At a large driving demonstration with some 20 Mercedes Arocs construction trucks in various configurations, Daimler recently showed off its new hydraulically operated front-wheel drive, named HAD. This was done under harsh conditions in the vast quarry Erkenbrechtsweiler, near Stuttgart in southern Germany. All trucks were equipped with the AMT PowerShift3, equivalent to the DT12 in the USA.

At the same time, I got to try a new Turbo Retarder Clutch from Voith. It was shown at the IAA show in Hanover last fall. Voith and Daimler have tried out this new device, and Mercedes Trucks has acquired the exclusive rights to use it for a certain period of time. After that, any OEM can offer it. This is a torque converter in pocket size that is perhaps even more revolutionary than the HAD drive.

On the test course

First on the rough-terrain course we drove a loaded 6x4 dump truck with the interaxle differential (between the two drive axles), locked. It made it up several steep gravel slopes without problems. Then we drove a fully loaded Arocs 8x8 rigid tipper (as they’re called in Europe) with a gross weight of 40 metric tons, or 90,000 pounds. Its permanent all-wheel drive crawled its way through every conceivable incline, obstacle and surface.

The eight-wheeler was fitted with the new Turbo Retarder Coupling, which has the Daimler product name on it. We put it to the test by stopping on a steep upward hill and then starting again, and without any problems.

Actually, we could drive uphill with the gear selector in position Drive then let the rig roll backwards down the hill and brought it to a stop by stepping on the accelerator, then immediately sent it climbing. With a conventional dry clutch there would at least be a smell of burning facing, if not worse.

Like a torque converter

The Turbo Retarder Coupling functions as a torque converter. It is placed between a truck’s normal friction clutch and the gearbox. When starting under load, the ordinary clutch engages then becomes passive while the turbo coupling handles start-up with the friction of its oil flow. The device also acts as a fluid retarder. Its maximum braking power is 476 hp in addition to the vehicle's Jake-type engine brake and its normal air service brakes.

This type of Turbo Retarder Coupling is now available as an option for Arocs trucks and is standard on the Mercedes Actros SLD heavy tractor, designed for a truck-train weight of up to 250 tons.

Hydraulic traction assist

Finally, we tested the main attraction of the day, namely HAD, which was mounted on an ordinary 4x2 tractor with a dump semi-trailer, loaded to more than 40 tons. This type of vehicle is primarily intended to run on paved roads. But when loading and unloading, the tractor sometimes has to make it through mud and other difficult surfaces, where all-wheel drive would help.HAS provides a hydraulic drive on the front wheels, which can be connected at speeds of up to 25 or 30 kmph (about 15 to 19 mph).

We started on a slippery uphill and activated the HAD-drive with the push of a special button. The truck started with a hydraulic whining and the situation was handled. Likewise, when we ran on 4x2 mode into a stop on loose macadam rubble, we easily got out with the HAD assistance.

Hydraulic drive on the front axle is not new in itself. Several competitors like MAN and Renault have offered this solution for several years. Poclain of France, a manufacturer of wheel excavators and agricultural equipment (and which introduced such a system in the U.S. in 2008), supplies the hydraulic hub motors for the front wheels. Mercedes has further developed the system, such as placing the oil pump on the engine PTO instead of on the gearbox.

As used by MAN, the pump is on the transmission and the front wheels alone can propel the vehicle and help brake on downhills. On a Mercedes, HAD can build up pressure for traction while at a standstill, but is not used not for downhill braking. And Mercedes mounts a cooler with fan on the frame, positioned so it does not affect body mounting.

The launch was preceded by thorough testing. Mercedes strongly believes that the HAD device is now commercially viable. HAD is delivered in combination with the Daimler’s automated manual transmission, Powershift 3. HAD is available for 4x2, 6x2 and 6x4 vehicles with 10.7- and 12.8-liter OM series diesels, with 326 to 510 hp.