Mack’s mDrive HD Transmission Tackles Vocational

June 24, 2015

Mack Trucks has been understandably conservative in approving its mDrive automated manual transmission for vocational applications. The rigors of running both on- and off-road under heavy loads can destroy a gearbox meant for easier highway operations. So the builder famous for dump and mixer trucks avoided selling the AMT into such uses and instead beefed it up specifically for them.

The result is the mDrive HD, for heavy duty, which Mack executives announced in February at the World of Concrete show in Las Vegas. They explained that the HD has extra-tough gears and synchronizers compared to the on-highway mDrive that came out in 2010, which more than half of all customers for road-going Macks now choose. A few months later, execs hosted a drive of the new product at their Customer Center (formerly the technical center) in Allentown, Pa.

Mack mDrive Test Set

Truck: 2015 Mack Granite mixer chassis w/ 11.1x300x105mm frame rails, conventional-cab, BBC 122 in., GVW 69,000 lb.
Engine: Mack MP7; 10.8 liters (659; 395 hp @ 1,500-1,700 rpm; 1,560 lb.-ft. @ 1,100 rpm; w/ PowerLeash engine brake and rear PTO
Transmission: Mack mDrive HD, 12-speed automated manual w/ 12th-overdrive
Front Axle: 23,000-lb. Mack FXL23 on taperleafs
Rear Axle: 46,000-lb. Mack S462 w/ 3.98 ratio on Hendrickson HMX460 mechanical
Wheelbase: 230 inches
Front tires & wheels: Michelin 445/65R22.5 L XZY3, on Alcoa polished aluminum discs
Rear tires & wheels: Michelin 11R22.5 H XZY3, on Hayes Lemerz steel discs
Brakes: Bendix S-cam w/ Bendix ABS and Bendix/Mack Road Stability Advantage
Fuel Tank: 88-gal. D-shaped aluminum
Body: 10.5-yd. Beck concrete mixer

There I drove several Macks—a Granite dumper and mixer and a Titan extra-heavy-duty tractor—and learned more about those models as well as the transmission. The HD uses higher-viscosity synthetic lube oil, said Curtis Dorwart, Mack’s vocational segment marketing manager. The mDrive HD is rated for up to 2,060 lb.-ft., and is available only with Mack MP7, MP8 and MP10 diesels. It will be standard in Granite vocational trucks and tractors and available in the Titan extra-heavy-duty model.

The heavier-duty mDrive was extensively tested, said Stu Russoli, the former vocational manager who recently moved into marketing highway vehicles.

“We didn’t want to put it in construction applications until we knew it would stand up to frequent shifts and heat seen in off-road running,” he said. Aside from the other changes, mDrive HD’s oil cooler was mounted to one side of the casing so it’s closer to the heat exchanger up at the radiator.

A 12-speed mDrive HD weighs 237 pounds less and costs “significantly less” than a similarly torque-rated Allison fully automatic torque-converter transmission, heretofore the only self-shifting alternative to a multispeed manual in Mack vocational models, Dorwart said. A single-countershaft mDrive weighs about the same as a comparable Eaton manual transmission with twin countershafts and less than a triple-countershaft Mack manual. He and Russoli declined to cite price details. An mDrive HD will handle most vocational applications, Russoli said, “but there are some where we’ll say, ‘You should stay with an 18-speed manual or stay with an Allison automatic,’ and we’ll still offer them all.”

Dorwart rode with me as I took a Granite mixer chassis around the paved test track and up and down hills, where startability was challenged by stiff grades but ably handled with smooth clutch engagement. Later, we went out on nearby streets and freeways for running at much higher speeds. Technicians had put several tons of stone in the mixer’s barrel to simulate a load of concrete, and this settled down the vehicle as we banged over broken and bowed concrete that’s in abundance in northeast Pennsylvania. We still experienced a lot of bouncing, though, because there’s not much else a stiffly sprung truck can do in such circumstances.

And in this running, the mDrive always seemed to pick the proper gear no matter how fast or slow I drove, and whether we were on a straightaway or turning corners. It operated much like other mDrives I’ve experienced, with smooth, positive shifts and all around proper behavior. Three programmable operating modes—Easy Shift, Enhanced Construction and Heavy Haul—are available. This one had the construction setting, so shift points were a little higher on the tachometer than I was used to. Technicians at Mack service departments can program a transmission’s electronic controls to suit the customer, Dorwart said.

Notable were the results when I pushed a “Perf” (for performance) button on the dash-mounted selector; this delays upshifts and adds power and torque as engine revs rise, making a truck noticeably livelier during acceleration. To me, it was especially effective as the transmission reached higher gears and road speed climbed. Punching the Perf button again returns the tranny to an economy mode. I’ll bet drivers will like this feature, but managers bent on saving fuel might want to disconnect it, or order trucks without it.

Maneuverability was fairly good, although wheel cut was limited by wide 445-series tires. With trucks like this, you need to spin the steering wheel quickly while entering sharp turns and then the truck goes where you want it to. I was satisfied with the Granite axle-back’s turning ability until I hopped into a Granite dump truck with a forward-set steer axle. Its wheelbase was longer by 7 inches, but to my surprise, it turned more sharply: the opposite of what I’d come to expect with axle-forward trucks. Dorwart said this was possible because engineers had relocated the steering gear from outside the frame rails to just inside. This, and narrower 315-series front tires, gave the wheels more cutting room and the truck a shorter turning circle.

Next, we walked down a grassy hill to where a Titan tractor sat, its engine shut down since it was parked several hours previous because no one else had driven it. It was hitched to a four-axle lowboy carrying a Volvo wheel loader, and the rig’s gross combination weight was 105,000 pounds, about 15,000 less than an mDrive-equipped Titan’s rated GCW of 120,000. We climbed in, I cranked over the engine, and after about a minute I punched Drive on the selector, released the brakes, and nudged the accelerator.

The rig was parked on a slight upgrade, which proved problematic. The D16 diesel surged and dropped off repeatedly as it and the transmission struggled to get the rig moving. The clutch overheated, illuminating a warning light and setting off an audible alarm. I eased up, punched Neutral and let the rig roll back down the grade and stopped. I speculated that the cold engine was causing the bogging, and Dorwart wondered if the axle ratio (3.79 to 1) was a little too “fast” for this type of load. After another minute or two, I punched D and we slowly moved forward again, and turned onto the track.

The engine smoothed out and the tranny began upshifting, and we accelerated around the oval, getting up to 45 and 50 mph. Even now I think the powertrain would have done OK if we’d let it warm up, but it would definitely have done better with a 4.10 or 4.30 axle ratio. Or was that a situation that might have been handled better by a manual 18-speed with a suitably short Low gear ratio, or by an Allison’s torque converter? As Russoli had said earlier, Mack will sell you those, too.

Nonetheless, the mDrive is an impressive component. As with the highway version, the heavy-duty model will improve fuel economy, greatly ease the workload on a driver, and widen the pool of potential employees who needn’t learn to operate a nonsynchronized manual transmission. So will an Allison, but at a stiffer price: an advantage that all automated transmissions have in the marketplace. With mechanical gears employed, AMTs also feel like they’re delivering more power to the wheels. That the mDrive HD is stronger is something that will prove out over time, miles and operating hours.