Tyred out ... Stopping in the wet.

Written by Auto Express
Posted in Technical Articles

Tyred Out! - For most drivers, a tyres life is over when the tread depth reaches the 1.6mm legal minimum. Thats great if you want to avoid a fine, but seriously flawed for preventing accidents.

Studies by the Motor Industry Research Association showed that in wet braking and cornering, the critical tread depth is around 3.5mm. From new until this point, a tyre has lost around 4mm of tread and less than 2% of it's wet braking ability. But for the next 2mm of wear they found that performance deteriorated by up to 35%. In real terms, the 50mph to 0 stopping distance increased from 23.8m with the full 8mm of tread to 24.2m with only 4mm of tread. At the legal minimum of 1.6mm of tread it increased to 36.5m - a full 9m further than new tyres. That's two car lengths further - easily the difference between crashing or coming safely to a stop. The story is even more worrying on a concrete surface as you find on some motorways where the speeds are even higher than the 50mph brake test. With new tyres it took 5m further to stop than on tarmac and with barely legal tyres it took a massive 13m extra - about the length of a 40 tonne lorry!!

In order to test these results AutoExpress Magazine together with Roadsafe, took 4 Rover 75's and kitted them out with tyres with 8, 4, 3 and 1.6mm. The speed was set to 50mph and done on a soaked tarmac surface. They also did a test to see how cornering in the wet was affected by tyre wear.

In the braking test the results were equally disturbing.

As expected, the car fitted with new tyres performed best with the tyres being effective as soon as the brakes were applied. They produced an overall stopping distance of 26.31m.

With 4mm of tread still on the tyres there was a period where the ABS was activated so the car took nearly 6m longer than with new tyres to come to a stop with a total of 32.75m.

The results were just as dramatic on the wet cornering test with the difference between new and barely legal tyres being huge. The slightest accelerator input with the 1.6mm tyres pushed the nose wide and required much more steering input to maintain the line. Enter a corner or roundabout too fast and the chances are you would not recover control before hitting something you'd sooner avoid.

In conclusion, it is crucial to keep a close eye on the tread depth on your vehicles tyres and consider changing them not as late as the bare minimum of 1.6mm but at 3mm as recommended by most road safety experts. It clearly makes a huge difference to how quickly you could stop in a hurry.