Small clouds drift in the sky, while on the ground, the S 518 LE test vehicle is clocking lap over lap on a rough circuit. The camouflage wrap on the body blurs the striking outline of the Setra.

The disguise conceals the new Setra MultiClass S 518 LE low-entry intercity bus. The XXL variant of the new series was put through its paces in rough-road tests. “Three axles, three doors and a double-width front entrance all combine to exert a comparatively high load on the vehicle structure”, explains test engineer Thomas Kuhn.

Calling Kuhn an expert is almost an understatement. The mechanical engineer has been working in the area of durability testing for 23 years.

The site where durability tests are conducted is the rough-road track at the Daimler Truck AG Development and Testing Centre (EVZ) in Wörth, not far from Karlsruhe in Germany's south west. Here, tomorrow’s buses are really put to the test. Nothing is left to chance. The numerous test tracks at EVZ represent typical road surfaces that have a particularly damaging effect on vehicle structures, such as cobblestones, ridges and potholes. The responsible testing team at Daimler Buses has put together very specific tracks for testing under rough-road conditions.

The combination of surfaces is derived from typical applications experienced by bus companies. In preparation, the test team previously examined stresses and strains on buses along various routes throughout Europe.

Special manoeuvres were also carried out at regular intervals. These included quick lane changes, for instance during an evasive manoeuvre, or emergency braking starting from different speeds. All of these are manoeuvres that buses have to cope with throughout their service life. and which a Setra MultiClass 500 will also have to deal with.

Holger Heidler took to the steering wheel. The trained construction engineer also has two dozen years of experience with durability testing. Heidler’s main job is to prepare the test vehicles. Like Kuhn, he enjoys his job: "It’s really varied. We deal with everything you may come into contact with – framework, engine, chassis." And: “We look at all products before they are launched." Whether it’s the Setra double-decker touring coach, high-decker bus, or now the Low Entry buses.

Heidler is currently steering the MultiClass 500 across track 5, which is littered with potholes. You think it can't get any worse? Well, think again: Track 6 is already waiting and it is utterly brutal. It features obstacles 70 millimetres high and arranged in a herringbone pattern. Buses have to drive over this at 10 km/h. Neither the materials nor any crew or passengers would be able to withstand higher speeds.

The rough-road test schedule has been carefully compiled. “We complete the course in the bus with it empty, partially loaded, full to capacity, and even overloaded”, explains Thomas Kuhn. A genuine durability test. Tubs of water strapped to the seats and a large number of ballast bags full of lead shot in the centre aisle simulate a bus that is over its maximum capacity.

The test vehicle is painted white and is not equipped with underbody protection. There is no panelling or insulation on the inside. This makes it easier to inspect the support structure. Anomalies, for example on weld seams, can be quickly identified. Kuhn says, "We pay particular attention to the side wall pillars and the substructure around the axle. because these are subjected to high loads."

The objective of the test is to simulate the fatigue of the body structure throughout the vehicle’s service life. As the vehicle is only driven on these simulated rough roads, ageing is accelerated by a factor of approximately 100. This means that one kilometre of rough-road track corresponds to 100 real kilometres.

Driving on rough-road tracks is extremely tough for the shock absorbers. For this reason, a special shock absorber cooling system is installed in the vehicle. Drivers are switched several times throughout the test period. because the demanding schedule is certainly challenging for the materials but also rattles the crew and passengers around. It would be too much to ask from anyone to be at the wheel for more than an hour at a time on this track. So the drivers switch round and take an hour’s break. “You have to be in a good physical and mental health”, confirms Thomas Kuhn.

Special rules apply on test sites. To be permitted to drive on the EVZ premises, drivers must be thoroughly instructed both theoretically and practically. Every test lap is made up of different sections of track. Drivers must be extremely attentive when changing lanes and pay attention to other test vehicles. A control station monitors the traffic. In the cockpit, drivers use a monitor to accurately track the specified drive program. The monitor displays both the specified route and the target speed. The speed is specified to an accuracy of 1 km/h and is supported by the cruise control system.

Rough-road testing can take up to a year. After one third and two thirds of the total distance has been completed, the bus is returned to the Daimler Buses Testing department in Neu-Ulm. where the entire body is thoroughly examined. Numerous vehicle components, such as the engine and tank, are removed to expose the body and refitted after inspection. Between these major inspections, minor, less extensive inspections take place at the EVZ site. For instance, wheels are removed to inspect the undercarriage and body.

“After rough-road testing is successfully completed, we issue a durability approval”, explains Thomas Kuhn. Then, he can be certain that “this is a really great bus." Holger Heidler is also impressed by the new MultiClass 500: “The vehicle drives well, even on rough roads.” And: “It’s a great feeling to complete the tests and be certain of a job well done”.

The test vehicle still has a few laps to go on the bumpy EVZ tracks before the series production of the new Setra MultiClass 500 can be initiated.