Allison's Low-Low Ratio Helps on a Hill

March 4, 2011

Here’s the situation: Your load of concrete has to be placed at the top of a steep incline, and you need to get up there with as little shaking and bouncing as possible. That means deep gearing, and mechanically there are a number of ways to do it. About the easiest for a driver is an Allison automatic transmission with a 2nd Reverse. The company announced the tranny last year, and this was my first opportunity to drive it.

Here’s the situation: Your load of concrete has to be placed at the top of a steep incline, and you need to get up there with as little shaking and bouncing as possible. That means deep gearing, and mechanically there are a number of ways to do it. About the easiest for a driver is an Allison automatic transmission with a 2nd Reverse. The company announced the tranny last year, and this was my first opportunity to drive it.

Allison and Western Star Trucks demonstrated the low-low Reverse feature during January’s World of Concrete show. Drivers with CDLs could try their luck maneuvering a dump or mixer truck through an orange-coned course laid out in a convoluted oval on a vacant parking lot near the Las Vegas Convention Center. The mixer had the Allison with 2nd Reverse, so that’s what I chose the day before the show opened, during a preview arranged by Akbar Ghous, Western Star’s southeastern regional manager.

The course included several backing exercises, and the final one was at a big pile of dirt with a 40-foot trail on a 15-percent grade. I scooted through the course—not much work with Allison’s 4700 RDS (Rugged Duty Series) with the familiar push-button selector on the dashboard. And the truck’s power steering made for quick, low-effort turns.

The ’Star 4900FA (for forward-set axle) had a 20,000-pound steer axle with wide tires. But the wheels cut surprisingly sharp in either direction, though I still had to spin the steering wheel early into any turn. All in all, I managed to get the bulky mixer chassis into backing slots without too many adjustments or rolling over too many cones.

“Rookies get a pull-ahead,” I joked to Ghous at one point as I punched D to move forward to better position the truck for a back up. Punching D or R is infinitely easier than punching a clutch pedal and manipulating a shift lever in a manual tranny, and I say this even though I really enjoy driving a burly construction truck with an Eaton Fuller 8LL gearbox.

I’ve heard fleet managers say that their automatic transmissions gave them access to new driver candidates, like men who’ve never driven a manual of any kind and women who prove to be kinder to equipment.

At the hill I got the truck into position, punched R and began backing. It would’ve been a little tricky with a manual transmission, as there’d have been a bit of clutch riding until I got used to the truck and the hill. But it was simple with the Allison, even if Ghous hiked to the top of the hill to direct me left or right so I wouldn’t drop the truck off the narrow trail’s edges. Does he really think I’m a rookie? I thought, but watched him in the mirrors anyway.

Before leaving the cab, Ghous had coached me into selecting 2nd Reverse: With foot on the brake pedal, punch the R button once, and “R R” – which I’ll call regular Reverse – appears on the selector screen. Punch R again and “r r” appears, and the truck lurches gently as the low-low ratio engages. Release the brakes and the truck begins moving back. Tap the gas pedal and the engine revs noticeably higher and the truck backs a bit faster.

“You can idle up the hill like that,” Ghous had said of 2nd Reverse, and he was right. With the Detroit DD13 engine idling at about 700 rpm, the truck slowly moved up the grade with no further nudging of the accelerator. I ran the hill several times, using the accelerator and not. If I had backed down the hill, “r r” would’ve given me better control.

For comparison purposes, I also used regular Reverse and saw that the truck would move up the hill if the engine were revved a bit, but not at idle speed. It would probably do that even with a load in the Kimble barrel, which was empty for this demo, but it wouldn’t be as smooth, and the wheels might spin in loose rock or dirt.

At 17.12 to 1, 2nd Reverse’s ratio is more than three and a half times deeper than regular Reverse’s 4.80 to 1. That can put a lot of torque through the driveline, especially at start out, when the Allison’s torque convertor does its multiplication. But torque drops fast as engine revs climb, so there’s not much chance for driveline damage unless the wheels are really bogged. Then it’s time to call for a dozer-tow.

Aside from 2nd Reverse, the 4700RDS has seven forward gears instead of an Allison’s usual six. First gear’s ratio is 7.63 to 1, followed by Second’s 3.51, Third’s 1.91, Fourth’s 1.43, Fifth’s 1.00, Sixth’s 0.74 and Seventh’s 0.64. So this truck will start a heavy load easily and then cruise with it on freeways at more relaxed engine speeds.

However, First is not engaged when D is punched. Second is, and the selector screen shows “2 7,” meaning the 2nd of 7 available gears is engaged. The first digit climbs as the tranny upshifts out on the street or the freeway. If you need First, punch Mode and push the down-arrow button, and “1” appears on the selector. Once you take off, punch the up-arrow and it’ll shift into Second and hold there, then go for Third and so on. Or just punch D and it’ll come out of First (or wherever the tranny is) and upshift normally.

With Ghous’s blessing, I took the truck off the course and onto nearby streets. We followed Convention Center Drive west to Las Vegas Boulevard—The Strip—where I hung a right and cruised north to Sahara Avenue, then east and south onto Paradise Road for a return to the convention center area. Doing this, I watched the tachometer swing up and down as the Allison went through its smooth shifting. On The Strip we got as high as Fifth gear as the truck accelerated to 40 or so miles per hour, but revs dropped quickly when my foot moved away from the gas pedal or only touched it lightly.

During this joy ride I could appreciate the Western Star’s attributes: roomy cab, nicely trimmed interior, impressive quietness, and great visibility in all directions. The 4900’s cab sits high, and you climb three big steps before getting inside. The wide hood is a constant reminder that this is a big truck, but the hood slopes downward, helping with visibility to the front. The windshield and side windows are about as big as any you’ll find on a heavy truck, so you know what’s going on around you.
I remarked on the cab’s quietness to Ghous, and on its attractive interior trim. “Not all of ’em are this nice, are they?” I asked.

“Yes, they are,” he answered. “This is a premium truck. You pay extra for that in a Western Star, and in return you get premium features, like the interior and the sound insulation.” Drivers lucky enough to get ’Stars will certainly appreciate the comfort.

Back at the course, we chatted with a couple of Allison representatives, Dan Murphy and Jerry Hacker, about the Optimized tuning that the builder’s been promoting. Others had tried to explain it to me, but I still didn’t understand why it differs from what’s come before. Optimization, they explained, pairs the automatic transmission and its electronic controls with the engine, taking the truck’s application fully into account, so everything operates at peak efficiency.

“Isn’t this done on any truck with an Allison?” I asked persistantly. After all, Allisons are available on most heavy and medium-duty trucks these days.

“It’s done more carefully with Allison Optimized,” they said almost in unison, noting that it’s an exclusive with Daimler Trucks North America, parent of Western Star and Freightliner, through March. Among its features is a standard five-year/unlimited-mile warranty on the Allison, which is several years longer than usual.  

Murphy further listed several performance features included with an Optimized Allison-DTNA truck combination: low base shift schedule, which causes the tranny to “short shift,” or upshift at low revs when the truck’s empty; shift energy management, which adjusts torque to allow smooth shifts; auto-Neutral with Parking, which prevents a forgetful driver from running the drum outside with the tranny in gear (I tried this later and sure enough, the tranny went from R to N when I set the parking brake); and prognostics that monitor the tranny’s fluid condition and advise techs when to change it.

After the DTNA exclusive runs out, the Optimized treatment is likely to spread to other truck makes. You ought to drive one, in forward and reverse.