Special applications need special vehicles. One important criterion when selecting the right vehicle is whether it is to be used on-road or off-road. Other criteria include haulage quantities and transport frequency. Using a vehicle for a mixed variety of transport tasks on varying surfaces rather than for a single task on unchanging routes places different demands on road vehicles.

When transporting large quantities of bulk such as gravel or sand on surfaced roads, for example, one factor above all is crucial: payload capacity. Lightweight vehicles are called for. The appropriate choice here is generally a particularly lightweight semitrailer tractor from the Actros family in the standard 4x2 drive configuration. An equally lightweight three-axle tipper semitrailer in tow will hold up to 28 tonnes of paid freight. Off-road applications are largely off-limits with this kind of truck/trailer combination - thanks to its standard differential lock, the Actros can cope with unsurfaced tracks at the loading or unloading site, but that is where the traction reaches its limits.

If a semitrailer combination of this kind regularly has to cope with more demanding terrain, a 4x4 semitrailer tractor should be considered. When it comes to the Mercedes-Benz commercial vehicle range, the all-wheel-drive Actros is the perfect choice. Thanks to permanent all-wheel drive - with standard inter-axle and inter-wheel locks - this tractor unit can master more arduous terrain than a standard rear-axle-driven vehicle.

A popular alternative to the semitrailer combination of two-axle tractor unit and three-axle semitrailer described above is the 6x4 tractor with a two-axle semi­trailer in tow, a combination where limited off-road usage is possible. Given the nature of the vehicle, the payload is somewhat lower, but ride comfort is considerably higher.

The three-axle 6x6 all-wheel-drive tractor unit offers better going on unsurfaced terrain. The permanently-driven front axle is particularly advantageous when it comes to cornering.

With the aforementioned semitrailer tractor, this particular semitrailer tractor unit reaches the limits of its suitability in construction-site use. For more arduous transport tasks on rougher terrain, there is no avoiding the four-axle model. The lower payload means that the large solo vehicle is not by nature the optimum choice for strictly on-road applications, but it is the perfect choice for mixed on and off-road usage and more restricted operating conditions.

It is quite simply easier to manoeuvre a solo vehicle backwards and forwards around corners to reach the loading or unloading point than a combination. In light to medium-duty applications the standard formula for the four-axle model is 8x4, while 8x8 is unbeatable for heavy-duty applications. 8x6 is also available as a compromise.

A size smaller, but slightly more versatile, is the classic three-axle model. The classic three-way tipper is a far more common variant here than it is with the four-axle model. The 6x4 features as the bread-and-butter tipper in many fleets. As a tractor vehicle - generally seen with a tandem trailer with a central axle in tow nowadays, rather than the fifth-wheel trailer it tended to be coupled with in the past - one day it might transport bulk or excavated material cost-effectively from A to B on the road before being used the next day as a solo vehicle on the cramped, rough terrain of construction site C. As a universal tipper, the three-axle vehicle will tow a medium-duty low-loader, with a medium-sized earth-moving vehicle strapped on, while - reinforced with 6x6 - it can cope with seemingly insurmountable obstacles on rough terrain.

Mercedes-Benz construction-site vehicles feature differentials that are all lockable. In addition, all the wheels are permanently driven in the case of all-wheel-drive vehicles. Mercedes-Benz all-wheel-drive trucks do not have selectable systems like some rival products. This means that specific locking of the inevitable differentials - there are already three in the 4x4 two-axle vehicles - becomes even more important.

Over the years an extremely logical engagement system has evolved which eliminates any incorrect operation of the differentials. Instead of the previous individual switches for the inter-axle and inter-wheel locks, their logical operation has now been combined in a rotary switch. From the "Off" position with completely open differentials, position 1 first shuts off the sometimes traction-reducing inter-axle compensation, followed by the rear inter-wheel locks. If this is not sufficient, position 3 also locks the front axle differential, though it is almost impossible to negotiate bends with a rigidly locked-up multi-axle truck. In the vast majority of cases the vehicle can at least be given the traction to free itself, and in such a precarious situation the wheels should be straight anyway.

Like the differential locks, an easily operated power shift for the obligatory 16‑speed transmission is standard equipment on the heavy-duty construction vehicles by Mercedes-Benz. The "Telligent gearshift" EPS practically shifts the gears at the touch of a button - preselect, operate the clutch and the gear is engaged. This can be the gear the driver chooses as just right on the basis of his experience, or the gear the networked engine/transmission electronics have calculated to be appropriate at that particular moment. 

To this end, the electronic brain uses numerous sensor-based parameters, with a heavy emphasis on rolling resistance. This is because these are crucial when selecting the correct gear, especially on difficult terrain. The technology makes good use of any help it receives from the driver: if the driver activates a differential lock, the shift computer logically interprets this as an indication of increased rolling resistance. The logical next step: accordingly it increases the gearshifting speed in main gears one to four.

The use of the new Mercedes-Benz PowerShift Offroad in the construction Actros vehicles is even more effective than the Telligent automated gearshift provided as standard in all construction Actros vehicles. In this, the most highly developed system for the completely automated operation of a manual transmission, the electronics not only control gear selection but also operate the clutch and control the engine speed.

In practice the experienced off-road driver with the benefit of this system will simply select Automatic or Manual mode, though occasionally preferring the latter when operating off-road. When a gearchange appears appropriate to the professional behind the wheel, simply nudging the shift lever is enough to let the machinery do the rest. Neither do load changes need to be taken into account, as the driver’s foot can remain firmly on the accelerator.

Once the vehicle and its equipment have been properly specified for the intended applications, there is another very important aspect to consider: the tyres. Whether the truck is to operate on the road or in gentle, intermediate or even adverse off-road conditions, every professional driver will tell you that choosing the right tyres is crucial off-road.

The basic principle is always that the heavier the going, the larger the tyres should be. The larger the tyre and the heavier the tread, the better. Compromises are possible, but in extreme cases it is difficult to make them work in practice. If a truck such as an Actros 4146 with an 8x8 configuration is to master even the toughest ground conditions, it cannot be fitted with tyres aimed at good ride comfort and the lowest possible rolling resistance. This is a job for tyres with a heavy tread, and this inevitably means a rough ride where the going is smoother.

In addition to sporting the right tread for the conditions, a good off-road tyre has sufficient resistance to external damage and enough internal strength to operate at a reduced air pressure. And finally, special-purpose tyres of this kind are also able to cope with special off-road chains. Fitted with these chains, the all-wheel-drive Actros can almost compete with a fully fledged tracked vehicle.