Tires: It’s all About the Tread

Author: Matthias Gaul

Dec 07, 2022 Safety on the road

Current DEKRA driving tests with professional test drivers at the Lausitzring in Klettwitz impressively demonstrate the weaknesses of worn tires. Among other things, the braking distance on wet roads is almost one-fifth greater than with new tires.

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.
“Overall, we found that with decreasing tread depth, the tires perform significantly worse in terms of power transmission between the vehicle and the road, especially in wet conditions,” Christian Koch sums up. As a result, critical situations can arise more easily and the ESP stability program becomes less effective. “In addition, with lower tread depth, the issue of aquaplaning becomes more relevant, meaning that the tire floats up on the water film on a wet road and loses contact with the road,” adds Koch. With greater tread depth, on the other hand, the tread is better able to absorb the water, which reduces the aquaplaning risk. At the same time, the tests confirmed that the minimum tread depth of 1.6 millimeters required by law for passenger cars in Europe is the absolute lower limit. “In the interest of your own safety, we recommend replacing the tires before they are worn down that far,” advises the DEKRA tire expert.
In real life, tire aging also plays a role
In all of this, the tests with the machine-treaded tires reveal only one side of tire wear. “In real life, you have to take into account that rubber tends to gradually harden with age, which also has a negative impact on performance,” says Christian Koch. Therefore, it can be assumed that real-world worn tires, which are already a few years old, would have performed even worse in these tests. “However, we specifically wanted to investigate only the effect of the tread depth – which is why the roughened tires were new in terms of the rubber compound,” emphasizes the DEKRA tire expert.
In the EU, incidentally, the approval and licensing of tires in terms of rolling resistance, noise emissions, and wet grip is still based exclusively on their brand new condition. How well a tire with reduced tread depth does its job has not yet been taken into account. This will change in the future, however, and the corresponding specifications and standards are currently in the coordination phase.