A transmission truly lives up to its name: it transmits the motor’s power and torque from engine to driveline and beyond. A manual transmission does so with sets of gears that the vehicle’s driver can shift to accommodate changing situations. Gears are mounted on shafts, which are parallel to the engine’s crankshaft; the transmission’s sliding clutches move along the main shaft fore and aft to take one set of gears out of the power flow and introduce another.

This box contains the single largest collection of gears in the whole vehicle, and its ratio is the most complex of the three driveline numerical ratios that affect a truck’s performance (the others are rear-axle ratio and tire/wheel size). Change one item and you impact the others, like it or not.

There are two basic facts that pertain to understanding the effect of shifting gears: more torque (power) means less speed; and more speed means less torque. As the gears increase torque, they decrease vehicle speed at about the same rate. This means the driver can change the ratio between the engine and the drive wheels to get either more speed or more reduction (torque). The lower the gear, the more torque, the higher the gear, the more speed.

On heavy-duty transmissions, dual countershafts are used, with one placed on either side of the main shaft. This sends power and torque through multiple sets of gears, which together can take the loads better than any single set could. Countershafts sometimes also drive power-take-off assemblies, which operate pumps, winches and other auxiliary equipment on the truck.

(The above general description applies to all types of manual and semi-automatic transmissions. Fully automatic units historically have used highly complex planetary-gear sets, along with hydraulic pumps, valves, and an oil-filled torque converter. Those are another story-we’ll leave them for another day.)

BASIC DESIGNS

In heavy trucks, much shifting work has been eliminated by the range-type transmission, which has been around for decades. It uses a five-speed main box that’s shifted with the usual lever, plus a two-speed “range box”, operated by a switch-controlled pneumatic mechanism. The driver starts out in low range and shifts through the first four or five gears, then flips the range switch into “high” and goes back through the same shift pattern. (A variation on this is the Eaton Fuller Super-10, where only five lever positions are used, and each is “split” via a thumb switch.) The nine- and 10-speed transmissions so popular with fleets are examples of this basic range-type box.

To get even more ratios, a “splitter box” can be added behind the range box. This is done with nine-speed transmissions to create 13- and 18-speed gearboxes. In a 13-speed, the driver splits the top four ratios in high range; with an 18-speed, ratios in both low and high range can be split. (Most of today’s diesels don’t need all the possible ratios to be split.) Splitters are also used on other units, such as 12- and 14-speed models.

On/off-road trucks need more low-end muscle, so specialized six-speed or “LL” (Low-Low) eight-speeds are common. A “deep reduction” range-accessed through a flip of an air switch-provides the two or three lowest ratios, only used for off-road demands. A variation is the 15-speed transmission, which is really a 10-speed with a deep-reduction range that gives five extra-low ratios. On-road, it’s driven like a standard 10-speed. These units have no splitter box, so the driver has to shift through each main gear.

SPEC’ING GUIDELINES

Which ratio is right for you? “The biggest thing in selecting a ratio package is to appreciate what that truck is going to do,” says Pete Messeroll, national accounts executive with Eaton Truck Components, Brampton, Ont. “What kind of weights will it haul? What’s the nature of the vocation? Does the operator care more about highway speed and having power in reserve than maximizing fuel economy? The selection of the engine is often based on these concerns, and that directly affects how you spec the transmission.”

So for the time being, forget about gears and assess these areas:

* What’s your typical gross vehicle weight or gross combination weight? The heavier the gross, the more powerful the engine and stronger all the drivetrain components must be. If your GWV will vary, list several target weights and tell your truck dealer about them.

* How many miles do you expect to cover per trip segment, and the typical operating environment (highway cruising, frequent stops, etc.)? What kind of terrain will the truck encounter, and what do you expect its average road speed to be?

* As a percentage, how much time or distance do you expect to spend at each vehicle weight? How many empty miles will you run?

* List the vehicle and engine configurations. Make special note of any aerodynamic aids, tire size and type, engine brakes and retardation.

GEARING

The expression “drivetrain” is one that looks beyond the transmission: it covers the clutch, transmission, drive shaft, and rear axle. Together, these components transmit power from the engine to the driving wheels. Like pieces of a puzzle, if the individual components aren’t compatible, they’re merely pieces. “Gearing” is the process by which all the key elements of the drivetrain are interconnected to give a truck the ability to generate and adjust its momentum.

To know which pieces fit best, look at a few basic terms and concepts:

* Geared Road Speed. This is the truck’s top speed possible, based on the transmission’s top gear and rear-axle ratios. As noted above, it’s important to define the speed at which you plan to cruise, and the gross weights your rig will haul, because you don’t want the engine to run out of revs just cruising down the highway. That wastes fuel.

* Speedability. The estimated actual road speed your truck can attain. This takes into account more real-world conditions: tire size, gross weight, and pavement friction. Speedability should be higher than your desired cruising speed.

* Maximum Gear Reduction. The highest multiplication of engine torque available, calculated by multiplying the transmission’s lowest (i.e., highest numerical) ratio by the rear-axle ratio. Trucks hauling heavy loads or operating in severe terrain need greater maximum gear reduction than others.

* Gear-Fast/Run-Slow. As engines become increasingly powerful (remember when 500 horsepower was just a dream?), that power is more and more coming from a high-torque/low-rpm setting rather than the traditional low-torque/high-rpm approach.

Under gear-fast/run-slow, geared speed is higher than the cruise speed, allowing the engine to cruise at a modest 1400 to 1600 rpm. However, even at these low revs, the axle must be able to run fast at highway speed-perhaps down in the 1:2.8 or 1:3.0 range for linehaul applications.

* Ratio Steps. Described in percentages, gear steps are like the spaces between the rungs of a ladder. If a person has strong legs, he can take steps that are placed far apart. If he’s not particularly strong, or has to lug a heavy load up the ladder, the rungs need to be closer together. If the rungs are too close, his progress up the ladder will be slow and he’ll become fatigued.

Your engine is the guy on the ladder. It needs properly spaced steps so it can generate enough rpm to move the truck. In most ratios, the steps should be close enough to provide at least 90% of the engine’s rated horsepower after each normal up-shift.

How many gears (or “speeds”) should your transmission have? The gear-fast/run- slow parameters tend to favor the abilities and economics of newer nine- and 10-speed units. Previous 13-speed users might find that they no longer need the close 15 to 17% steps in top gear, in view of today’s flatter horsepower curves-and a 10-speed transmission will cost less, too.

“I’d say 10-speed transmissions are pretty standard for fleets, but owner-operators still prefer 13-, 15- or 18-speed boxes,” Pete Messeroll observes. “In my opinion, a lot of owner-ops focus on road speed too much. They’ll gear their vehicle to get a fast operating speed, like by taking an overdrive 13-speed or even 18-speed transmission and putting a 1:3.9 or 1:3.7-ratio drive axle in, and end up with very high road speed capabilities.

“The thing is, though,” he continues, “they could change the spec and use that muscle in a more economic manner, say by achieving more gradeability so they’d do less shifting and thereby save fuel.”

THREE KEYS

And that brings us to three key concepts in the process of spec’ing a transmission: gradeability, startability, and the direct-drive versus overdrive question.

* Gradeability simply measures the ability of the top gear to allow the truck to climb a certain amount of grade before the driver has to downshift. The guidelines at one typical transmission OEM state that a fully loaded vehicle should have a 0.3% gradeability at governed engine speed, or a 1% gradeability at peak torque in the cruise gear. This basically means you shouldn’t have to downshift if you encounter a 1% incline while in your top gear.

* Startability is the drivetrain’s ability to get the vehicle and load moving. The lowest ratio’s gears must be numerically high enough to get the truck moving on a specified (in percent of grade) upgrade in “low” gear, and on flat terrain (first gear) without abusing the driveline or clutch. Startability is likewise expressed in a percentage number-although the percent sign is usually deleted-that refers to the amount of grade on which, if you stopped, you should be able to start moving again without causing any drivetrain damage.

According to one OEM, startability should be spec’d as a rating of at least 16 for general on-highway operations; 25 for moderate on/off highway service; and 30 for severe on/off-highway environments.

Calculating startability (referred to as “S” in the following formula) involves a formula that’s simpler to use than it looks:

S = T800 x Ra x Rt x M/(10.7 x GW)

In this case, T800 represents engine torque at clutch engagement (800 pound feet); Ra is the axle ratio (let’s use a value of 3.73 for this example); Rt is the transmission’s low-gear ratio (we’ll call it 12.45); M is the tire rpm (we’ll use 474), and GW is gross vehicle weight (we’ll assume 80,000 pounds here). The 10.7 figure is an engineering constant.

So, the math becomes: S = 800 x 3.73 x 12.45 x 474/(10.7 x 80,000) Or: 17,609,479/856,000 = 20.6

Recalling our guidelines above, the startability factor of 20.6 wouldn’t be advisable for more demanding environments, but is more than adequate for general linehaul work. Plug in your own numbers and see what you get.

o Direct drive or overdrive? With a direct-drive transmission design, the input and output shafts spin at the same rate in top gear, passing torque from the engine straight through the transmission. Overdrive uses an extra set of gears to speed up the output shaft, meaning that the engine turns slower than the output and drive shafts. (Overdrive ratios vary from 0.72:1 to 0.86:1.)

Direct-drive enthusiasts say that system gives your fuel economy a boost because torque in direct-drive transmissions travels straight through the input shaft to the main shaft and on to the output shaft in top gear.a true 1:1 ratio. Proponents of overdrive say the lower engine revs of their approach compensate in less wear and tear on the drivetrain, offsetting any fuel-economy benefits of direct- drive.

Manufacturers know which engine and transmission models work best together. Still, check the torque and horsepower curves carefully to ensure that the transmission you want will match the engine’s performance characteristics.

However, it’s useful to recall a key point made in Recommended Practice 1107 of The Maintenance Council of the American Trucking Associations, since it addresses a fundamental choice that every operator must think about in choosing a transmission: “Be prepared to trade performance to get good fuel economy. It takes energy to accelerate quickly, to go fast, and to maintain speed up hills. That energy equates to fuel burned. By lowering your performance demands, and thus your energy requirements, you will achieve good fuel economy with reasonable performance.”