The payload master
We test drive Mercedes-Benz’s all-conquering Zetros
It was a rainy day in Oetigheim, at the truck test driving ground for Mercedes Benz.
Bad news for comfort, but good for testing the capabilities of a vehicle. And while drivers in the Middle East don’t typically see many days of rain, a vehicle’s traction – tested by the presence of water – is all important in the off-road conditions and demanding applications common to our region.
For the test drive Mercedes Benz had supplied CMME with the Zetros, in its three variants (the 4×4 1833, the 6×6 2733 and the 6×6 3343 tractor unit). Probably not the best known truck in their truck range (that would be the Actros), but a signifcant vehicle nonetheless in an important segment of the market.
First shown at a defence show in Paris in 2008, the Zetros is one of the newest vehicles in Mercedes Benz’s heavy vehicles line-up.
At a glance its applications are diverse: fire truck, support vehicle for mining or oil and gas, transport and logistics and disaster relief and military. While the heavier 6×6 was shown as a dedicated tractor unit, both the other vehicle options can also be configured to tow loads.Customers can choose to fit the vehicle with additional equipment such as cranes, hoists, or water tanks. The truck is relatively svelte and lightweight, meaning it is able to be carried inside a number of commonly-used transport planes, or even on a railway carriage
Only 30 minutes drive from the Mercedes Benz truck factory at Woerth, where the Zetros is built, the testing ground at Oetigheim is as demanding as can be imagined for heavy vehicles: steep slopes (up to 38°), pot holes, water traps and hills and stoney ground. Watching the Mercedes Benz test drivers traversing the course it was clear that they were seasoned professionals: hills were taken at speed, corners roared around and tight turns completed with only a narrow gap between obstacles. But that’s partly a reflection of the Zetros’ capabilities – you wouldn’t want to experience the same ride in a vehicle designed primarily for on-road driving.
The Zetros is built with a cab-behind-axle design (also called cab-behind-engine), which positions the driver behind the front axle rather than over it, and also at a lower position (similar in theory to a passenger car). As a result, thedriver is subjected to significantly lower vertical and horizontal loads, and the speed limit at which the driver is still able to remain in control in his seat is much higher. While a truck designed with a cab-over-axle may be physically strong enough to traverse off-road terrain, the driver will be shaken and bounced-about, unable to drive competently. Indeed the difference between cab-behind-axle and cab-over-axle is not always appreciated in the industry, even to the detriment of Mercedes Benz at test drive events: drivers who first experience a cab-behind-axle vehicle such as the Zetros will then try to drive a cabover- axle truck at the same speed, and on an offroad course the difference is immediately noticed: in short it’s a bouncy, uncomfortable ride. And it is precisely this ability to drive at speed over rough terrain that makes the Zetros such a good emergency response vehicle, whether for civil services or for mining or oil and gas.
T&FME put the truck through its paces. Due to time limitations, and with its greater terrain flexibility, we drove only the 4×4 model.
One of the more popular applications for the Zetros is fire engine, and with its 9-speed manual hydraulic-pneumatic G 131-9 transmission (8 forward gears plus its constant mesh crawler reverse gear) it is easy to see why: the truck accelerates fast. If desired it can also be equipped with a six-speed Allison 3000 SP/PR fullyautomatic transmission.
The two most common variants of the Zetros are built with the Mercedes Benz 7.2-litre inline six-cylinder OM 926 LA engine, capable of producing 240 kW (326 hp) and a maximum torque of 1300 Nm between 1200 and 1600/ rpm (supplied to the Middle East as a Euro 3 emissions engine). The heavy duty 6×6 tractor is supplied with the heavier and more powerful OM457-6 engine, a 12L putting out 315 kW (428 hp). (Some of these figures can be guessed from the Mercedes Benz model naming convention: the first two numbers are its GVW in tonnes, and the second two its engine power in horsepower, with a zero added.)
The design philosophy of the Zetros was to build a truck that could carry heavy loads across any terrain, as well as drive fast. The success of the Mercedes Benz engineers can be seen in the high gross combined vehicle weights (GVWs) of the different variants and their respective payloads, for what is essentially an off-road truck: up to 18 tonnes GVW for the 4×4, and up to 27 tonnes for the standard 6×6, while the 6×6 tractor can tow a load for a GVW of 43 tonnes, though it is built with heaver components and features twin-tyre assemblies on the two rear axles, also making it wider.
The powertrain and chassis of the Zetros are based primarily on the Actros and Axor model series, but its high payloads are possible in part because the truck design incorporates a number of lightweight components (including the engine). Payload (the carry load or GVW excluding body) is high, around 10 tonnes for the 4×4, and 16 for the 6×6. It’s also worth mentioning that operators in the Middle East may be able to put on extra weight, depending on their needs and any applicable road restrictions.
The Zetros we drove wasn’t fully laden, but the 7.2L puts out a decent burst of power, and at no stage of each gear interval did it feel underpowered. On the contrary, steep hills were taken with ease, and once the gear shifting was mastered (no mean feat as the box is configured with a 2-4-4 layout, requiring a decent shove to cross from the first 4 gears into the 4 higher ones) the Zetros began to feel that it could go anywhere, no matter how tough the conditions.
One of the best measures of off-road capability for a truck is its approach angle, and here the Zetros scores high points, especially when compared with other vehicles in its weight classes. It has a front angle of approach of 34 degrees, while at the rear angle of approach of around 30 degrees for the two-axle model, and a good 35 degrees in the case of the three-axle version. In the case of the vehicle we drove, it was able to power up steep hills in the wet weather, and only the steepest hill of 38° (the hills were actually tiered concrete to prevent ground damage from the wheel) proved too slippery forit, the truck sliding back down, though it had managed it earlier in the morning before the rain set in. Visibility from inside the cab was good, though as can be imagined on steep hills, as the ground falls away so is visibility reduced.
The Zetros relies on drum brakes and an engine brake, the latter is engaged by a dashboard control and then automatically activates when the accelerator is not engaged: this means that on steep hills the truck will crawl down slowly at a constant pace without the driver needing to engage any of the foot pedals, but only steer the wheel.
With an off-road gear ratio of 1.690 the truck is able to travel at extremely low speeds, a particularly important feature when driving downhill on the steepest of gradients.
Taking on such a demanding course would not be possible without use of the differential lock (indeed certain features were designed to demonstrate the diff lock), which is activated by a turn switch on the stripped-back dashboard, in easy reach of the driver. It has three modes – interaxle, rear interwheel and finally front interwheel. Other design features which aid off-road terrain driving are its planetary axles for high ground clearance, with high placement of the fuel tanks for good clearance.
There’s a number of features of the Zetos that have been designed with maintenance and longer vehicle life in mind, not least the forward opening engine bonnet which gives the maintenance teameasy access to the vehicle’s vital parts (but is still strong enough to walk on).
The frame and chassis are all built with Mercedes Benz’ proven quality, and its customers in the Middle East can vouch for the long life of their vehicles. All components have been tested and verified for the most difficult off-road applications in accordance with the demanding quality standards, using simulations, test bench trials and also extreme summer and winter road tests.
And good news for the Middle East is the vertical air intake, sucking in clean air at the top of the cab, which is particularly beneficial in dusty conditions. The air intake is fitted with a cyclonic air filter or separator inside the engine bay, while on the 6×6 4333 there is also a filter at the top of the air intake that can be manually (and regularly) cleaned by the driver.
Furthermore, and essential for desert driving if you don’t want to lug around a compressor, is an (optional) tyre pressure control system which enables the tyre pressure to be adjusted from inside the cab while driving.
Verdict
With a front end design that echoes the popular GL and G models, the Zetros is instantly recognisable as a vehicle designed for off-road use. And when it’s put to the test it doesn’t disappoint, with its powerful engine mated with an effective gearbox, and easy control of the differential lock. The smooth drive-feel of the cab-behind-axle design means that the driver is impressively insulated against the shocks and bounces you’d normally feel driving over rough terrain, allowing you to focus on the turns ahead rather than clutching at the wheel. The vehicle’s impressive payload capabilities underline its versatility, and it’s hard to imagine an off-road fleet that wouldn’t be bolstered by the addition of a Zetros. And for customers in demanding applications the ability to fully specify the vehicle prior to production can make a crucial contribution when it comes to having a vehicle that is fully fit for purpose. This makes the Zetros an important vehicle to be considered when needing to add speed or off-road capability to your vehicle fleet.
