01/12/2012

• Prototype tread carved by hand
• Experience and sure instincts are keys to success
• 25 million test kilometers per year

The silver-grey BMW races forward against the backdrop of the setting sun. The headlights are on full beam. The contours of the Bavarian sedan, dark and sharp, stand out against the flame red sky. Travelling at exactly 100 kilometers per hour, it reaches the target marker. The driver takes a firmer hold of the steering wheel and slams on the brake. The ABS system lets out a rapid-fire staccato as the tires lock. Small clouds of smoke are released by the wheel casings.  The sophisticated on-board electronics now kick in to calculate the braking distance to the nearest centimeter. The computer calculates a distance of 36 meters from the brake marker until the tires have come to a complete standstill – around four meters less than was recorded in comparable tests with the previous model. When the test driver reads the result on the display, he knows that the development work on the new tires has taken a significant step forward. He maneuvers the BMW into the waiting area at the testing ground. Precision braking is only one half of the job, and tire changing the other – for the umpteenth and last time today.

All that effort was worth it in the end. Another item in the technical specifications can be ticked off. The specifications detail all the objectives the project team wants to achieve by the end of development. Various technicians and chemists have previously pooled their know-how to develop proposed solutions to overcome goal conflicts and produce optimized contour concepts. The first tread variants are then developed from this information. But after material tests in the lab and countless computer simulations, the moment of truth is out on the test circuit. Only now do the new ideas prove their worth, because although a lot of tire development progress is made via virtual simulations, there is no real substitute for the original measurement data and the subjective judgments of the test drivers with their many years of experience. So with every product innovation comes a range of tests with different treads to identify the strengths and weaknesses of the individual prototypes.

The special thing about this initial test phase is that the products are not yet available in their vulcanized form. The vehicles are instead fitted with prototypes, whose tread has either been carved by hand or by robots. A tire carver requires around six hours for a summer tire; complicated winter treads take longer to carve – sometimes up to 40 hours. This is extremely time-consuming but still a great deal cheaper than using a tire mold, even though up to 800 of these hand-carved tires are required to develop a new product. The best treads are selected, optimized further and subjected to more tests until finally a tread is developed that will later go into series production. And only then can the first tire mold be produced.

Continental tire tests generally take place at the Contidrom. The corporation’s own test circuit in Jeversen, a small town in the southern part of the Lüneburg Heath, is the mother of all company test circuits. It entered operation back in 1967 and has been continuously modernized ever since to respond to the ever increasing requirements of tire and vehicle testing. Today, the Contidrom is one of the most modern test circuits anywhere in the world and is considered a benchmark by many customers in the automotive industry. Extending over 160 hectares, the testing ground offers every conceivable scenario for tire testing. Speeds of up to 250 kilometers per hour can be reached on tracks approximately 10 kilometers long and with many different surfaces, some of them fully irrigable. The Contidrom also has circuits for testing chassis elements. A team of around 60 staff – including test drivers, mechanics, technicians, engineers, data processors and firefighters – are on permanent standby to ensure that everything goes without a hitch.

The testers select the vehicles in a target-oriented manner. A key selection criterion is for the chassis to be as neutral and also as “well-behaved” as possible at the limits, because the measurement data should reflect tire performance in an undistorted and reproducible manner that is free from chassis influences. To avoid accidental influences, every single test is repeated several times. Dozens of sets of four tires are used for each test and tire type – a considerable expense when you consider that each and every one of the tires is first run in for about 500 kilometers.

The vehicle road tests can be broken down into two areas: Subjective judgments and objective standard tests. Experienced drivers subjectively rate the handling of the tires on wet and dry surfaces, the comfort level and tire noise. The standard tests determine a major proportion of the tire’s measurable performance characteristics.

These include:

• Wet grip performance on wet surfaces (straight lines and cornering)
• Braking performance on dry road surfaces
• Resistance to aquaplaning in straight lines and when cornering
• Various winter suitability tests
• Resistance to tire roll-off

Both types of tests are required to form a complete assessment of a new tire. How well the characteristics work together can be determined by comparing the tire noise and road surface noise measurements for example. The values measured in the lab and on the noise measurement circuit at the Contidrom can show that a tire meets the legal requirements, but the subjective assessment given by the test driver can sometimes provide a conflicting picture. Whether a tire produces unpleasant tire or road surface noise that irritates the driver cannot be fully determined by objective measurement methods alone. The personal impressions, sense of hearing and experience of the test driver are indispensable here. On the contrary, other tire characteristics can only be determined objectively or subjectively. Vehicle braking distance for example is always measured objectively. But different handling results can be recorded with comparable lap times, so it is important that a tire demonstrates gradual breakaway to give the driver adequate reaction time. A tire with more abrupt breakaway may come out on top when it comes to lap times, but receive a bad assessment from the test driver.

Despite standardized procedures, these test drives are always particularly expensive for winter tires because more than 60,000 additional individual measurements per year are involved in the “starting off on snow” test. The figure is almost 800 kilometers if you add together all the individual braking distances when “braking from100 km/h to standstill”. A total of 18,000 corners are tackled on ice and snow to test the cornering stability of the individual testing tire models – this amounts to around 700 kilometers. No wonder then that the timeframes extend over many months and sometimes even years. Not to mention the adverse weather conditions that make it impossible to carry out tests at the Contidrom. But even in such circumstances, nothing is allowed to stand in the way of tire testing with its tightly planned schedule. In such cases, the crew just hops on a plane and finishes off the testing program on another circuit (always accompanied by a part of its total 25-tonne cargo of tires, testing devices and test cars). And they even travel to a testing circuit on another continent if need be. For this purpose, Continental owns similar testing grounds in Spain, Asia and the US, among other places. So that work can resume as smoothly as possible and because the engineers require reproducible measurement procedures for testing in the various development stages, the conditions must correspond exactly to those at the Contidrom. The test circuits belonging to the Continental Corporation therefore have virtually identical road-surface properties across the globe.

New tire models have normally covered around 25 million test kilometers a year before start of production – this corresponds to an astonishing 625 times around the earth. 

Too much effort and expense you might say. Well, think again! On the way from the idea to the actual product, tire tests are only a small part, but a particularly important one. And although the job description of a test driver may sound like a great adventure to most people, test drivers themselves would describe their job in all honesty as mostly unspectacular. But they are convinced that even average drivers could spot the differences in performance between two different tire models in practice. Perhaps not so much the handling characteristics on dry roads, but certainly the typical wet-grip characteristics: Braking performance, wet handling and aquaplaning speed. Depending on their experience behind the wheel, drivers who switch from a premium to a low-quality product when buying a new tire would notice the difference on a wet surface as soon as they enter the first corner.