Spec’ing a heavy-duty transmission is not exactly an art, but it’s actually not a simple job.

The choices are multiplying, for one thing, with more and better automated gearboxes on the market and a pronounced swing toward them in many linehaul fleets. Manual transmissions still dominate the on-highway world, with about 60 percent of the market, but that’s changing at a fair clip. Bison Transport, Challenger Motor Freight, Transport Groupe Robert, QwikX… they’ve all gone 100 percent automated. Countless others have at least a few, and most who are giving them a try are apparently pleased. Maybe surprised as well.

And then there’s the issue of new engine characteristics that mean some of your other past assumptions aren’t necessarily valid any longer either. In some cases the engine’s sweet spot has moved lower down the tach by a couple of hundred rpm, and in every case it’s in a narrower rpm band, so conceiving the powertrain at large demands a new approach.

The precision with which engine designers have had to manage emissions compliance demands more or less equal precision in how you mate motor with gearbox, rear axle, and tires. A precision you didn’t have to employ when you spec’d a truck in 2001 and earlier. The differences aren’t always huge, but they certainly exist, and if you miss the gearing mark you’ll regret it when you or your drivers complain of low power or when you see your fuel economy heading dramatically in the wrong direction.

One western fleet, for example, has gone from a rear-axle ratio of 4.33 to 3.55 in tandem tractors pulling single trailers. That’s Big Freight Systems in Steinbach, Man., with Cummins engines that used to do a 62-mph cruise at 1680 rpm. Nowadays, their newer engines are doing their best work at 1450 rpm. The gearing had to change.

Cummins testing shows that an engine geared to run 1450 rpm at 65 mph gets approximately 4 percent better fuel economy than one geared for 1600 rpm. Other engine builders agree.

In fact, cruising at 1450 rpm seems to be the new norm for run-of-the-mill 80,000-lb highway trucks, except with current Caterpillar ACERT engines, which are at their fuel-efficient best way down at 1300.

Cat’s official line on this came from John Campbell, director of the company’s on-highway engine products, a couple of years ago, who said you’ll need a faster (numerically higher) rear-axle ratio. For example, a 550-hp engine with 1850 lb ft of torque geared traditionally with 3.70 rears prior to 2004/2007 technology would achieve a certain fuel-mileage figure running 65 mph at 1500 rpm. A current 550 Cat will demand that you select two gear ratios higher — a 3.36 or 3.25 — to get the same result.

Get your gearing wrong and you could see too
many bills for driveline maintenance

“This ‘gear fast, run super-slow’ gearing philosophy will not affect startability or gradeability to any great degree because of the increased displacement and higher turbocharger boost pressure of ACERT engines,” Campbell promised.

That said, things change when the weight goes up. Pulling anything much beyond 80,000 lb, as we do all the time here in Canada, still demands cruising at 1600 or above if you’re going to have any torque to fall back on when you meet a grade.

Truck sales people have information on how to gear a truck properly, and they all have computerized configuration programs to help out. But if they seem unsure, grab product literature off a rack or go online to see what the engine people themselves recommend. Call the factory’s sales-engineering reps if you have any doubts. Once ratios are built into transmissions and axles, they can’t be easily altered.

Direct Drive:

Is a direct-drive transmission better than an overdrive version? This is an old issue and neither side of the debate is right or wrong, but overdrive dominates in a big way. Not for a couple of U.S. mega fleets, however. Both Wal-Mart and Schneider National spec direct drive exclusively, or nearly so, which means there must be something in it.

You can get direct-drive units from some manufacturers, though not in all models. At Eaton, for example, there’s only one direct-drive automated mechanical — the UltraShift 10-speed LST. The advantage in that case is actually useful because you get additional torque capacity in the top two gears-200 lb ft as opposed to just 100 lb ft for the overdrive model, improving efficiency and gradeability. Volvo recently introduced its I-Shift automated mechanical transmission here, with three models. All are 12-speeds, one of them a direct-drive.

The automated direct-drive choices stop there but you have a few more options on the manual front, from Eaton and — if you can find one — from Transmission Technologies Corp., which bought the Spicer transmission lineup and moved it to Mexico a decade ago. ArvinMeritor has left the manual world behind in favor of concentrating on its automated FreedomLine offerings, and all of Mack’s transmissions are overdrives.

Direct-drive proponents say there are fuel-economy gains to be had in spec’ing high-torque engines with direct-drive transmissions and fast axle ratios, potentially in the 5 percent range. Not bad, but Eaton’s Pete Messerroll says he has yet to see a real-world, on-the-road test to confirm such a saving.

If you go to direct drive you have to spec the correct driveline components, or you may lose more in maintenance costs than you’re winning at the fuel island. Why? Too much startup torque can play havoc with U-joints and driveshafts. The typical results of an under-spec’d driveline are broken or brunnelled universal joints and generally unsatisfactory life.

The challenge of start-up torque is greater with a direct-drive transmission than an overdrive. Look at an overdrive transmission with a reduction of 12.56:1 in first gear. Take the engine’s peak-torque rating — say, 1400 lb ft — and multiply it by 12.56 to get the actual output torque in that low gear. So you’d see 17,584 lb ft of torque in startup mode.

Now try the same engine with a direct-drive gearbox where the first-gear ratio is 15.02:1. Do the same math and you get startup torque of 21,028 lb ft. Quite a difference, especially if you haven’t spec’d a driveline to accommodate that extra 3444 lb ft of tube-twisting torque.

Once in high gear, then the driveline torque is the engine’s peak torque with direct drive — 1400 lb ft in this case. There’s no multiplication going on. That’s easier to deal with, but it’s still lower with overdrive — a typical 0.74:1 overdrive would give you only 1036 lb ft (.74 x 1400) to challenge the driveshaft and U-joints.

The obvious solution is to spec the right driveshaft. It’ll cost more but Dana Spicer says the direct-drive/fast-axle combination can deliver efficiency of 98.5 percent, compared to just 96 percent for the overdrive/slow-axle pairing.