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A bonnetted off-road truck series in 4x4 and 6x6 configurations

Unashamedly military in appearance the civilian version of the Mercedes-Benz Zetros range is aimed at the Australian mining, exploration and fire-fighting sectors. 

Having been trained as a military vehicle demonstration driver by Volvo Truck in my past life, I have a fondness for purpose-designed machinery. Army trucks have to function in a life and death environment, as do those that operate in remote areas.

No matter how well a maker off-road-modifies an on-road vehicle, the result is never as good as a clean-sheet-of-paper design exercise. (Look at the miners and construction companies who’ve tried various brands of popular medium 4×4 utes for heavy duty work and have had to go back to the ‘old fashioned’
LandCruiser 70 Series.)

The case for a purpose-designed off-road truck is even stronger, because the loads are greater and the price of failure is much higher.

The Truck

The Zetros was launched in 2008 as a family of 4×4 and 6×6 trucks, sharing common cab, engine, transmissions, drivetrain, chassis and axles.

The design concept was capability, so each Zetros comes fully equipped for off-roading, with three-seat air-conditioned cab, a manual synchro box or Allison auto, ample ground clearance, deep-reduction gearing and multiple mechanical differential locks.

All that gear on a limited-production vehicle doesn’t come cheaply: the 4×4 1833 sells for $222,000 + GST, and the 6×6 2733 for $240,000 + GST.

There is an options list, but it’s confined to things like a ROPS structure over the already impact-tested cab, or a roof-top hatch with machine gun ring!

For a brand like Mercedes-Benz that has been associated with COE trucks for many years a bonnetted ‘Benz may seem a retro step, but the cab style is for function, not fashion.

A bonnetted configuration moves cab weight behind the front axle, for optimum front axle weight and also reduces overall height. A lower-height cab – under three metres – with chamfered top sections lets a military Zetros fit into air and road transports and in civilian life allows mechanical ladders, water jet monitors, concrete pump booms, crane jibs or drill-rig masts to stow above the cab.

The Zetros is built on a single-skin chassis, with a sizeable rail thickness of 9.5mm and reinforcing sections behind the cab and at the front of the chassis.

The loaded truck can be lifted clear of the ground at the front, using reinforced chassis points.

The frame is designed to twist in off-road conditions and bodywork bracketry allows 500mm of diagonal movement. A three-piece steel bumper is fitted up front.

Planetary hub reductions are fitted to all axles, giving two-stage final drive gearing and allowing a smaller diff bowl in each axle housing. The front axle has a nine-tonnes rating and each rear axle, 13 tonnes.

Long, taper-leaf springs are fitted, for optimum ride quality and wheel travel. The 6×6 has an inverted-spring, six-rod tandem arrangement. Brakes are practical drums.

The standard wheel and tyre package is singles all around, but for most Australian applications the Zetros will be specified with duals at the rear, to maximise legal payload. Our test truck was shod with 385/65R22.5s up front and 315/80R22.5 rears, for a maker’s GVM of 25 tonnes.

The test truck carried a 10-tonne concrete block and weighed in around two tonnes under GVM.

Power to move all this comes from a Euro V OM926LA, 7.2-litre six, with outputs of 240kW (326hp) and 1300Nm from 1200rpm to 1600rpm. There’s a choice of a manual nine-speed, synchromesh transmission or an Allison 3000P six-speed, torque-converter automatic. The test vehicle had the Allison. (‘Benz has its own auto, but the Allison is a better fit with many off-road customers’ fleets.)

There is a choice of engine power take offs and a range of Allison PTOs.

Drive goes into a Mercedes-Benz VG1700 two-speed transfer case with a low ratio of 1.69:1 and centre differential, allowing full-time all-wheel-drive. There’s also an inter-axle differential in the leading rear drive axle.

The cab is an extended type, providing storage space behind the three seats. Outboard seating is suspended and the central perch has a fixed base. The dashboard is straight from the Axxor  COE truck.

There are folding footsteps on the bumper and friction-surface wear pads on the top of the steel mudguards and the steel tilting bonnet. This allows easy clambering onto the bonnet, for access to a roof rack or any equipment that may be overhanging the cab.

A nice touch is a positive lock on the raised bonnet, so it can’t drop down unexpectedly.

Another maintenance item I liked was the electrical arrangement, with traditional fuses and circuit breakers and individually wired circuits – not a CAN Bus system that requires diagnostic equipment.

The Test

Daily checks of chassis componentry are easy, because the tall rubber  and raised suspension puts everything at chest height. Physical engine checks require a climb, or the driver can rely on the truck’s electronic check program.

Getting in and out was a climb that would be easier if there were external grab rails behind each door.

Once aboard I found it very easy to adjust the seat, steering wheel and mirrors, while the engine warmed up. The spring parking brake was a Euro-style graduated-release type, making smooth lift offs on a steep grades a doddle.

Vision over the raked bonnet was very good and large mirrors revealed gaping pedestrians and motorists behind. The Zetros 6×6 certainly has presence!

The truck was easy to drive around town and on the highway, and although the box didn’t have an optional retarder, engine/exhaust braking was powerful enough to slow the loaded truck effectively.

Performance was on a par with road-going rigids and mechanical noise was commendably low: three of us could chat easily.

Bitumen-road and gravel-road grip was good; steering predictable and light and high-range gearing allowed easy cruising up to 100km/h.

However, the real fun began when the roads turned to fire trails.

I don’t know if you’ve driven in the bush around the picturesque Victorian seaside town of Anglesea, but this Bass Strait frontage is usually freezing cold or blazing hot and clouds are either soft, fluffy white or forbidding grey and scudding at ground level. We had the latter. (I know the photos show dust, but they were shot the previous day.)

The topography is hilly, with weather resistant tea-tree scrub and hardy eucalypt forest rooted in sandy soil and clay. There had been just enough light rain on the tracks to turn the powdered-clay-dust surface to something akin to soggy soap.

Mercedes-Benz’ Special Vehicles manager Philip Leslie showed me how all the bits worked and then assumed an advisory position in the left hand seat.

Manoeuvring a 23-tonnes machine around narrow fire tracks on a slimy surface brought back memories of doing demo drives for Australian Army evaluation in Volvo C306 and G88 vehicles, back more years than I care to remember.

I appreciated the ease of control of the big Zetros, with a T-bar auto box lever, wand for engine braking, a simple dial for low-range selection and all diff lock functions progressively arranged on another rotary dial.

Click One on that rotary switch locked the centre diff and the tandem-axle power divider, meaning that all three axle diffs had to rotate at the same speed. Click Two locked the two across-axle rear diff locks and Click Three locked the front across-axle diff lock.

Diff lock action was quick, with no ‘blocking’ or ‘hang-up’ that plagues some systems and makes safe manoeuvring difficult.

It was brought home to us just how slippery the tracks were when we came across Old Mate in his Range Rover Sport, towing a dirt bike trailer. He was ruefully inspecting some expensive mudguard damage, caused when this capable machine had slid sideways on a cambered bend.

He and his white-faced kids were finding it difficult just to stand up on this slimy surface. Being nice blokes we offered to tow him out.

Despite its compromise bitumen-gravel tyres the big Zetros steered accurately most of the time, with only heavily cambered sections forcing sideways slips into the gravel gutters.

The ‘Benz took steep uphill and downhill sections in its stride and engine braking was much stronger than I expected, making an expensive retarder option unnecessary.

When it came time to return to the gravel and black top, the two rotary switches had us instantly back in on-road mode. Control familiarisation shouldn’t be a problem for owners.



It’s not a big market, with only 1108 new 4×4, 6×6 and 8×8 van and truck sales in 2015, but the off-road truck business is interesting and, as with the road train market, image building. A bonnetted 4×4 or 6×6 truck sends a message of brand strength.

Mercedes-Benz has been knocking at the off-road truck door for many years, but the Unimog is too expensive and too complex for most customers, and the COE ‘Benz models have tall cabs that restrict many applications.

The Zetros 4×4 and 6×6 pair could just be the ticket.

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