Many factors play a role in vehicle road safety. Anticipatory driving, observance of rules, appropriate speed, and the use of seat belts are just as important as active safety systems. Another vital component often does not receive the attention it deserves: tires and their tread depth. After all, inadequate tires are named as the cause in 38 percent of cases in the official German accident statistics. “It may certainly be a factor that worn tires are relatively easy for police officers to recognize at an accident scene,” says Peter Rücker, Head of DEKRA Accident Research and Analysis. So it is possible that tires are statistically overrepresented. That aside, it is clear that tire defects play a significant role in accidents.
But what influence does tire tread depth truly have on accident-relevant driving behavior? To answer this question, DEKRA had new sets of tires from well-known manufacturers in two different sizes mechanically roughened, i.e. the tread depth reduced, by a retreader, and carried out several tests. Professional drivers drove the vehicles and tested the driving behavior of tires with tread depths of seven to eight millimeters (new condition) as well as four to five and two to three millimeters on various test tracks at the DEKRA Lausitzring in Klettwitz.
Braking distance is 16 to 18 percent greater than with new tires
In the first series of tests, the driver fully braked a passenger car with the different tire sets from 100 km/h, both on wet and dry roads (ABS braking to DIN 70028). In the process, the braking distance with the lowest tread depth increased significantly compared to the tires in new condition, especially on wet roads. “We measured an increase in braking distance of between 16 and 18 percent for the two different tire dimensions,” explains DEKRA tire expert Christian Koch. In an emergency, this difference could be the difference between life and death. At the point where you come to a stop with tires in mint condition, the vehicle with the tires reduced to two to three millimeters of tread depth still has a residual speed of around 30 km/h. “If, for example, you crash into the rear of a truck at this speed, serious injuries are possible,” accident expert Peter Rücker points out. If you collide with a cyclist, they are likely to suffer serious or, in the worst case, fatal injuries. On dry roads, the braking distance was also greater with the abraded tires, but only by 2.4 to 8.5 percent.
Cornering limit speed decreases with dwindling tread depth
The second series of tests involved a stationary circular drive (according to ISO 4138). In this test, a professional test driver gradually accelerates a vehicle in a fixed curve radius until the lateral force transmission reaches the limit and the vehicle breaks away – i.e. up to the so-called cornering limit speed. In these tests, the tread depth made little difference on dry roads. On wet roads, however, things were different: “The cornering limit speed was between 10 and 18 percent lower with two to three millimeters of tread depth than with the new tires,” reports tire expert Koch. The vehicle even reached unstable driving conditions at significantly lower speeds, which in turn could easily lead to an accident. The third test series, the double lane change (according to ISO 3888-1), also known as the “moose test”, substantiated these results.