|---- Specifications ----|
|Engine||12.8 liter inline-6 diesel||Weight||--|
|Aspiration||turbocharged||Torque||1844 lb-ft @ 1100 rpm|
|HP||510 hp @ 1800 hp||HP/Weight||--|
|HP/Liter||39.8 hp per liter||1/4 mile||--|
|0-62 mph||--||Top Speed||--|
There is certainly no denying that the Arocs is an imposing presence, exuding power and robustness even when stationary. The radiator grille with its "bucket tooth" look visualises the link between technology and terrain – this truck is clearly in its element off the beaten track. It demonstrated its credentials at the Rheinkalk surface mining site in Dornap.
The 4151 8x8/4 was chosen for the test drive on the so-called extreme circuit. In keeping with the established Mercedes-Benz nomenclature, this is a 41-tonner with four axles, all driven, two of which – the front ones, of course – are steered. The engine is rated at 375 kW (510 hp), as is also indicated in the model designation. The engine in question is the top-of-the-range 12.8 l six-cylinder in-line engine from Mercedes-Benz, which delivers its full output at 1800 rpm and summons up maximum tractive power of 2500 Nm at 1100 rpm.
The fully automated Mercedes PowerShift 3 transmission is normally flange-mounted on the standard-fit Euro VI-compliant BlueTec engine. The selected test vehicle was fitted with the sixteen-gear manual transmission which is available as a cost option, however. This optional manual transmission lends itself to precise operation, thanks to a newly developed shift-by-wire system with noticeably more effective power-assisted gear shifting. The light clutch pedal also plays its part here.
In the course of the first test drive across the off-road site, the main focus is less on the transmission and rather more on the impressive engine, however, which demonstrates hefty pulling power right from the low rev range combined with an innate liveliness. This often avoids the need to change gear.
Operating this high-end construction vehicle proves as ergonomic as drivers might expect, thanks to the hallmark Mercedes set-up for the differential locks on the new Arocs which sees them connected in series as necessary for the purposes of traction and dynamics. In addition, the clearly laid-out dashboard provides precise information on the current operating conditions.
The instrumentation includes a speedometer of course, but this is less important in difficult terrain. Equal prominence is given to the rev counter, which at peak levels of driving resistance indicates whether the engine is up to negotiating the terrain ahead.
When the Arocs 4151 8x8/4 is operating at full load, the rev counter needle points stubbornly to 1100 rpm. Even the steep part of the demanding off-road circuit – which was extremely slippery as a result of previous rainfall – fails to tame the high-capacity engine. The driver actually has a little space left between accelerator pedal and floor.
The all-terrain chassis displays a deftness of articulation under the dumping platform that is a joy for the photographers on the scene to behold. The atmosphere remains relaxed at the wheel of the Arocs, too. Axle load compensation provides for the best possible weight balance between the two front axles, with traction almost at 100 percent. The new all-wheel-drive dumper also benefits from the high-grip off-road 13 R 22.5 tyres here.
The steering operates with noticeable precision in these demanding manoeuvres. But there is no surprise here as it is attributable to a world first in the commercial vehicles sector which is on board the new Arocs: the electro-hydraulic Servotwin steering system, which is initially being introduced exclusively on four-axle vehicles. The benefits for Arocs drivers are speed-sensitive power assistance and active return movement.
This newly developed technology, which features as standard on 8x6 and 8x8 vehicles and as an option on 8x4 vehicles, opens up a world of unprecedented steering comfort combined with the utmost in steering precision.
The test vehicles on the second circuit also feature Servotwin. Gear-shifting is entrusted to the standard-fit Mercedes PowerShift 3 transmission on these trucks. Upline of the transmission, the powerhouse comes in the form of the new OM 470 10.7 l six-cylinder in-line engine, which is available in four output variants ranging from 240 kW (326 hp) to 315 kW (428 hp). The test vehicle employed here, a four-axle 3240 with an 8x4/4 drive configuration, features the second most powerful engine version, rated at 290 kW (394 hp) and generating up to 1800 Nm of tractive power. The three-way dump truck is fitted with 315/80 R 22.5 tyres, which are suitable for on-road and light off-road use.
The candidate for this test drive, equipped with a slightly smaller engine, half-length cab and less sturdy tyres, is nevertheless put through its paces under a full load of 32 tonnes. Only the hallmark brawny Arocs looks remain unchanged.
The workplace is laid out as on the previously tested 8x8 model. The parking brake is ergonomically integrated in the instrument panel next to the steering wheel for ideal accessibility. On all-wheel-drive vehicles, this location is reserved for operation of the differential locks, which are used more frequently in off-road terrain.
The Arocs 3240 8x4/4 is ergonomic and lends itself to intuitive operation, thanks not least of all to the Mercedes PowerShift 3. The steering-column lever for the Mercedes PowerShift 3 enables stress-free driving both on- and off-road. The on-board computer is operated via the standard-fit multifunction steering wheel. The instruments are clearly laid out and offer a wealth of information and setting options. The seat and mirror adjustment options are arranged in an ideal manner, with every switch positioned just where it belongs. Even gearshifting appears identical to on-road use with Mercedes PowerShift Offroad – driving the supposedly rugged construction vehicle is highly comfortable right from the word go.
The transmission computer has defined second gear as ideal for moving off. In view of what for 394 hp (290 kW) and 1800 Nm is a rather high vehicle mass of almost 32 tonnes, and a short final drive ratio tailored to off-road use, this is an indication of a configuration philosophy with an onus on avoiding unnecessary strain on the clutch. Power transmission is established at idling speed.
The Mercedes PowerShift 3 always selects the appropriate gear for moving off – according to the load, gradient and active driving mode. During acceleration, the Mercedes PowerShift 3 shifts gear according to the established principle of large gear steps. Where possible, the 3240 8x4/4 shifts from third into sixth gear and then directly into eighth gear. The transmission computer knows just when and how to shift gear, without wasting time or fuel.
A brief sprint is sufficient for now – after all, we are not on a paved road here, but at the Rheinkalk surface mining site featuring terrain of varying severity. The Mercedes PowerShift 3 automated transmission which comes as standard on all Arocs models amply demonstrates its off-road capabilities here. In addition to the power drive program there is a special offroad mode. Each drive program mode offers a choice of three modes (standard, manual and either power or offroad).
The test vehicle is equipped with the offroad drive program for the Mercedes PowerShift 3. When this program is activated by a tap of the finger on the steering-column lever, off-road terrain should not pose any problems. At least as far as gear selection is concerned.
In response to even slight rises in driving resistance, the gear selector computer sets the shifting thresholds for up and downshifts to a very high level. The offroad drive program with the Mercedes PowerShift 3 reliably prevents the vehicle from getting stuck as a result of over-optimistic changes of gear ratios.
Standard driving mode is ideal for the further course of the test drive in the medium-difficulty terrain at the surface mining site. Tuned to 1800 Nm in this mode, the 10.7 l engine flexes its muscles and is instantly raring to go. Similar to the previously tested 12.8 l engine, this variant with a smaller displacement takes all the relevant hurdles that the terrain has to offer in its stride.
On encountering a sudden slow stretch on the circuit, Mercedes PowerShift 3 shifts down directly from fifth to first gear with an assurance suggesting it can do this sort of thing in its sleep. The traction at the drive wheels is instantly adapted to the current driving resistance. The OM 470 pulls the Arocs through the deep, muddy water without getting stuck.
The swiftly activated inter-wheel differential lock is also instrumental in maintaining the tractive power. Deactivating rotational speed compensation could not be more simple, even for off-road novices. The unavoidable gear-shifting manoeuvre is carried out without any need for major operations involving a gearshift lever or sensitive use of a clutch pedal. This leaves the driver all the time he needs to simply turn the differential lock switch down to the next level.
Off-road driving is never quite that simple, however. Before we tackle the steepest uphill stretches, the test supervisor warns: "The driver's know-how and skills remain indispensable in difficult terrain." By which he means that even the best automated transmission does not have the power of vision - even if the new PowerShift offroad mode's highly sensitive sensor system appears to anticipate impending changes to driving resistance with its inclination sensor, which has undergone further optimisation for off-road operations.
The tester thus has an opportunity to switch to the manual shift programme and try out the manual driving mode. In fixed third gear the four-axle truck clambers up an uphill gradient with its fully locked tandem drive, almost as if to indicate to the off-road greenhorn that it really has no need for all-wheel drive. Know-how plus basic Arocs technology suffices. The six-cylinder engine attends to the rest, responding truly elegantly to the accelerator pedal.
Downhill – and it feels like a sheer drop – we proceed in first gear, which remains locked to prevent upshifts, and with uncompromising intervention by the reinforced continuous brake. The latter extracts a level of braking power from this 10.7 l engine which would formerly have been the reserve of a ten-cylinder naturally aspirated engine.
After the impressive trip with the all-terrain, all-wheel-drive Arocs 4151 8x8/4 and the essentially equally off-road-capable Arocs 3240 8x4/4 for varying operations, a road-oriented test drive with an Arocs 2630 6x4 rounds off the comprehensive test programme.
The three-axle dump truck is powered by a compact 7.7 l engine which is instrumental in providing the Arocs concrete mixer Loader with a quite sensational transport volume of 8 cubic metres. Its power spectrum ranges from 175 kW (238 hp) to 260 kW (354 hp).
What's good for a concrete mixer truck can't be bad for a bulk freighter that is always striving for the highest possible payload. We hit the road in the three-way dumper with the mid-range light-duty engine rated at 220 kW (299 hp) and 1200 Nm on board.
Like the OM 470 from the second test drive, the OM 936 engine concealed under the medium-length Arocs cab is a totally new development. The light-duty engine bears up well to comparison with its big brothers, boasting similar technical refinements such as a maximum common rail injection pressure of 2100 bar or the world's first adjustable exhaust camshaft in a diesel engine.
Turbocharging for the power variants up to 220 kW (299 hp) is carried out by an asymmetric exhaust gas turbocharger with double-flow turbine. Two-stage charging with two turbochargers is provided for the two highest power variants.
The impressive performance of the OM 936 is in evidence right from the approach road to the motorway. Around 90 percent of the maximum engine output is already available from an engine speed of 1600 rpm. The engine also offers high dynamic performance right from the low rev range.
The maximum torque is available throughout the main driving range from 1200 rpm to 1600 rpm. The engine also displays a high level of performance at engine speeds below 1000 rpm, however. A further impressive aspect in practice is the engine's spontaneous response to movements of the accelerator pedal.
The standard-fit Mercedes PowerShift 3 also gives an excellent account of itself once again. The automated gearshift strengthens the impression that this "small" engine is well able to punch above its weight. The Mercedes PowerShift 3 whizzes through the gear ratios at appropriate gear steps, always at pains to keep the engine speed down in the manner of a twelve-litre engine.
This is no surprise at first, as the test Arocs is initially on the road in unladen state, so it is only to be expected that engine power and manual transmission will harmonise to perfection. After the first brief trip on the motorway, the dump truck calls in at a building materials depot to take three full wheel loader shovels on board, however.
Will our nimble Arocs 2630 6x4 now be revealed as a lame duck? Not a bit of it: on the quite hilly stretch of country road back to the "discharge point", the compact engine and automated transmission prove an impressively high-calibre duo.
The shift points are spot-on with the small engine, too. Whenever the driver is about to tap on the touchshift button to carry out a manual gear change, the automatic Mercedes PowerShift 3 steps in beforehand to shift the gear ratio. This usually involves a single gear step, with occasional double steps, too.
The Servotwin steering system with partial electric assistance impresses the test driver once again on the smoothly negotiated stretch of country road. While the lightness of touch comes to the fore in off-road terrain, it is the increasingly direct configuration that is appealing on fast stretches of country road. This leads to a firm feel at the wheel with clear feedback from the road surface.
Operation of the vehicle using the lever on the right of the steering column, where the engine brake is also activated, is intuitive. There's not a great deal for the driver to do, however. The drive and brake management system designed to handle the operating conditions encountered in practice takes much of the work off the driver's hands.
The driver operates important assistance systems such as cruise control and the user-definable speed limiter via the multifunction steering wheel.
A particularly useful feature is a user-programmable hotkey, to which each driver can allocate the submenu which he considers to be most important (driving time or trip consumption, for example). A push of the button is then sufficient to call up the desired information on the display.
On the subject of driving time: we have clocked up almost half the legally permissible limit after the three test drives. Back at the Rheinkalk site, the test driver manoeuvres the Arocs 2630 6x4 precisely into position at the discharge point and dumps the bulk materials with due aplomb.
A short move forwards followed by an abrupt stop is enough to send the rest of the material sliding down the load surface. And so the tester ends the final test drive with the new Arocs models with a degree of professionalism akin to that demonstrated by the newcomer in all its different guises.