The right gearing can make or break a truck.
It can provide consistent power going through the gears, good hill climbing ability and good fuel economy. As a bonus, it will also help the clutch last.
There are four basic steps to getting the right gearing:
Make sure your transmission’s torque rating matches that of the engine (people have been known to underspec a transmission to save dollars). This is not too much of a concern today because the factory is likely to refuse to build the wrong combination of components, according to Bill Batten, product line manager of heavy-duty transmissions at the Fuller Transmission Division of Eaton Corp.
If a salesperson should try to spec the truck out this way, or you come upon a used vehicle with components mismatched this way, stay away. Charlie Allen, director of sales engineering at ArvinMeritor, reports that a transmission with 1,850 pounds-feet of torque “has different bearings at the front of the countershaft and different processing of the gears and shafts” from one rated at 1,650. Using the wrong box will be likely to yield unwarranted bearing and geartooth wear.
Both experts say transmissions and other drivetrain components are strong, which means “nothing is to be gained” by overspec’ing, as Allen put it. But, there is an exception to that rule – you may want to turn the engine power up at resale. Batten says this depends upon a situation in which “the engine has the iron for uprating. A fleet may want its drivers to have 475-hp engines with 1,650 pounds-feet of torque, and then uprate the engine for 500 hp and 1,850 pounds-feet of torque for resale into the performance market.” If you uprate the engine at resale without properly matched drivetrain components, the warranty automatically becomes invalid, reducing the vehicle’s market value.
Both manufacturers also offer transmissions capable of handling an extra amount of torque in just the top two gears, which handle all highway cruise situations. According to Allen, the ArvinMeritor Torque 2 10-speed works with Cummins Smart Torque to produce an engine torque output rating of 1,650 pounds-feet in gears one through eight, and then ups it to 1,850 in the top two gears. The Eaton Fuller Lightning and FR-Series add 100 pounds-feet in their top two gears.
The engine and transmission communicate electronically to ensure the torque only shows up in the gears that can handle it. This gives very good highway performance, yet allows a much less expensive drivetrain, since the real money goes to make the parts strong enough for starts in the lowest gear. This means you can spec an engine with the highway performance formerly reserved for trucks equipped with 13- and 18-speed gearboxes, yet use a 10-speed.
The torque ratings for the drive axles and driveshafts are also critical. This is a complex calculation based on the engine’s torque output, the lowest gear ratio in the transmission, the axle ratio, the tire size and other factors related to vehicle weight using the truck manufacturer’s software. Let the truck factory do the difficult calculation, but, says Gerard DeVito, manager of global Roadranger product planning, specify the traditional overdrive transmission – not direct drive – to get “the right driveline balance.” Direct drive typically reduces the rotating speed of the driveshafts nearly 25 percent, which increases the torque they and the axles must carry by the same percentage. When you specify this kind of drivetrain, says DeVito, “The driveshaft yokes, and crosses get really big. Even then, they still may not do the job because there is too little margin for driving errors that stress the drivetrain. Drivers are not perfect.” Direct drive saves a little fuel because of reduced transmission friction in top gear, but it may not save you money in the end.
Consistent power output going through the gears gives a much better driving feel and encourages the driver to upshift and save fuel. But the engine and transmission must be matched for this to happen.
Some engines need fewer gears than others because of torque rise. Torque rise means the engine’s torque increases as rpm falls. Torque rise is the key to upshifts that still give enough power after the shift. It’s ideal if power after the shift is 90 percent of what it was at the top of the last gear.
What a driver feels is horsepower. And horsepower comes from both torque and rpm – the two are interchangeable. Suppose an engine makes 825 pounds-feet of torque at 2,000 rpm. The driver shifts up, skipping a gear, and gets 1,000 rpm in the next gear. But suppose the engine produces 1,650 pounds-feet of torque at the lower rpm? He will feel exactly the same power as he felt at the top of the previous gear. This is high torque rise – what people are really talking about when they discuss an engine with “lots of torque.”
This technician is getting ready to pull out a transmission. Properly matching the transmission torque rating to the engine’s torque output goes a long way in guaranteeing a long, reliable life. But underspec’ing or uprating an engine to make more torque than a transmission is designed for can make short work of both bearings and gearteeth.
The truck OEMs offer sophisticated computer programs to figure out how well any engine-transmission combination will work together. But you can get a good idea of how well the two components will match all by yourself. All you need is the horsepower chart for the engine and the transmission manufacturer’s list of ratios and steps for the gearbox. Transmission specs give the percent step between each gear and the next.
See “Matching Engine and Tranny” for examples.
“The peaky curves on older engines have changed,” Allen says. “Engines now have much flatter torque and power curves. The result is a wide operating range, suitable for 10-speeds. You don’t need to be afraid to let it lug – it will feel good. Plot the power curve and you’ll see what I mean.”
Allen’s argument certainly makes sense when it comes to performance, and the horsepower charts prove the point. But Eaton’s Bill Batten doesn’t agree that the flatter curves of the latest engines rule out one advantage of using the traditional 13- or 18-speed. Batten says, “The new environmental rules made the fuel economy sweet spot narrower, and it will get narrower still.” Eaton’s DeVito added: “The fuel islands are a lot smaller than they used to be, though the feel of the torque comes on the same as it did. The driver can’t tell the difference by the seat of his pants.” (Fuel islands are engine-operating areas where fuel consumption stays within a narrow range.)
Mack’s David McKenna, product manager marketing for Mack engines, agrees that sweet spots are getting smaller. Mack recommends cruising at 1,550-1,650 rpm with their latest highway engines. The new engines won’t give the best cruise fuel economy unless held in a narrow rpm range.
So should you spec a 10-speed or a 13-or 18-speed? It depends on driver skill, load and terrain. If your trucks have enough power, and road conditions are right for them to maintain a consistent cruising speed most of the time, you gear the truck to hold the rpm in the sweet spot at that speed and run a 10-speed. If your cruise speeds vary much of the time, and you are a skilled driver, you may want to invest in a transmission that offers narrow gearsplits at cruise speeds. Then the driver can split down when climbing hills and keep the engine in the sweet spot.
Getting the right balance of fuel economy and performance depends upon cruising at the right rpm. To pick the right gearing, you need to first decide on your cruise speed. Then, consult the manufacturer for the recommended cruise rpm. It will be something like 1,600 plus or minus 50 rpm with smaller engines, and 1,500 plus or minus 50 rpm with larger engines. Following their recommendation will put the engine near the sweet spot without lugging it down so it won’t climb normal highway grades.
Cruise rpm is determined by multiplying tire revolutions per mile, top transmission gear ratio and rear axle ratio together.
Gradeability is calculated somewhat similarly in terms of gearing. But it uses engine peak torque and tire rolling radius together with some efficiency factors. It’s another calculation you should ask your component makers to make for you to ensure that the transmission will be up to the job of climbing the steepest hill you’re likely to encounter, especially if you run off-road.
With the right rear axle and top gear ratios, your cruise rpm will give good fuel economy. With a transmission matched to the engine, downshifting will be minimized and upshifts will never disappoint. The right starting gear will give you the grunt you need for handling the steepest loading dock driveway without clutch abuse, making you smile every mile.
This technician is using a pilot mainshaft to locate and install a clutch onto an engine flywheel. The transmission has been removed. As you can see, replacing or working on a clutch is a major operation. For fleets, using the right specifications for your clutch and drivetrain and training drivers in proper starts can help guarantee long clutch life.
Don’t take your third pedal for granted – make sure it is built to handle the job
Choosing the right clutch with the right components, and operating it the way it’s meant to be operated, can make your ride smoother, keep your maintenance expenses down and minimize downtime and frustration.
Fortunately, clutches are easier to specify than most other drivetrain parts. Charlie Allen, ArvinMeritor’s director of sales engineering, says the most important key to clutch spec’ing is the engine’s torque capacity. This brings up the issue of clamp load – the spring tension that holds the clutch in the engaged position till you step on the pedal to release it. Clamp load is directly related to engine torque. The clamp load literally clamps the two clutch-driven discs (that turn the transmission input shaft) between the engine flywheel, an intermediate plate and the clutch pressure plate. The friction generated by the clamp load stops the slippage that occurs during engagement – as you slowly release the clutch to get started. When you depress the clutch pedal, you move the pressure plate backward, away from the flywheel, against spring tension. So, Allen points out, the pedal effort goes up when the clutch torque rating goes up.
The best ways to keep the clamp load and pedal effort low enough for driver comfort are to make sure to specify a cerametallic clutch lining (which Allen points out needs less clamp load than an organic one), and to use a modern clutch design which has its pressure plate springs designed so the highest clamp load occurs just at the point where the clutch is fully engaged.
Handling the torque
A clutch must be rated for its engine’s peak torque, not clutch engagement torque. A clutch rated only for clutch engagement torque would be likely to start slipping even without your foot on the clutch pedal as the engine rpm rose from 800 rpm to its torque peak of about 1,200 rpm. Engine output torque will typically rise from less than 800 pounds feet at 800 rpm to 1,550, 1,650 or even more as the engine moves up to 1,200 rpm.
Select a clutch that will handle the higher torque that often comes when uprating. Even if the torque were only to rise 100 pounds-feet, you could have a “significant risk of slippage,” says Allen, “especially if the clutch is not in perfect adjustment.” Clutches can experience something called “micro-slippage,” which is a condition even the most experienced driver will not be aware of. With micro-slippage, the clutch discs creep around on the flywheel and pressure plate so slowly the driver does not notice a change in engine rpm. But the wear can ruin the clutch.
Note also that torque from a diesel comes in uneven spikes, which can cause torsional vibration to damage the drivetrain. Make sure your clutch has long-travel, low-frequency dampening springs suitable for a modern diesel engine.
Fit and startability
The factory will pick the exact clutch model based on the particular engine and transmission you’ve specified, so the clutch will bolt up to the flywheel, and the transmission input shaft will fit correctly into the splines in the center of the clutch driven discs. But you should make sure a drivetrain calculation has been used to evaluate startability (see “Geared to Go”). Even when a clutch has the ability to lock up and stop the engine if fully released, it’s inherently vulnerable as the driver slowly engages it during initial start because he is limiting the clamp load with his foot to make it slip. The key to minimizing slippage is a starting gear that multiplies the engine’s torque sufficiently. This allows the driver to start with the engine at a low rpm – preferably idle – where the torque output is limited. And it allows the clutch to lock up before the truck has picked up a lot of speed, minimizing slip time. Since the startability calculation considers all the factors, including the vocation of the truck and even the tire size, making sure it’s been used to give you the right starting gear will go a long way toward guaranteeing your clutch will live long.
Another choice to consider is the type of lining – cerametallic or organic. Cerametallic linings were developed to give long life as diesel engine torque ratings escalated in the late 1980s and 1990s. They are much harder than traditional organic linings, so wear is considerably less – about half that of organic linings. The downsides are more wear to the flywheel and pressure plate surfaces, and, for some drivers, an annoying tendency to grab too fast for a smooth start. Allen points out that the ArvinMeritor Freedomline transmission, which operates the clutch electronically, pampers it to the point where use of a single, organic disc is practical, giving silky smooth starts.
His view is that organic linings have almost disappeared from the Class 8 truck market, in part because ceramic linings allow not only long life but also lower pedal effort (needing less clamp load to handle the torque).
Drivers who experience rough starts with ceramic linings can often improve the smoothness of their performance just by making sure to start in the right gear, which also keeps the clutch parts from overheating.
Upgrading wear capacity
Even in clutches of a given torque rating, you may be able to upgrade the capacity for wear by spec’ing a clutch with more total friction surface, or extra clamp load. Eaton’s customer service representative Vince Fuleki says that on many of their clutches, that means six rather than four wear paddles. The Eaton guidelines cite these factors as reasons to upgrade beyond the standard clutch:
Premium clutches from both Eaton and ArvinMeritor offer automatic adjustment capability. Keep in mind that self-adjusting clutches do more than save a lot of downtime and hours under the truck with a wrench. Since it’s difficult to ensure clutches are always perfectly adjusted in the real trucking world, chances are a self-adjusting clutch will last longer than a manually adjusted one because it stays in adjustment until its life is over.
Doing all these things will make sure you’ve got a clutch in the truck that can live long. Follow the maintenance and operating suggestions mentioned in the sidebar, and you’ll be spending time and money doing things other than working on your clutch.
Matching Engine and Tranny
You can take the numbers you already know about your engine and figure out what kind of transmission step it can handle.
Consider an engine like the Mack AC-427, with 427 peak horsepower at 1,800 rpm and maximum torque of 1,560 pounds-feet at 1,200 rpm. First take the peak power rating of 427 and multiply it by .9 (for 90 percent). That gives 384.3 hp. Look at the horsepower chart, and follow the horsepower line down to the left toward the torque peak until it reaches 385 hp. For this engine, that occurs at about 1,325 rpm (just read straight down from this point on the hp line to the rpm scale). Next, divide the governed speed of 1,800 rpm by this rpm – 1,325 rpm. This gives 1.3584. All you need to do to find the percent step is to throw away the 1 and then move the decimal point two numbers to the right. This number – 35.84 – rounds off to just about 36 percent. This engine can gracefully handle a transmission step of about 36 percent.
The high torque-rise Mack AC-380/410 produces 410 peak horsepower and 380 hp at its governed speed of 1,800 rpm with 1,660 pounds-feet of torque at 1,100 rpm. To get the 90 percent power figure, multiply 410 by .9, and you get 369. If you read this engine’s horsepower chart, you will find that 370 hp occurs at 1,180 rpm. Now, divide the governed speed of 1,800 rpm by 1,180, giving 1.5254. Taking out the 1 and moving the decimal two spaces right, you can see this engine can handle a step of 52.54 or nearly 53 percent!
An ArvinMeritor paper Allen wrote states that the ArvinMeritor “C” or “A” ratio 10-speed transmissions have steps of 37-38 percent. So they would match well with either of these 2002-model engines. The paper also shows horsepower charts of typical engines of years ago and today, demonstrating how high torque rise has broadened the power band of modern engines.
Calculating Your Cruising Numbers
Trucks today normally run an overdrive top gear ratio around .74:1. Typical low profile tires turn about 502 revolutions per mile. Since a truck is traveling a mile per minute at 60 mph, this number also gives your wheel rpm at 60. Let’s say you have chosen to cruise at 65 and that your engine manufacturer recommends cruising at 1,450-1,550 rpm.
First, divide your cruise speed of 65 by 60, and you get 1.0833. Multiply your tire revolutions per mile by this factor to find how fast the wheels will be turning at your cruise speed: 502 x 1.0833 = 543.8, or 544 rpm. Now, try several axle ratios to see which works best. The most commonly used are 3.90:1, 3.70:1 and 3.55:1. Multiply 544 rpm by .74 (the top gear ratio) by 3.90 (the axle ratio), and you get 1569 – a cruising rpm that’s too high. Try multiplying again with an axle ratio of 3.7, and you get a cruising rpm of 1489, within the recommended range.
You will need a low gear ratio powerful enough to get you started on the steepest expected grade. This helps the durability of clutch and other drivetrain parts. We’ll use the numbers from the truck we chose the axle ratio for just above. Our end result will be a fraction that gives us a two-digit startability number. This number represents how much starting power the truck has.
For the upper number, start with the engine’s clutch engagement torque (“TC”) at 800 rpm in pounds-feet, which is on the engine spec’ sheet. A typical figure would be 750 pounds-feet, so we will use 750. Next, we need to determine the ratio of the lowest gear. R is the axle ratio multiplied by the ratio of the lowest transmission gear. An Eaton Fuller B ratio 10-speed has a first gear ratio of 11.06. Multiply 11.06 x 3.7 (the axle ratio). This equals 40.92, which we will round off to 40.9. You also need M, which is tire revolutions per mile or 502. Now, multiply TC x R x M: 750 x 40.92 x 502 = 15,406,380.
Now, multiply the factor 10.7 x the GCW of 80,000 lb, which gives 856,000. To get the startability you divide the upper number or 15,406,380 by the lower number – 856,000. The result of this is 17.9. The startability number required for highway trucks is 16. Since this truck gives us 17.9, it is good to go with this transmission, axle ratio and tire size. For off-road trucks, the startability factor should be 25, and for severe service applications, a startability factor of 30 is recommended.