New tyre test: Replace summer tyres when tread depth is below three millimetres
Munich. Driving properties of summer tyres deteriorate rapidly when tread depth falls below three millimetres, according to the Swiss car magazine auto-illustrierte. The publication joined forces with the experts from safety partner TÜV SÜD to conduct extensive tyre tests in Mireval in southern France. Their conclusion? “When the tread depth reaches three millimetres, consider replacing your tyres”, says Thomas Salzinger, head of TÜV SÜD’s Tyres team.
Worn tyres, incorrect tyre sizes, non-approved rims: in annual roadworthiness tests, almost two million vehicles were submitted with wheel and tyre faults in the first six months of 2013 alone. This prompted auto-illustrierte to collaborate with TÜV SÜD over providing hard facts on wheel-related topics and alerting drivers to the role of tyres in road safety. Wet driving characteristics were a special focus of the test. The results clearly showed that braking performance, water displacement, traction, grip and lateral stability all plummet at treads of less than three millimetres. In wet braking, the braking distance from 80 kilometres down to 20 was measured at 33.2 metres for a tread depth of 2.6 millimetres, compared to 28.6 metres for new tyres. Salzinger, who directed the tests, comments, “When a car with new tyres has already come to a stop, a vehicle with worn tyres will still be travelling at around 30 kilometres per hour and has almost five metres more of braking distance.”
Getting a grip on aquaplaning
The tests were carried out using summer tyres of the brand Dunlop Sport Maxx RT. All tyres had the same production date but different tread depths, comprising new tyres and tyres worn down to 4.4 and 2.6 millimetres. The car used was a 230-hp VW Golf GTI. The test involved wet and dry braking, straight aquaplaning at a water depth of seven millimetres, circular track, wet and dry handling, and noise measurements in the vehicle interior. The location was the Goodyear/Dunlop test facility at Mireval in southern France. The tests were conducted and accompanied by TÜV SÜD experts serving as independent, impartial partners.
The worn tyres showed glaring weaknesses, particularly in wet conditions. Further critical areas in addition to their longer braking distances were their poorer handling overall – when fitted with new tyres, the GTI completed the soaked circular track four seconds and four-and-a-half kilometres per hour faster – and the increased danger of aquaplaning. The latter test highlighted the strength of new tyres, which were able to tackle the flooded aquaplaning test strip at an average of twelve kilometres per hour faster without ‘floating’ than tyres with a tread depth of 2.6 millimetres. Tyre expert Salzinger concludes, “Even tyres with 2.6 millimetres of tread come to the end of their safety reserves in wet conditions. Drivers should thus never wait until they reach the legal minimum tread depth of 1.6 millimetres, but should preferably make an appointment to have their tyres replaced as soon as the tread reaches three millimetres.”
At Garching near Munich, TÜV SÜD operates the largest independent tyre and wheel laboratory of its kind in Europe. The specialists are long-standing expert partners to the tyre and automotive industry on all matters concerning tyres and wheels.
Detailed results of the tyre tests can be found in the August issue of auto-illustrierte and at www.auto-illustrierte.ch .