Roger Sanders

Looking at the tremendous advances in vehicle technology over the last 10 years, you would expect that the role of the tyre would be less important to road safety. In fact, quite the opposite is true, especially in critical conditions such as wet and cold weather. Yes, there have been some tremendous advances in brake and chassis development with ABS, electronic stability programmes and, indeed, tyre construction, compounds and tread patterns. However, none of these can be fully effective unless the tyre makes good contact with the road.

Wet weather performance

The greatest risk in wet weather is aquaplaning, which occurs when the tyre is unable to channel the volume of water away from the area in contact with the road. The pressure of water builds up until it lifts the wheel off the road at which point the driver has little or no control of the car. The pressure build-up is a function of the speed and weight of the vehicle, the depth of water on the road, the depth of grooves in the tyre, and the area of the contact patch.

The depth of grooves in the tyre in the main drainage channels must be at least 1.6mm, but this law was introduced in January 1992. Since then, the speed capability of cars has increased such that even the smallest cars are now capable of 100mph and vehicle mileage on motorways has increased by 31% since 1995*. However, of greater significance is the decrease in ground pressure generated by modern cars as, while vehicle weights have gone up, this has been more than outweighed by increases in tyre size.

The trend in vehicle design over the past five years has shown a rapid move towards more powerful vehicles requiring larger brakes, which in turn require larger rims to accommodate the brakes and wider tyres to transmit the power and improve dry handling. Cosmetic appearance also plays a large part in the trend towards larger tyres. For example, the largest wheel size available as standard on a Ford Escort in 1992 was 15in yet now there are models of Focus with 18in wheels as standard. Larger wheels and tyres reduce ground pressure and increase the risk of aquaplaning.

Tests conducted at MIRA confirm historical data that the tyre’s ability to cope with wet weather conditions diminishes as the tread pattern depth reduces. The chart shows by how much braking distances increase as the tread depth reduces from 3mm to 1.6mm and the potential speed differential for two vehicles braking at exactly the same point.

The increase in the wet stopping distance of a tyre at the legal minimum depth of 1.6mm over a tyre with 3mm can be an astonishing 44m (132ft) from 70mph – equivalent to the length of three full-size articulated lorries. This could easily be the difference between incurring a severe accident or not. From motorway speeds (70mph) a car with 1.6mm of tread will still be travelling at 40-50mph, while a car with 3mm would have stopped. Our statistics come from tests conducted at MIRA with several different vehicles and makes of tyre and the results are consistent.

Matt Watson of Auto Express looks at tyres from a 1992 Ford Escort and a 2006 Focus, revealing how tyre sizes have grown since the 1990s

Notable variations occur only where the vehicle is significantly heavier relative to the area of tyre tread in contact with the road which therefore exerts greater pressure to disperse the water in the contact patch. Similar findings are contained in a report published jointly by the AA Motoring Trust and the County Surveyors’ Society in June 2005 from tests at the TRRL, which concluded that “grip on wet roads is markedly reduced below 3mm” and that “the legal limit of 1.6mm tread depth is the point at which tyres are so bad that they break the law and (motorists) put themselves and other road users at greater risk”.

In conclusion, despite the advances in tyre technology the tread depth of tyres continues to play a critical role in road safety in wet conditions and the latest tyre and vehicle technology has not diminished the need for adequate tread depth. The recent trend towards larger wheels and tyres has exacerbated the problem and motorists should be advised to consider changing their tyres at 3mm and well before they reach the legal minimum of 1.6mm.

Cold weather

In a similar way, the advanced technology of cars can be most effective when tyres suitable for the ambient conditions are fitted. Most people would associate cold weather tyres with snow and ice, but their performance is superior to normal tyres when the temperature falls below 7˚C. In the UK, average monthly temperatures are below 7˚C between October and April and at these temperatures the molecules in tread rubber progressively harden. The tyre is less able to adapt to the contour of the road surface and this reduces grip levels.

With cold weather tyres, rubber compounds remain pliant at low temperatures allowing the tyre to key into the road surface, thus giving higher levels of grip, and with their highly-developed multi sipe tread patterns and pliant compounds no summer tyre can match their stopping distances in cold wet conditions.

<< back to contents page