All basic models undergo intensive testing

Given the wide range of Mercedes-Benz and Setra models available, it is not possible to scrutinise each individual length, height, engine and transmission variant. The engineers therefore define representative basic models for the tests. They cover all models, such as two- or three-axle vehicles, solo buses and articulated buses. Weight distribution and vehicle height are likewise taken into account – a Setra TopClass 500 model, for instance, is a super-high-deck touring coach and naturally has completely different handling characteristics to a low-floor urban bus such as the Mercedes-Benz Citaro.

Focus: correct functioning in the cold, driving on snow and ice

The focal points of the annual winter tests are correct functioning in a cold environment and driving on low-friction surfaces, i.e. ice and snow.

Winter tests obviously cover the heating, ventilation and climate control for the driver's and tour guide's areas as well as the passenger compartment. Defrosting of the windscreen is likewise important, as is the correct functioning of the windscreen wipers and the leak-tightness of doors and flaps at extreme sub-zero temperatures. How easy is it to fit snow chains, are the automatic snow chains guaranteed to work on the driven axle?

Are air-operated and electrically operated doors sure to open and close even if there is a severe frost? Do the air lines remain leak-tight at temperatures down to minus 30 degrees? Can very fine snow penetrate through the intake opening and clog the air filter? Does the AdBlue preheating work? The list of functional tests is a long one.

Some key aspects only become apparent at second glance: in the case of the panelling, for example, how do different materials such as sheet steel and plastic behave together, since each has a different expansion coefficient, depending on temperature?

How do the flexible adhesives on a side panel react at extreme sub-zero temperatures, when the steel frame remains rigid whereas a plastic panel contracts?

Tests are performed on the engine, the exhaust gas aftertreatment system, the transmission and all the drive system peripherals. Prior to the cold-starting test, the test vehicles are left with the engine flap open over the weekend so that the components cool right down. Even in the oil sump, the temperature then drops to minus 25 degrees or lower. And even the most experienced engineers are pleased when, under such conditions and at an outside temperature of minus 30 degrees Celsius, a heavy-duty diesel engine starts reliably after just a few seconds and runs smoothly straight away.

In the case of the chassis and suspension, the primary concern is ESP and its numerous subfunctions, the brakes and the steering. How does the power-steering pump operate at a temperature of minus 25 degrees Celsius, for example? How early is ESP supposed to intervene?