Although the rolling resistance of a tyre is becoming an increasingly important selection criterion for operating cost reasons, grip remains the most important criterion for buyers. In European tyre labelling, this safety aspect concentrates on wet grip.
A criteria among others
Of the three criteria adopted for European tyre labelling, wet grip corresponds to the safety aspect. For the consumer, this criterion indicates the tyre’s ability to stop a vehicle in low grip conditions. Labelling, of course, is not exhaustive and many other criteria need to be taken into account to evaluate how a tyre handles: Dry braking, response when entering corners… Premium tyre manufacturers incorporate all these elements in the tyredesign but labelling settles for a value that the buyer can easily understand.
Comparison with the benchmark
Unlike rolling resistance, which is measured in a laboratory, wet grip is subject to a track test. Tyres are fitted on a vehicle (like in normal use) or on a trailer to simulate braking. The driving test is performed on a standard road surface covered with a 1 mm water film, with a tolerance of 0.5mm. The standard also requires compliance with an outside temperature depending on the type of tyre: between 5 and 35°C for summer tyres and between 2 and 20°C for winter tyres.
A braking distance is then measured to decelerate from 40 to 10 mph, with ABS, or a grip coefficient at a continuous speed for a simulation trailer.
The grade marked on the label does not directly reflect the test result. The tested tyre is in fact compared with a benchmark tyre in the same conditions, in order to eliminate any differences between tyre manufacturers in the choice of test track, weather conditions… European tyre labelling therefore reflects the performance of a tyre in relation to a benchmark tyre. For trucks, the grade ranges from A to F, where A is the highest rating and F is the lowest rating, G is not used. However, unlike for passenger car tyres and van tyres, the D grade is used.
Truck tyres are separated into two categories, depending on whether their width is greater than or strictly less than 285 mm. There is therefore a benchmark tyre for each family.
At 50 mph, between a whole rig fully fitted with A-rated tyres and another with F-rated tyres, the braking distance to bring the vehicle to a complete stop varies by about 38 metres.
The tyre’s braking ability in wet conditions depends on several factors, starting with the tread pattern. Grooves are designed to allow water toflow through them and improve water drainage, when driving and braking. When tyre manufacturers test their own tyres, they do not take the distance travelled before stopping into account. They consider the application of brakes and the tyre’s response throughout the deceleration, with its drift and consistency.
By Renaud Lacroix