• Hydraulic additional drive system weighs only 400 kilograms
  • 10% fuel savings over mechanical all-wheel drive
  • HAD easily masters unpaved uphill grades and loose ground

At first glance it's almost impossible to see what is special about this truck. The badge on the driver's door of the Arocs 1845 LS also provides no indication of what distinguishes this tractor unit. The first small sign that something is different is the unusual bulge on the wheel ends of the front axle.

The additional components that distinguish the new Arocs HAD can finally be found on the co-driver's side: between the emission control unit and the rear axle is an oil reservoir with cooler and a valve block for the additional drive system. Oil lines run to the high-pressure pump mounted behind the engine. The pump has an output of up to 112 kW and delivers a flow rate of up to 350 litres of oil per minute at a maximum pressure of 450 bar.From here, high-pressure lines run to the hydraulic wheel hub motors in the front wheels - a quasi special additional drive system: on-demand, lightweight and fuel efficient. If you look behind the front wheels in the wheel wells, you see that the lines cannot twist when steering, thereby ensuring a long service life.

HAD: ideal for construction site vehicles in mixed operations

Mercedes-Benz offers this new all-wheel-drive variant under the name Hydraulic Auxiliary Drive (HAD) for the Arocs. It is designed for all construction site vehicles that travel predominantly on the road but still occasionally need to navigate in challenging terrain to pick up or drop off a load. These include an Arocs tractor unit with 4x2 axle formula and tipper semitrailer, for example. This vehicle configuration is perfect for transporting construction materials quickly and efficiently from point A to point B or from the gravel plant to the construction site. A high payload and road-worthiness are therefore of special importance.

HAD shows off its full potential in the gravel mill and the rock quarry

Before entering the gravel mill, every vehicle has to get past the incorruptible scales to determine the kerb weight. In the case of the Mercedes-Benz Actros 1845 LS with three-axle tipper semitrailer, the scales show a weight of 14.8 tonnes, of which the traitor unit accounts for only 8.2 tonnes - thanks to the lightweight HAD front-wheel drive weighing just 400 kilograms. With a classic all-wheel-drive system, that is to say with transfer case, differential, drive shaft and thicker frame (9 mm instead of 8 mm), the digital scales would have indicated 400 kg more.

After weighing, the vehicle pulls away already in off-road mode because traction on the grounds of the gravel mill is significantly reduced. Paved roads turn into simple gravel roads, which also fits in better with the surroundings. This entices rolling down into a depression in low gear with a slightly higher engine speed to meet the waiting wheel loader. Especially when it means having to return fully loaded afterwards.

By the way, off-road mode is engaged by pushing the shift knob down slightly, which signals the fully automated Mercedes PowerShift 3 transmission to raise the shift points by about 200 rpm. This applies to upshifts and downshifts.

40 tonnes: no problem with 2200 Nm from the OM 471

With each bucket load, the wheel loader dumps more than seven tonnes of 11-22 mm gravel into the tipper semitrailer. With a combination weight of almost 40 tonnes, the Mercedes-Benz Arocs 1845 LS starts in 1st gear - without rolling back even a millimetre in the depression. In the Arocs this assistance comes courtesy of the so-called hill holder, which is activated with a push of a button on the centre control panel and releases the brakes automatically when the accelerator is depressed.

The truck picks up speed effortlessly. The long-stroke six-cylinder in-line engine (stroke: 156 mm, bore: 132 mm) produces 330 kW (449 hp) at 1800 rpm from a displacement of 12.8 litres. The usable full-power rev range goes down all the way to around 800 rpm, which is especially appreciated in off-road operations. The diesel engine delivers close to its full power already at a low 1400 rpm. The peak torque of 2200 Nm is already available at 1100 rpm. The OM 471 engine series comes in four output levels ranging from 310 kW (421 hp) and 2100 Nm to 375 kW (510 hp) and 2500 Nm.

The OM 471 engine and the automated Mercedes PowerShift 3 transmission are perfectly matched to each other and the harmonious gear changes are accompanied by a virtually unnoticeable interruption in tractive force. Hats off to the gear selection computer. The clutch also deserves strong praise at this point. With two discs, it has twice the friction contact surface and is able to transmit twice the forces. But because even a double-disc clutch has its limits, all Mercedes PowerShift 3 transmissions are equipped with overload protection and a warning system.

Hydraulic Auxiliary Drive masters unpaved 15% grade

The next hill is recognised as if the automatic transmission had known it was coming. Within the shortest possible shift time, the transmission operates again in first gear. The test of Hydraulic Auxiliary Drive does not take us on the road, but from the gravel mill to the adjacent rock quarry that spreads over multiple terraces of different heights.

Ahead of the Arocs is a grade of approximately 15 percent. And nobody would call what lies ahead of the Arocs a road: gravel and rocks on a compacted mixture of dirt and sand. Pulling 40 tonnes uphill on this stuff won't be an easy task. The engaged rear differential lock only helps to some extent. Now the Arocs HAD is called upon to prove its mettle.

A push of a button - again on the centre control panel - is all it takes and a front-axle symbol lights up on the central display. When the accelerator is depressed, the symbol changes colour from white to blue. The new lightweight all-wheel drive is now active. Oil is pumped into the wheel hub motors at high pressure, providing an additional output of 40 kW per front wheel.

In the cockpit, the operation of the hydraulic high-pressure pump is at first only evident in a faint additional sound. But then it is hard to believe your eyes: slowly but surely the tractor-semitrailer picks up speed and pulls up the hill faster and faster. Forward progress could have been even swifter if the accelerator had been depressed further. Because contrary to other systems, Hydraulic Auxiliary Drive is dependent on the engine torque. This means, the more the accelerator is depressed, the higher the output of the HAD high-pressure pump, thereby increasing the torque on the front wheels.

Buried in loose ground: keep going with HAD

Once at the top, the next test already awaits the Arocs HAD. It has to drive through a gravel pit about 200 metres long. Nothing unusual for a construction site vehicle that delivers gravel or crushed stone for road construction. Also not a demanding challenge as long as there is only a little bit of gravel on the ground and there is sufficient traction for the drive axle. But the situation here requires all-wheel drive: the gravel is loose and in some places up to 50 centimetres deep.

It is not supposed to be easy for the 40-tonne truck. That is why the gravel pit is entered without locked differential and without HAD assistance. The driver provides the only assistance by switching the transmission to manual operation and steering onto the hard-to-navigate ground in first gear at about 1000 rpm. The tractor-semitrailer combination gets stuck after about 50 metres. Despite engaged differential lock, the rear axle keeps burying itself deeper and deeper into the gravel.

Now HAD demonstrates its superior effect: thanks to 450 bar oil pressure, each wheel hub motor produces the peak torque of 6250 Nm. In concert with the torque of the rear axle, there is enough power to get the tractor-semitrailer moving again and out of the gravel pit. This is even easier in reverse: the weight of the three-axle tipper semitrailer in combination with the front-wheel torque helps to free the buried rear axle quickly from the deep gravel hole.

Hydraulic Auxiliary Drive active in forward and reverse gears

Hydraulic Auxiliary Drive is programmed to allow operation of the system in reverse up to second gear. It is a different story when driving forward: dependent on the speed, the auxiliary drive is active to 5th or 6th gear. The upper limit, at which point the system switches to passive mode at the very latest, is 25 km/h. If HAD is then not fully deactivated with a push of a button, the auxiliary drive automatically springs back into action once the speed drops below 25 km/h again.

Excellent handling in on-road operations

On the road, where front-wheel drive is not required, HAD is completely inconspicuous and the sophisticated drive system of the 1845 LS does most of the work for the driver. Despite the some 25 tonnes of gravel in the tipper semitrailer, the automated transmission selects to start in second gear. During the continued acceleration, the Mercedes-Benz PowerShift 3 transmission doesn't follow logical number sequences, but logical gear sequences. This means that three isn't necessarily followed by four, but rather by five and then seven or eight. The sensitive shift sensor system ensures precise gear selection in line with the particular driving and load situation. In this way, 12th gear is engaged quickly and the Mercedes-Benz Arocs switches into fuel-saving 'Eco Roll' mode, at a low 650 rpm.

Rarely does the engine speed exceed 1000 rpm - if at all, then only when accelerating. Consequently, the noise level in the interior of the well-insulated cab is always in a very pleasant range. Making phone calls using the hands-free system or listening to music is absolutely easy! The same is true for operating the switches and levers in the vehicle by the way. Everything is in the right place, starting with the steering-column lever, which also operates the triple high-performance engine brake (braking power: impressive 400 kW), to the multifunction steering wheel whose function keys control the various menus on the central display, among other things.

Steering sensation like in a rear-wheel-drive truck

Speaking of steering: contrary to mechanical all-wheel-drive systems, the new HAD does not restrict the steering angle. And the steering forces are also identical to those of a truck with rear-wheel drive.

Consequently, Hydraulic Auxiliary Drive demonstrates its advantages also on the road and represents a real drive alternative in particular for trucks in mixed operations. These vehicles mostly supply construction sites and often operate on the road. They rarely face tough conditions in rough terrain, which is why a mechanical all-wheel-drive system - regardless of on-demand or permanent - would be the wrong choice. The ideal solution is HAD. Compared with mechanical all-wheel drive, it weighs up to 575 kilograms less, offers 10 percent lower fuel consumption and does not require maintenance for the first 600,000 kilometres.

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