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14/05/2019
It's all about thickness

Tread

 

"Safe when new, safe when used" some say. "The thicker the safer", according to others. 

Paolo Ferrini

"Safe when new, safe when used" some say. "The thicker the safer", according to others. On the subject of minimum tread depth compatible with safety, the main tire manufacturers have for decades expressed their official position, shared through the "recommendations" published by their European technical body, Etrto (European tire and rim technical organisation), where it is specified: "In addition to the features of the pattern itself, tire construction, the chemical composition of the tread, road surface, weather conditions, mechanical features of the car, driving style and particularly speed, are all important factors affecting tire/road adhesion. Tread depth is not the principal factor in tire adhesion. These factors (…) make it practically impossible to define precisely the minimum tread depth compatible with safety (…) for all types of tires. 

Michelin started making waves already back in 2016 at the Paris Motor Show reminding everyone that "those who insist that tires be replaced when the tread pattern reaches 3 or 4 mm, should remember that the current legal limit of 1.6 mm perfectly meets the needs of modern mobility". And all hell broke loose!

"Our company produces and markets tires that offer not only a high and consistent level of safety, but also significant fuel savings, thanks to their low rolling resistance and long life cycle," they say at Michelin. "This limits the use of raw materials, reduces CO2 emissions and makes it possible to use tires longer and in complete safety. Some of our winter tires are approved for use in countries where these tires are mandatory and offer very high performance down to the last mile, i.e. up to a tread wear level of 1.6 mm. That's why we are against changing the current regulations on tread depth limit and for three good reasons: safety, cost, environmental impact”.

In this context, still according to Michelin, “new tires may also greatly differ in grip and performance according to brand, manufacturer, model and size. A premium tire with a 1.6 mm tread pattern can still outdo a new or almost new 'budget' tire”.

Continental, on the other hand, maintains a different position. "The thickness of the tread can influence braking distance and consequently the impact speed in the event of an accident. Wearing tires are also prone to aquaplaning and drastically reduce wet braking performance, which is why it is essential to replace worn tires when the tread depth is about to reach the legal limit.

Yokohama is more or less on the same wavelength. "Let's start from the idea that, even if the minimum tread allowed by law is 1.6 mm, we firmly believe it is important to communicate that tires suffer a significant deterioration as the tread wears" says the Italian representatives of the Japanese tire maker. "We need to point out that if we talk about use on a dry surface in summer the matter is not so relevant. Actually, a worn tire could sometimes outperform a new one due to having a larger contact surface on the ground and more stable lower tread blocks, but in wet conditions, with a layer of water on the road, then obviously a deeper tread will guarantee a greater water displacement, which means a reduced risk of losing grip due to aquaplaning. In conclusion, Yokohama, while making it clear that this is by no means an obligation, recommends replacing the tires when they have reached 3.0 mm of residual tread".

CST Tires for its part "recommends changing the tires when the original thickness of the new tread (8 mm) is reduced to: 3 mm for a summer tire and 4 mm in case of a winter/four season product. In fact, besides legal requirements (1.6 mm minimum thickness for all types of tires), ignoring the indicated tread depths would jeopardize (to some extent) safety on snow-clad roads. Therefore, waiting to reach the legal tread limit is always a mistake if we consider our safety and that of others a priority”.

Tread depth has a significant impact on a vehicle’s performance. Even just a few kilometres can take a toll on our tires. Current regulations require that the data we find on European labels should be the result of tests carried out on new tires only. It is sufficient to compare different labels to see that the market offers tires with considerably different performance levels. However, even though several tires can boast the same characteristics when new, they can change dramatically when worn. In reality, all tires on the road are worn to some degree.

As tires wear out, some of their characteristics improve on dry roads. In particular, braking (up to 10%) and rolling resistance (up to 20%). In fact, in dry conditions, the closer you are to the legal tread limit (1.6 mm), safety and fuel consumption improve as a result of less rolling resistance. On the other hand, on wet roads, braking is the main safety-related feature that tends to deteriorate. "It is clear that nowadays, there is a widespread awareness of the role that tires play in the safety of road users," says Simone Miatton, president and CEO of Michelin Italy. "Unfortunately, however, there is no information available to the driver about the performance and safety of his tires when these are worn, a condition we all have to face sooner or later. Michelin therefore launched the idea of tests to be carried out on worn tires. Providing correct data and information to all drivers is vital if we want to promote a safer and better informed mobility.

 

 

EUROPEAN REGULATIONS

The European regulation, according to Directive 89/459/EEC, defines at 1.6 mm the minimum legal tread depth allowed on a vehicles belonging to the following international categories M1 (cars), N1 (vans) and O1/O2 (trailers). In our Highway Code (Legislative Decree no. 285 dated 30 April 1992) this limit has been extended to heavier vehicles such as busses, trucks and trailers (cat. M2,M3,N2,N3,O3,O4).

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