Mercedes-Benz Zetros press trial drive
Mar 30, 2010
All-wheel drive competence: Unimog and Zetros
  • Zetros benefits from low-cost large-scale truck production technology
  • Uncompromising off-road solutions
  • Unique vehicle design to meet special customer requirements
  • Unique position thanks to the all-wheel drive competence of the Zetros, the Unimog and all-terrain vehicles
The Mercedes-Benz Zetros is portrayed as the "continuation of the Unimog". The basic difference is that the cab-behind-engine Zetros is based extensively on large-scale truck production technology and is therefore a size bigger, while the Unimog – which next year celebrates its 60th birthday in the Mercedes-Benz fold – has for decades been the measure of all things off-road with its all-terrain model series. They both have one thing in common, however: they get through and reach their goal, even in the most adverse conditions.
A great deal of know-how from the Unimog sector has been incorporated into the Zetros design. Decades of experience with making vehicles capable of fording, for example, especially the Unimog, lie behind all of the measures which enable the Zetros easily to achieve a standard fording depth of 0.8 metre and even, with appropriate additional measures, an optional fording depth of 1.2 metres (as is the case with the Unimog). The tremendous axial flexibility of the Zetros is another area in which it has also benefited from the wealth of experience gained in the Unimog sector.
Tons of work, no roads is the mission of the Zetros
Nevertheless, the technical concepts behind both vehicles are sometimes very different. First and foremost, the Mercedes-Benz Zetros serves as a transport vehicle in off-road conditions. It has therefore been designed for gross vehicle weights based on the maximum permitted weights for heavy-duty trucks.
Among other things, the Zetros therefore has a particularly high payload because it is based on lightweight components: a 7.2-litre engine, a correspondingly slim transmission and single-tyre axles instead of twin-tyre assemblies are the determining factors for the low kerb weight of the Zetros specialist off-road vehicle.
The low unladen axle loads associated with this low kerb weight, among other things, are a decisive requirement in it being able to be loaded in transport planes, which have strict limits in this respect.
Absolute all-terrain credentials take priority for the Unimog
If the focus of the Zetros therefore lies on the highest possible transport capacity even in difficult off-road conditions, then in the case of the Unimog the highest possible degree of mobility takes priority over transport capacity. The all-terrain Unimog also differs from the Zetros both in terms of gross vehicle weight and payload (up to 16.0 tonnes without body). As a result, the all-wheel drive family has been supplemented in the best possible way to meet customer demand.
The all-terrain Unimog U 5000, for example, has a gross vehicle weight of 14.1 tonnes and, due to its different basic purpose, its dimensions and payloads are also completely different to those of the Zetros: the kerb weight of the U 5000 ranges from approximately 5.3 to 5.9 tonnes, and it is available with a wheelbase of either 3250 or 3850 millimetres. In the case of the Zetros, the wheelbase is 4800 (two-axle model) or 4750 + 1450 millimetres, respectively, and the kerb weight of the chassis is 8110/10,500 kilograms.
The Unimog has been specifically designed to be extremely compact, manoeuvrable and narrow to guarantee continued mobility even in the most adverse and restricted conditions. Its vehicle frame (which incidentally was also the driving force behind the design of the G-Class chassis frame) features a box-section design for exceptional torsional flexibility and provides a low overall centre of gravity with high ground clearance.
Unimog-typical refinements
The portal axles of the Unimog are also characterised by high ground clearance. They enable the axle tube and differential to be positioned over the centre of the axle. In the Unimog, the portal axles are connected to the chassis frame with the aid of coiled springs featuring progressive characteristics and which allow for short frame overhangs (and therefore high angles of approach and departure). The maximum possible axle twist with these coiled springs is 30 degrees.
The axles are connected to each other in the Unimog by means of a drive tube - making longitudinal control arms redundant - which transfers the tractive power via a torque ball on the transmission housing. It also contributes towards enabling the Unimog to benefit from the best possible diagonal distortion as well as immense spring travel.
In addition, the articulated shaft is located in the drive tube and is therefore protected against dirt and damage. This drive tube also helps to eliminate the effects of the suspension system on the drive, which translates into constant traction without chattering, as well as better traction than with leaf springs.
The all-terrain Unimog (U 4000 and U 5000) is available optionally with a fast or very fast power take-off unit on the transmission and also two hydraulic connections at both the front and the rear. The implement-carrying Unimog (U 300 to U 500) also has a power-engaged front PTO shaft driven directly by the engine.
Modified truck technology in the Zetros
All these are Unimog-typical refinements differentiating it from the Zetros, which instead is based on off-road truck technology - in an appropriately modified form wherever the application requirements so demand: front and rear parabolic springs, exposed articulated shaft, knuckle yoke axles typical of trucks, as well as an even chassis frame with open C-profiles, are the most important features of the Zetros chassis.
Nevertheless, the Mercedes-Benz Zetros makes provision for diagonal axial twisting of up to 500 millimetres and has appropriate clearances built into the chassis. As such, overall the Zetros is almost level with the Unimog in terms of its axial flexibility.
When taking the angle of approach as a measure of off-road capability, the all-terrain Unimog and the Zetros differ from each other in the following aspects (fully laden): the Unimog has a front angle of approach of 44 degrees, while for the Zetros it is 34 degrees. At the rear, the maximum angle of approach of the Unimog is between 44 and 47 degrees, compared with the Zetros' angle of approach of around 30 degrees for the two-axle model, and a good 35 degrees in the case of the three-axle Zetros.
The cab-behind-engine Zetros does not have the front and rear hydraulic connections of the Unimog. However, both the G 131-9 manual transmission and the Allison fully automatic transmission have power take-off units to which hydraulic systems can be connected.
Similarities in driver's cab and entry access
The cab mounting in both the Zetros and the Unimog is based on the Unimog's tried-and-tested three-point bearing principle. This type of mounting is stress and torsion-free, and moreover it reduces the load caused by road surface-induced shocks. In general, the seating position behind the front axle (as is the case in both the Unimog and the Zetros) significantly reduces the vertical and horizontal loads which may occur compared with a seating position over or even in front of the front axle.
Both the Zetros and Unimog are finally in step with each other when it comes to entry access design. In both cases entry access is positioned behind the front axle and sufficiently far enough away from the door hinge assembly to make getting into and out of the vehicle possible (compared with a cab-over-engine vehicle) even with a relatively moderate door opening angle.
This is a significant advantage in confined conditions. The Unimog and Zetros are also in step in terms of the conventional design of the driver's cab, the single tyres on all axles, which are particularly suited to off-road use, and also the vertical air intake, which is particularly beneficial in dusty conditions. The Zetros has inherited its doors and the sliding window in the cab rear panel directly from the Unimog.
Differences in brake technology
In terms of brakes, on the other hand, the Unimog has opted for elaborate pneumatic-hydraulic fixed-calliper brakes, while the Zetros slows down with the aid of conventional drum brakes. The disc brakes of the Unimog do not fade and also do not freeze in the severest frost - nor are they affected by driving through mud and water.
However, pneumatic-hydraulic systems and fixed calliper brakes have not become established systems in the case of heavy-duty trucks - the former due to the two different media which they use (and also complex handling), and the latter due to their increased space requirements.
As a result, for several good reasons the Zetros has opted for conventional drum brakes: for one, their maintenance requirements are extremely low. In addition, the system is simple and much less complex than the floating-calliper disc brakes normally found in trucks - so much so that the drum system is generally preferred throughout the world basically on account of it being simpler to maintain and repair.
In addition, unlike floating-calliper disc brakes, drum brakes are to a large extent immune to freezing, for example following a night of heavy frost. They are also barely affected by driving through copious amounts of mud and water, whereas floating-calliper disc brakes can suffer problems as a result of water ingress.
UG 100 or eight-speed transmission for the Unimog
The Zetros and Unimog differ from each other in another fundamental aspect: the transmission. The Unimog UG 100 gearbox (fitted as standard in the U 5000) is noted for meeting the most complex requirements in terms of off-road mobility. It comes with up to 8 forward and six reverse gears as standard, plus additional off-road and working gears (up to 16 in total), covering a very broad spectrum of final speeds from 1.5 up to 90 km/h.
The electro-pneumatic shift operation of this transmission also enables the gear stick in the cab to be dispensed with, meaning that no vibrations are transferred into the driver's cab. The driver can switch directly between forward and reverse gears by means of Electronic Quick Reverse (EQR).
The all-terrain Unimog U 4000 on the other hand (maximum gross vehicle weight of 10.0 tonnes), is available with either a manual or automatic eight-speed synchromesh transmission, which has automatic as well as manual modes and an automatic clutch.
Nine-speed manual or fully automatic transmission for the Zetros
The Zetros, meanwhile, comes with a choice of either the G 131-9 manual transmission or the 3000 SP/PR Allison fully automatic transmission.
The optional fully automatic transmission enables switching to take place without any interruption in the tractive power, even in difficult terrain. And the manual transmission provides a final speed of 3.8 km/h with off-road crawler gear engaged, which is absolutely ideal for all situations in off-road applications (with rear axle ratio of 5.33 and 14.00 R20 tyres). As such, even the steepest of slopes can be descended safely using engine braking.
Climbing ability leaves nothing to be desired
Reasonable reserves are also available for climbing up the steepest sections: the maximum climbing ability of both the standard specification two and three-axle Zetros - with manual transmission and off-road gear engaged - is theoretically well over 100 percent using the crawler gear and while fully laden.
Even in first gear (plus off-road ratio), the two axle Zetros still achieves a theoretical value of more than 100 percent, while the three-axle model, at a full weight of 27 tonnes and in first gear (plus off-road ratio), is also still able to boast a climbing ability of 70 percent. The corresponding theoretical values for the Zetros variants with Allison fully automatic transmissions and with off-road gear engaged are more than 100 percent for the two-axle model and around 65 percent for the three-axle model.
Just out of interest, compare this with a fully-laden Unimog U 5000 (gross vehicle weight of 14.1 tonnes): it also has a climbing ability of around 70 percent in first road gear. With working/off-road gears engaged, it needs to move up to fifth gear to be able to tackle gradients of 100 percent.
In practical terms the maximum climbing ability of the Zetros is roughly 80 percent, while for the Unimog it is 100 percent.
Competence and tradition: more than 100 years of Daimler trucks with all-wheel drive technology
Whether early vehicles suitable for the deserts and jungles of distant lands, all-wheel drive workhorses primarily for the construction industry, or universal off-road machines such as the Unimog in all its many facets: Daimler can look back over a wealth of experience with all manner of all-wheel drive vehicles which is not only unparalleled, but which has also contributed to the development of the latest product in its extreme all-terrain cab-behind engine range - the Zetros.
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