Mercedes-Benz Zetros press trial drive
Mar 30, 2010
The all-terrain suspension system
  • Optimised spring damper system
  • Diagonal axial twist of up to 500 millimetres
  • Fording capability of up to 1200 millimetres
  • Particularly robust chassis frame
  • Maximum traction
For a genuine off-roader, single tyres fitted all round are a natural first choice because the wheels of the rear axle(s) can follow in the track of the front axle without building up any unnecessary driving resistance, something which would occur with rear axles fitted with twin tyres. In any event, unlike somewhat smaller twin tyres, large single tyres are also more beneficial from aspects of weight and ground clearance.
The Mercedes-Benz Zetros comes fitted with single 14.00 R20-size tyres as standard, which thanks to their larger diameter add to the vehicle's ground clearance by around a further 75 millimetres compared with the largest regular truck tyres (13 R22.5). The 14.00 R20 tyres also help to achieve a maximum load-carrying capacity of 9.0 tonnes per single-tyre axle, whereas with single 315 R22.5 tyres the maximum would be precisely 8.0 tonnes.
Wheels are available both for tubeless and tube type tyres. As an alternative to 14.00 R20-size tyres, the Mercedes-Benz Zetros can also be ordered with 395/85 R20-size tyres.
In the event that twin tyres are required, the factory can also supply the axles with corresponding 22.5-inch wheels.
Optional tyre pressure control system
As an option, the Mercedes-Benz Zetros is available with a tyre pressure control system which enables the tyre pressure to be adjusted, depending on the demands of the terrain and the ground conditions, direct from the cab whilst driving. Often it is impossible to make any progress, particularly on loose ground such as sandy surfaces for example, without reducing the tyre pressure and therefore reducing the ground pressure which is generated. A rule of thumb from the off-road world captures this principle in a nutshell: "reduced tyre pressure is almost as good for traction as loading".
When it comes to brakes, the Mercedes-Benz Zetros has opted for classic drum brakes all round, which are both easy to maintain and function reliably even in the harshest of off-road applications and whatever the weather conditions. An ABS anti-lock brake system is fitted as standard – however this particular ABS system can be disengaged if necessary (useful for example when driving downhill off-road, e.g. on gravel or stones, so that a braking wedge can build up in front of the tyres).
Wide gap parabolic springs on all axles
Axle suspension at both front and rear is taken care of with wide gap parabolic springs in conjunction with stabilisers and shock absorbers. The spring damper system has been specially designed for the higher loads of off-road applications, and the axle suspension has been matched to this accordingly. For example, the characteristics and stress distribution of the parabolic springs of the front and rear axle(s) have been optimised precisely for harsh off-road use.
In addition, the spring eye bearings and stabiliser back bearings have been enhanced specifically to meet harsh off-road requirements. The Mercedes-Benz Zetros features shock absorbers with a particularly high thermal capability and which also benefit from a wider damper gauge than regular construction vehicles. Even the rear axle stabiliser has been specifically modified for more demanding requirements.
Off-road properties in a class of their own
The result of all of these measures is that both the recovery capacity and response of the springs are in a class of their own in terms of ultimate off-road properties. Both make a significant contribution to the extremely high level of ride comfort of the Zetros, even in the harshest of conditions. In turn, this also results not least in a reduction in deviation from tyre uniformity, which reduces wear and tear on corresponding auxiliary equipment and components. This highly effective, detailed fine-tuning is based on lengthy and painstaking tuning work which has benefited significantly from the off-road know-how gathered from the Unimog sector. Variations of the spring damper system suitable for different axle loads are also available.
When it comes to the chassis frame, especially in the case of off-road vehicles, in order to achieve the necessary diagonal axial twist (a requirement for maximum traction when off-road), a high degree of torsional elasticity is required. The chassis frame of the Mercedes-Benz Zetros (9.5 millimetres thick) is a ladder-type frame which is resistant to bending (important for on-road use) but torsionally soft (important for off-road use), therefore giving it very good torsional flexibility. It also features open C-profile longitudinal members as well as bolted-on C-profile crossmembers. Originating from the Actros model series, its rigidity and stability have been specifically enhanced even further with the addition of internal reinforcement brackets and plates on the crossmembers (each 8.0 millimetres thick). The bolted-on frame head has also been reinforced and enables the attachment of front-mounted implements.
Particularly robust chassis frame
Overall the chassis frame and frame head of the Zetros are designed in such a way that even when fully laden, the vehicle is not only able to be towed, but also raised via both the frame head and the rear. The rear underride guard features a folding design so that is can be folded up when driving off-road and does not unnecessarily restrict the angle of approach.
At the front, the radiator of the Zetros is protected by a robust radiator guard plate made of solid steel. The angles of approach of the standard specification vehicle are 27.6 degrees at the front and 32.6 degrees at the rear. The chassis and clearances (wheel housing for example) of the Mercedes-Benz Zetros are basically designed in such a way that no interferences will occur even with a diagonal axial twist of 500 millimetres.
Furthermore, as standard the Mercedes-Benz Zetros is designed to cope with a maximum fording depth of 800 millimetres. The fording capability can be increased to a maximum of 1200 millimetres as an option.
Fording depth up to a water line of 1.2 metres
This has been possible thanks to a variety of adjustments being made both to individual components as well as to overall assemblies. Two criteria always come into play here: sealing and venting. On the one hand, components which may come into contact with water, dirt and mud have to be sealed against such elements.
On the other hand, all of these components must of course be allowed to continue to vent. This is accomplished with the aid of a sophisticated venting system, composed primarily of a network of vent lines stretching through the entire vehicle. The individual lines lead to the outside above the water line. The cab rear wall acts as the connection point, given its central location.
The most important assemblies to be made capable of fording include the brake system, axles, manual transmission and transfer case, and the steering. The front and rear lights are also sealed against water ingress, with regard both to their housings and electrical connections, and are fitted with vent lines.
In addition, the flywheel housing and the generator, for example, are also enclosed using a special method. With the generator in particular it has been observed that dirt can otherwise collect there and lead to the alternator failing. Venting of the enclosed Zetros generator is achieved by means of snorkel. The cooling air piping system has a siphon above the water line.
The standard-fit air prefilter on the A-pillar is not just another requirement to enhance the fording capability of the Zetros - it also has the added benefit of ensuring that significantly less dust which may be stirred up is sucked in. The manner in which the cab-behind-engine Zetros has been made capable of fording is based on almost 60 years of experience with such designs on the Unimog.
Identical interfaces for bodies
The interfaces for mounting bodies on the Mercedes-Benz Zetros are identical to those used on the Actros and Axor series. In principle, adapting the widest variety of implement and attachment systems does not therefore involve any additional expense. At the same time, the even, straight and extremely durable chassis frame is suitable for all kinds of tailor-made and individual body/equipment solutions. Furthermore, the reinforced and bolted-on frame head of the Mercedes-Benz Zetros has been designed especially for the attachment of front-mounted implements.
One feature is also worth noting: the axial twisting of the axles caused when driving on uneven terrain is not only made possible thanks to the vehicle suspension, but to a great extent it is also down to the torsional flexibility of the chassis frame. To retain full mobility off-road and neutralise the effects of chassis frame distortion on the body, Mercedes-Benz recommends using a torsion-free body suspension for off-road applications, such as a double three-point bearing, for example. Basically, the body suspension on the Mercedes-Benz Zetros is an area that has been left to the responsibility of the body manufacturer. In terms of the design of the connection, within certain limits body manufacturers are therefore free to choose for themselves between a chassis frame structure and superstructure.
Clearly structured model series
The basic wheelbase of the two-axle Zetros model is 4800 millimetres; the dimensions of the corresponding pickup body are 5100 millimetres in length by 2450 millimetres in width, with 500-millimetre-high side panels. The overall length of the two-axle model (without body) is 7806 millimetres. The gross vehicle weight is 16.5 or 18.0 tonnes, respectively, depending on whether the front axles are 7.5 or 9.0-tonne axles. The turning circle of the two-axle model is 20.5 metres.
The basic wheelbase of the three-axle Zetros model is 4750 + 1450 millimetres; the dimensions of the corresponding standard pickup body are 6200 millimetres in length by 2450 millimetres in width, with 500-millimetre-high side panels. The overall length of the three-axle model (without body) is 8956 millimetres. The gross vehicle weight is 25.0 or 27.0 tonnes, respectively, depending on whether the front axles are 7.0 or 9.0-tonne axles. The turning circle of the three-axle model is 23.5 metres.
Product safety developed to the highest standard
For Mercedes-Benz, product safety ranks foremost, from the creation of an initial design right through to the overall vehicle ready for approval. Although based to a large extent on established, proven large-scale production technology, overall the Mercedes-Benz Zetros has been subjected to the same meticulous development and product validation process as all vehicles bearing the star on their grille. The most important elements of this process include:
Validation by means of numerical simulation (FEM): the setting up of a finite element model by way of example of the chassis frame and based on the given CAD geometry, followed by definition of the boundary conditions (bearing) as well as the loads to be sustained. The distortion and displacement resulting from the twisting which occurs are calculated. These results are used in turn to project whether the geometry of the components is sufficient for the requirement or whether components or materials will require further changes to be made.
Validation by means of hydropulse testing: the chassis frame is subjected to load on a servo-hydraulic test bench ("hydropulse" for short) in a manner which is equivalent to 500 hours of the most extreme off-road driving. Within a short period of time, chassis and cabs can be subjected to loads which in a practical road test would correspond to a workload of several hundred thousand kilometres. The advantages of conducting tests on a test bench in this way include clearly definable boundary conditions and environmental conditions, as well as time-savings.
Comprehensive road test for extreme efficiency
Practical road test: not everything about the overall vehicle can be uncovered by means of computerised simulations and test benches. In the end, criteria such as handling, tactile qualities or the overall tuning of the systems can only have their finishing touches applied in the real world to ensure harmonious, trouble-free operation. The Mercedes-Benz Zetros has been extensively tested in both summer and winter trials: from the Sierra Nevada of southern Spain, to Rovaniemi in the Finnish polar circle, the new cab-behind-engine vehicle has been tested to see how efficiently it copes with extremes of heat and cold.
Other elements of the practical road tests include intensive rough-road testing in the newly established Development and Testing Centre (EVZ) in Wörth, covering a total of 14 different road surface profiles. There are also functional tests, such as the fording capability, initially carried out in precisely defined conditions in water pools, followed by tests in mud. Off-road tests also take place at an early stage on extremely demanding tracks in Tunisia, where optimum conditions prevail for determining dust entry in the air which is drawn in, and examining the effective­ness of the cyclone moisture separator in the prefilter. Then there are yet further road tests on off-road sections at Horstwalde and Welzow Süd (brown coal surface mines), which provide ideal conditions for testing the handling on steep route sections and sunken terrain.
Total integration in the Mercedes-Benz service world
A further element of product safety which is as equally important for trouble-free operation and of benefit to the customer comes courtesy of the fact that the Mercedes-Benz Zetros is able to draw extensively on the large-scale production pool of Mercedes-Benz, and is therefore fully integrated into the global world of Mercedes-Benz spare parts and servicing. More than 5000 outlets in 160 coun­tries ensure worldwide availability, pricing in line with the market, and quick, professional servicing, even in the remotest corners of the globe.
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