25 August 2019 18:49
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Dave Whyte: I have no shame in saying that my experience behind the wheel is almost entirely made up of time spent on sealed roads. Mostly I was behind the steering wheel of a truck designed for long-distance transport, more recently with trailers with low ground clearance in tow. Imagine my mood as I sat down in the cab of a truck and saw a narrow firebreak full of ruts in front of me. My feelings were quite mixed when I boarded a brand new Mercedes-Benz Zetros in the bush to test its off-road capability…
Available in 4×4 and 6×6 configurations, it is Mercedes-Benz’ offering to those who are looking to go places that big trucks shouldn’t go, including fire fighting and remote access applications. It’s a long way from its flagship Actros highway truck, but the jobs the Zetros is intended for, and the terrain it’s designed to cover, would likely stop most highway trucks in their tracks.
In saying that, what lies beneath the Zetros is also available under an Actros cab, but the Zetros has a few tricks up its sleeve. There are a few places the Zetros will go that a 6×6 Actros won’t, including the belly of a Hercules freight plane. In fact, that was one of the design criteria, and is the reason for the oddly shaped roofline that angles down at either side. While the vast majority of buyers will never test this for themselves, those that buy in big numbers will likely take this into consideration.
The Zetros is a fine example of a manufacturer pulling together readily available parts to make a specialised machine. It is powered by the same OM 926 LA 7.2-litre in-line six-cylinder engine that can be found under the Axor cab, and the dash panels from the Axor and Atego range are used inside the cab. While the cab itself is specific to the Zetros, the doors are taken from the Unimog parts list. What this means for the buyer is that parts are readily available, and any Mercedes-Benz truck dealership can offer servicing and support.
“Every aspect of the Zetros underlines the toughness and durability of components for use in off-road applications where 4×4 and 6×6 all-wheel-drives are operating.”
– Dave Whyte
The driveline from the engine back consists of an Allison 3000P six-speed automatic transmission, which is driven through a two-speed Mercedes-Benz transfer case. Power is then sent to the front and rear wheels, with the standard on-road program sending 40% to the front and 60% to the rear wheels. Once off-road mode is selected the power is spread evenly, 1:1, between the front and rear wheels. This also engages a lower 1.69:1 ratio in the transfer case, giving much better speed control and engine braking performance. Diff locks add to the tractive power, and are easily activated from inside the cab. Suspension comes in the form of parabolic springs all round, with the three-leaf front setup rated to 9.0 tonnes, and the four-leaf rear up to 9.0 tonnes per axle (depending on wheel and tyre size). Planetary hub reduction diffs (with aforementioned diff locks) put the power to the ground through huge 14.00 R 20 tyres.
All of this is mounted to 9.5 mm thick chassis rails that provide both strength and flexibility, as was evident over the drive. The heavy-duty hardware is certainly that – heavy. In cab/chassis form, the 4×4 Zetros 1833 A weighs in at just over 8.0 tonnes, while the 6×6 2733 A tips the scales at 10.5 tonnes. This may not seem too bad to those who operate heavy-duty road train prime movers, but the Allison automatic does limit their GCM, and these trucks won’t pull 130 tonnes. A manual gearbox is available for those heavier GCM tasks though. That said, they are still easy to operate, and agile enough to get where most big dollar 4×4 wagons won’t.
While the Zetros is designed for off-road use, it is also quite driver friendly on the tarmac. The off-road driving activities for the day were planned to take place out the back of Anglesea, about an hour’s drive west of Melbourne. The hour-long drive involved freeways and some B roads, before hitting the Great Ocean Road for the last few Ks. This was a great opportunity to get acquainted with the dynamics of the Zetros, including steering and braking, before hitting the rough stuff.
As is common practice, the Zetros is speed limited to 100 km/h, but the gearing and my mechanical sympathy limited road speed to a maximum of 90 km/h. At this speed, the engine was working at 2000 rpm, and, while it would’ve done 100 km/h, I didn’t see the need to push any harder. The Zetros handled very well on the road, however the lack of weight on the 4×4 model made for a good demonstration of the stiffness in the rear springs. The 6×6, with a heavy tray and 4.0-tonne of concrete on the back, gave a much better ride, and, consequently, a better demonstration of handling.
Eventually the time came to select off-road mode and push our way through the trees. As we drove along the fire trails, there were a couple of places where the tracks of smaller 4×4 vehicles came to an abrupt end. One of these was a steep bank where it was clear someone had made multiple attempts to get their 4×4 to the top, but never made it. Both the 4×4 and 6×6 Zetros trucks simply walked over it like an ant hill and powered on, giving a clear demonstration of just what these trucks could do. This was one point where I was able to watch the suspension on the 6×6 really working, and the flex in the chassis. The flexibility in both components meant that all six wheels were firmly on the ground, until the rear axle lifted off as the truck levelled out at the top.
While most of the trails we covered didn’t seem to really test the Zetros, certain sections demonstrated the benefits of different aspects of their design. Ground clearance, approach angles and turning circles were all highlighted over the course of the day, with me, for one, left amazed at where these trucks went. What’s more amazing is that they did this stuff with me at the wheel, in unfamiliar territory and unfamiliar trucks, yet with great ease and comfort.
There is no doubt that the Allison automatic had a huge impact on the ease of operation, with no break in the delivery of power during gear changes. This meant seamless power on climbs, even where gear changes were necessary, and no loss of momentum. Sure, they may have looked a little second hand after pushing their way through the scrub all day, but that’s what they’re designed to do, right?
The Zetros is fighting for a part in a niche market where buyers really do buy on necessity, not looks (which I happen to like anyway). While I have driven a few competitors before, none of them would have fitted in a Hercules freight plane. All jokes aside, most other models in the off-road market use a cabover design, and, while the difference may be minimal, the experience in a bonneted truck is totally different. Not only is there that protection out in front, from low hanging trees and such, but the improved ride and lower noise levels make life much easier on a driver.
Make no mistake, the Zetros is capable, the service and backup is available, and you can buy one from any Mercedes-Benz truck dealer across the country. Whether you’re after a fire truck, a water cart or an off-road camper, the Zetros is well worth a look.
Source: Mercedes-Benz Special Trucks World