The tread depth determines how much water a tire can take up and disperse from beneath the contact patch. The less tread you have remaining, the more likely it is that a wedge of water will build up ahead of the tire. This leads to aquaplaning as the tire loses contact with the road surface. When aquaplaning occurs, the tires can no longer transmit steering commands or braking forces to the road and the car drifts out of control.
An in-vehicle test conducted recently by AutoBild has revealed just how dramatically the braking distance increases on very wet road surfaces. In the test, a VW Golf was fitted with 225/45 R 17 tires from Continental, Goodyear and Michelin with different tread depths. The findings are shocking.
The best outcome, braking from 80 km/h with anti-lock brakes, was achieved by the Continental tire, which in brand-new condition brought the car to a stop in 25.8 meters. With a residual tread depth of 3 mm, the braking distance was 31.0 meters. And with the minimum legal tread of 1.6 mm, the car finally stopped at 36.2 meters.
The differences were greater still when braking from 120 km/h. On brand new Continental tires, the Golf stopped after 59.1 meters. With 3 mm of tread remaining, the braking distance was 124 meters and with the minimum legal tread, it took all of 154.9 meters to stop the car.
To be perfectly clear: This means that where a car with new tires comes to a safe halt just short of an obstacle, a car with only 1.6 mm of tread left on its tires strikes the obstacle virtually unbraked, still traveling at 106 km/h.
The test drivers’ conclusion was unequivocal: Only new tires offer the greatest reserves of safety in the wet. Running tires down to 3 mm of tread is acceptable. But with the legal minimum tread depth of 1.6 mm, things get dangerous.
The findings were published by AutoBild on August 22, 2019.