If you’re fond of spec’ing high-torque engines combined with direct-drive transmissions and fast axle ratios, you’re likely looking for fuel-economy gains. Compared to medium-power engines with overdrive gearboxes and slow axles, those gains are indeed available, potentially in the 5% to 6% range. Not bad.

But if you haven’t spec’d the correct driveline components, you may be losing more in maintenance costs than you’re winning at the fuel island. The reason? Too much startup torque can play havoc with critical driveline components.

It’s a surprisingly common problem, according to Bob Morgillo, warranty and technical support manager at Dana’s Spicer Driveshaft Division.

In fact, he says, some fleet managers are not spec’ing direct-drive the way they’d like because they’re not aware that a driveshaft is available to match. In some cases they had unwittingly under-spec’d the driveline, suffered higher maintenance costs as a result, and then resolved not to use the direct-drive combination outlined above.

The typical results of an improperly spec’d driveline are broken or brunnelled universal joints and generally unsatisfactory life.

This situation only recently came to light, Morgillo explains, when Spicer learned that a major truckload carrier had gone back to spec’ing overdrive boxes and lesser engines for exactly those reasons. Simple ignorance was the culprit, because Spicer does have a driveshaft ready for the high-torque task.

The challenge of start-up torque is greater with a direct-drive transmission than an overdrive. Continuous torque at road speed is also a factor, but less so.

For instance, look at an overdrive transmission with a reduction of 12.56:1 in first gear. You would take the peak-torque rating of the engine-say, 1400 pound feet-and multiply it by 12.56 to get the actual output torque in that low gear. So you’d see 17,584 pound feet of torque in startup mode.

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

Once in high gear, the driveline torque becomes the engine’s peak torque with direct drive-1400 pound feet in this case-because the transmission ratio in use is 1:1. There’s no multiplication going on. That’s certainly easier to deal with, but it’s still lower with overdrive-a typical 0.74:1 overdrive would give you only 1036 pound feet (0.74 x 1400) to challenge the driveshaft and u-joints.

The obvious solution is to spec the right driveshaft, considering three key factors in the process: startup torque, continuous torque required to maintain vehicle speed, and maximum driveshaft speed.

There’s a higher cost for the heavier driveshaft, of course, but the direct-drive/fast-axle combination can deliver efficiency of 98.5%, says Morgillo, compared to just 96% for the overdrive/slow-axle pairing.

There’s another solution as well, namely an engine that will manage torque output.

Many Cummins models are available with the inexpensive “Smart Torque” feature, which provides extra torque in the two upper gears. You would buy a 1450-pound-foot engine but get up to 1650 pound feet in the top two cogs for better performance with fewer shifts.

That also means buying a cheaper-and lighter-driveline to match. In fact, direct-drives have been limited to 1450 pound feet for many years, so “ST” engines can make a significant difference here.

Using the same technology, “Powertrain Protection” is an active feature on all Cummins diesels with “CM” controllers (basically all engines except the N14 CELECT Plus). It offers the opportunity to customize the zero-km/h torque and torque output of the engine in the lower starting gears to any predetermined level whether it be at or even below the ultimate strength of the propshaft.

There’s another similar option available, namely the “STX” versions of Cummins ISX and Signature engines. The technology isn’t presently aimed at the direct-drive challenge, rather at allowing 40,000-pound axles to be used with engines in the 565- and 600-horsepower range. With an 18-speed Eaton Fuller transmission, it does so by limiting torque output in three stages-1650 pound feet in the first five gears, 1850 in the next four, and the full 2050 pound feet in the remaining gears. It also manages torque in similar ways during coast mode.

As we understand it, because its broad purpose is powertrain protection with optimized performance, there’s no reason why the STX idea could not be utilized to maximize direct-drive spec’ing flexibility. Bob Morgillo agrees.

With or without such engine programming innovations, the bottom line here is simple: there are more options with direct-drives than you might think, but the spec’ing must be done carefully.

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SIDEBAR: Make Inspection Part of Your PM Routine

Spec’ing the right driveshaft and universal joints in the first place is a fine way to start a long life for your drivelines, but regular inspection is the way to guarantee it. Here’s what you should be doing:

1. Check the output and input end-yokes for looseness. 2. Check for excessive radial looseness of the output/input shaft. 3. Check for looseness across the ends of the U-joints. 4. Check the slip spline for excessive radial movement. 5. Check the shaft for damage, bent tubing, or missing balance weights. 6. Check for loose or missing plug.

This information, along with many photos of failed driveline parts, is contained in a small but useful Spicer pamphlet-“Failure Analysis Guide: Driveshaft Components.” It’s Form #3272, available from your Dana Spicer distributor. For more detailed servicing instructions, refer to the Spicer Universal Joint and Driveshafts Service Manual, #3264, available free of charge.

And the entire manual is also available on CD-ROM, where it’s designed to be searchable, as well as being accessible on-line. The Spicer Service Manual is not designed to be downloaded; rather, users can print specific pages or sections.