Going off road to make a delivery or pick up a load? Most of the time you can do it in a heavy-duty or medium-duty truck with a conventional axle configuration — one or two drive axles and a non-driving steer axle. But you might get stuck if the ground is soft or muddy. Yes, you can send another truck to pull the first one out, or call a wrecker. Either way costs money and time, and you risk damage to the vehicle.
That's why some operators specify front-driving axles, typically on concrete-mixer chassis or trucks that haul blocks, pipe, wallboard or other construction materials. An alternative is a central tire inflation system (also called tire-pressure control), which can deflate tires for travel over rough terrain, then air them up when it's time to return to paved or graded roads. This is especially effective in deep sand, in places like Florida and eastern Minnesota, and we've described this product in previous Hands-On Trucking articles.
Now there's another alternative: hydrostatic front-wheel drive, which sends power and torque to the truck's front end to enhance traction and give it a more secure stance on slopes and hills. It's called Addi-Drive by Poclain Hydraulics and EZ Trac by Tuthill Drive Systems, which collaborated on its development. Poclain supplies a hydraulic pump and motors and electronic controls; while Tuthill, the maker of Mud Hog hydraulic drives for agricultural implements, machines hubs that house the motors at the front wheels.
Both companies are promoting it; Tuthill has begun selling kits to retrofit existing trucks and Poclain is talking with truck manufacturers and expects some to soon offer the system as a factory-installed option.
The system turns a 4x2 or 6x4 truck into a 4x4 or 6x6. It places hydraulic motors in front-wheel hubs and powers them with pressurized fluid from a pump driven by an engine or transmission-mounted PTO. Addi-Drive/EZ Trac can thus take the place of a heavy, bulky mechanical front-driving axle and transfer case. Hydraulic apparatus is comparatively compact, so a truck's standard chassis and cab height can be maintained. Lower stance means the truck is less top heavy, and less likely to roll over in turns on the street or highway, or on hills during off-road running.
Poclain has installed a system on a Sterling L-Line 6x2 truck to demonstrate the concept, and did so during the recent ConExpo-Con/Agg in Las Vegas. That's where I got to drive it, on a gravel lot on the city's south side. About a dozen people watched it run during one demo, and while there was nothing rough about the conditions, we did see how the system improves traction.
Poclain's truck also had another new product, called Creep-Drive, which provides very smooth propulsion at very low speeds. This uses a hydraulic motor on the drive shaft; it, too, is spun by pressurized fluid from the PTO pump, and can move the truck from a barely perceptible forward or rearward motion to 5 or 6 mph. This allows a highway truck to double as a low-speed work truck.
"Hydrostatic ground drive is used separately from tools," explained Homer Hawk, a Poclain application engineer. "You can set your engine speed to run the tools — pavement cutter or groover, bridge inspection boom, paint striper, or street sweeper. Then you independently move the truck with the Creep-Drive."
After preliminarily driving the truck with its normal power train, which included a Mercedes diesel and an Allison automatic transmission, I began operating the creeper. With Hawk as the instructor, I first put the Allison's floor-mounted shifter in Neutral, turned on the PTO and set engine speed at about 1,000 rpm, then turned a rotary switch on an adjacent control box to Rear Creep. This activated the driveline motor.
I grasped a small joystick and moved it forward or rearward, and the truck moved accordingly. Speed adjustment is infinite, with road speed (what there is of it) depending on how much up or back I moved the stick. Speed would also depend on how fast the engine is running, though that is pretty much limited to the PTO pump's designed speed.
Hawk said creeping speed is smoother and slower than what it would be with a 5- or 6-speed Allison and normal rear-axle gearing, without dragging the brakes. And a manual transmission wouldn't have to be one with a low-low range.
On the dashboard was a color LCD panel with gauge-shaped displays showing road speed, horsepower, and system pressure, but I didn't pay much attention to it.
Okay, now I was ready for front-wheel drive, so he directed me to turn the rotary switch to Assist. Tuthill calls this Automatic mode, because the system operates automatically, in conjunction with the truck's power train. We turned off the fast idle and I shifted the Allison into Drive, then pushed the accelerator. Off we went, to the end of the gravel lot, then backwards in Reverse to the other end. The truck went as fast as the engine and transmission would allow.
I turned the truck around and ventured onto a nearby street, accelerating past the system's 20-mph cut-off speed. It disconnected smoothly, and I braked, did another turn-around, and returned to the lot with the front wheels again pulling, though there was no pull in the steering wheel.
Hawk explained that Addi-Drive senses the speed of the rear wheels and reacts by driving the front wheels at a matching speed, or just a little faster. This way it pulls the truck when the rear wheels begin spinning.
They set up the Sterling to simulate such conditions by equipping it with an adjustable tag axle; adding pressure to its air bags raised the rear of the chassis and lifted the driving wheels so they spun in the gravel, or completely off the ground, so only the front wheels propelled the truck. Sure enough, it worked.
Addi-Drive can also double as a creeper, and the rotary switch had a Front Creep setting. Here I could use the joystick to move the truck using only the front-drive system. This setting locks in the system (Tuthill calls it Manual mode for EZ Trac) and, as in automatic Assist, operates in conjunction with the normal power train. So someone buying the front-drive system wouldn't need the driveline-mounted Creep-Drive, though he could buy it that way, and could operate the two together if he wanted four- or six-wheel traction in Creep mode.
Sales representative Don Quigley said about 5,000 front-drive systems are at work on trucks in Europe, many of them on MAN 4x2s that, of course, are now 4x4s. Some are on highway trucks whose owners bought an Addi-Drive "just to have it." They operate in hilly terrain where conditions are sometimes slippery, and this ensures that they'll keep going.
He related that when the Sterling was being loaded onto a transporter trailer for the trip from Poclain's home in Sturdivant, Wis., to Las Vegas, its rear wheels lost traction; but he directed the driver to use the Addi-Drive and it just walked up the ramps. "Boy, I'd like to have this on my tractor," the driver said.
Aside from on/off-road construction applications, an Addi-Drive might allow use of a less brawny truck for snow plowing (it will operate at speeds faster than 20 mph), and would help get trash trucks into and out of spongy landfills. But, Hawk said, it's not meant for constant off-road use. He explained that hydraulic motors act much like electric motors in that they apply high torque at zero rpm. The two 55-horsepower motors in Addi-Drive/EZ Trac make up to 5,500 pounds-feet initially, but that falls off as the motors begin turning. So Addi-Drive is muscular.
And while it's not light weight, it saves about 800 pounds compared to a mechanical front-driving axle, transfer case and forward driveline, he said. Total system weight includes the two 64-pound motors and the special hubs (they don't differ much in weight from stock hubs), plus the PTO gearbox, P90 pump, and associated hoses and controls, which might add another 150 pounds.
Tuthill now makes hubs and steering knuckle ends for 20,000- and 14,600-pound-capacity steer axles; they are machined to fit most steer axles, and have a 10-stud 335-millimeter-diameter pattern for hub-pilot-mount disc wheels.
All the above details constitute the upside of an Addi-Drive. Its downside is that it isn't cheap, and in fact might cost close to what a mechanical front-drive system does. That depends on what the truck manufacturer charges, but it's in the neighborhood of $20,000 for a Class 8 truck. Quigley said a truck builder would also set the price of an Addi-Drive, so he wouldn't guesstimate any dollar figures.
Tuthill was vague on the price of an EZ Trac retrofit kit, which includes all instructions, manuals and service support. For comparison, Dana Spicer's Tire Pressure Control system costs thousands less, but we're told it doesn't work as well in mud as in sand.
So if you really must get off road, or move really slowly on the pavement, you now have more ways to do it. Take your choice, pay your money and go.