciding on road speed. You may well decide that 100 km/h is the optimum speed for safety and fuel economy, in which case you might want to add 5 km/h and design the drivetrain to cruise at that speed in high gear.

Remember that the sweet spot has moved in modern engines, and it’s usually a narrow place that leaves little room for spec’ing error.

They’re harder to find than in days gone by, but you’d do well to ask your salesman for engine ‘maps’ detailing torque and horsepower curves for the engines you’re looking at. Look at where torque peaks on the tach and where horsepower falls off, and keep them in mind when you’re thinking about transmission and rear-end ratios.

Consider the possibility that you don’t need a 14- or 15-liter diesel, even though they bring better resale value. The fact is, today’s 11-, 12- and 13-liter motors develop healthy power and torque, in ratings of 425 hp or more, and torque as high as 1,650 lb ft.

They accelerate and climb hills well and the lower displacement can deliver about a half-a-mile per gallon better fuel economy, maybe more. As well, the smaller engines weigh 400 to 600 lb less than the big bores engine, and they’ll cost several thousand dollars less.

You might want to think about variable-power, multi-torque engines. They’ll deliver serious power when it’s called for — usually at highway speeds in higher gears — and cut back when it’s not. This preserves performance but saves fuel, and in some cases it could allow you to spec lighter drivetrain components to match the engine’s lowest torque output in lower gears where damage is most likely to be done. But make sure your drivers know about the feature so they can let it work.

Capacity: Weight capacity isn’t the only consideration here, because axles are rated both by their load-carrying capacity and by GCW (drawbar load). Capacity also applies to the torque and horsepower rating of the components, so the minimum torque requirement for the job becomes your baseline. You can safely add 10 or 15 percent to that figure but don’t go light, whatever you do.

You should figure out the maximum and average weight of the loads you’ll be carrying, and don’t ignore the potential for inadvertent overload. Which suggests that you ought to estimate maximum payloads on the high side. That might mean a more expensive axle, but the cost to replace a single carrier destroyed by overload abuse might be higher still.

Gradeability: This is the measure of how steep a hill the truck can climb without losing speed in a given gear. If you’re to optimize driver satisfaction, safety, and productivity, you can’t afford a truck that dies on a three-percent grade. Calculate gradeability for the top three gears to assess highway performance, or get your salesman to do it, and make sure to include the steepest grade the truck will comfortably climb at highway speed.

Engine rpm and tire size are the main
determinants of differential ratios.

The standard recommendation calls for a fully loaded vehicle to show 0.3 percent gradeability at full engine speed or one-percent at peak torque in cruise gear. It should maintain that standard at cruising rpm, which is commonly 50 rpm either side of 1,400 these days.

Not surprisingly, drivers hate piloting slow trucks with lousy hill-climbing power, which add minutes or even hours to a long trip, and that can easily push a driver out of logbook hours. There’s a safety issue too when the truck is significantly slower on a grade than everyone else. It should be able to keep up with traffic on all but the worst hills.

One of the factors in gearing is wheel and tire diameter, so once the truck’s in service, don’t change tire sizes without considering any effect on road and engine speed.

Startability: This is the steepest percentage grade on which a vehicle can be started from rest in first or low gear and accelerated. It’s based on clutch-engagement torque at 800 rpm. The standard recommendation is a minimum startability factor of 15 percent for moderate on-highway trucks and 20 to 30 percent for severe off-highway trucks.

Gear steps: These are important in terms of minimizing the number of shifts a driver has to make routinely and of keeping the engine in its most fuel-efficient range. As well, gears that are too tall will strain the entire drivetrain.

Multi-speed transmissions — those with 13 to 18 speeds — are often spec’d without much question when heavy gross weights are in the mix, but they’re not always necessary. Your operating conditions—especially the terrain your trucks roll across — may well allow something different.

The fact is there’s no single ideal drivetrain spec for any particular truck or job, so we suggest you and your dealer salesman go over this carefully. If there’s anything unique about your work — or if you simply want to explore spec’ing possibilities — it would make sense to have him consult with the transmission maker and ask for their computer models to spit out workable combinations. You might be surprised.

Miscellany: Among the remaining factors, durability is pretty important, so it makes a ton of sense to spec things just a little heavier than you need. You don’t want to design weakness into the mix before you pull pound one. You can get a bit clever here and check the durability of various components by inquiring with local remanufacturers. What wears out first? What breaks most often?

Another item: your differential choice, which is crucial to your driveshaft spec. If the rear end ratio is tall, you’ll find that the driveshaft is under extreme torque stress, meaning you’ll likely need a heavy and expensive one. For the sake of better balance, you might want to spec a deeper-reduction diff and move up to a taller overdrive transmission.

Engine rpm and tire size are the main determinants of differential ratios. Smaller tires demand a taller diff or a tall overdrive transmission.

And then there’s the desired weak link in the driveshaft assembly, which should be an easily accessible component deliberately spec’d for exactly the load you’ll be pulling. The idea is to prevent the catastrophic failure of expensive components like the engine or differentials by sacrificing a lesser, cheaper one like the clutch or U-joints.

Aside from gearing calculations, and your dealership can handle those, drivetrain spec’ing is not overly complex. Not as simple as it once was, but far more important if you’re to get the most out of your equipment. And there’s no room for anything but precision.

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Rolf Lockwood is editor emeritus of Today's Trucking and a regular contributor to

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