Michelin - Talking technical on the asphalt tyre

TALKING TECHNICAL -- THE MICHELIN N 'FP' ASPHALT TYRE Delivering high performance-- consistently Competing on dry asphalt places the focus very firmly on the outright performance potential of tyres. But at the same time, the drivers call for ...

Delivering high performance-- consistently

Competing on dry asphalt places the focus very firmly on the outright performance potential of tyres. But at the same time, the drivers call for tyres that are capable of giving their very best from stage start to stage finish. In other words, not only do they demand high performance, but they also want consistent delivery of that performance. This is indeed one of the keys to the success of Michelin's current generation WRC asphalt range which has won 28 of the 30 world class rallies contested on this type of surface in the course of the past seven years*. The Michelin N FP's most recent success was on last week's Rallye de France - Tour de Corse which Sebastien Loeb won for the first time. The same tyre is expected to play a similarly dominant role in Spain.

Used for the first time in anger on the Italian Riviera stages of the Sanremo Rally in October 1998, the Michelin N FP generation of tyres ('FP' for 'Fort Potentiel') revolutionised competition on dry asphalt, a type of surface on which tyres are exposed to enormous constraints, including forces of up to 1.8G under braking and extremely high cornering speeds with lateral forces of up to 1.5G.

Even so, at the time of its launch, the new Michelin N FP provided so much grip that the chassis engineers of the French firm's partner teams were forced to up -rate the rigidity of their cars by 20%. For the first time in rallying history, it was necessary to adapt the cars to the tyres and not the other way round!

At the same time, the tyres in the new dry asphalt range featured, and continue to feature outstanding consistency in the delivery of their performance potential. This characteristic of the Michelin N FP is a fitting reward for the tireless groundwork and research put in by Michelin's chemists and engineers .


In compliance with regulations introduced by the FIA in 1995, the dimensions of today's WRC asphalt tyres are somewhat undersized in light of the fact that they are asked to withstand the constraints generated by engines of more than 300hp powering cars of more than 1 tonne at high speed over hostile terrains, including jumps, broken-up asphalt and corner- cutting, etc.. As a result of these constraints, combined with the inevitable rise in the temperature of the covers and the presence inside them of ATS mousse, the air pressure inside rally tyres climbs significantly and this in turn leads to a reduction in the size of the contact patch with the ground as the driver advances in the stage.

This phenomenon is unique to the world of rallying. In Formula 1, for example, the dimensions of the tyres are more suited to the characteristics of the cars. Indeed, the variation in air pressure inside a F1 tyre is minimal which effectively ensures that the contact patch remains identical lap after lap.

In the course of a rally stage, however, the area of the contact patch of all four tyres falls by some 20%. To grasp the importance of this, consider that the contact patch of each tyre is approximately 100 sq. cm, which we will say is equivalent to 100 million molecules of rubber in direct contact with the asphalt.

The grip produced by the tyre is dependent on two cumulative phenomena: indentation, the result of micro-irregularities in the road surface, and adhesion. Indentation is the result of interaction between the particles of bitumen and the particles of rubber, a process which accounts for 70% of tyres' total grip and which is measured in millimetres.The remaining 30% are the result of adhesion which is the product of electro-chemical bonding between the asphalt and elastomer molecules. The unit of measurement in this case is angstroms.

A drop-off in the molecular bonding

Some 15km after the start of the stage, and on top of the increase in the temperature of the rubber molecules, there are 20 million less of the latter serving as a bonding agent and this means a smaller contact patch. The 80 million molecules still in contact with the asphalt are not only hotter but they must also produce a bigger effort in order to make up for the loss of molecular bonding and ensure the consistency of the tyres' performance .

Michelin has consequently sought to delay this variation in the size of the contact patch and the drop-off of molecular bonding as long as possible. In 1996, Michelin's engineers introduced an air pressure control system but this solution was immediately outlawed by the FIA. Today, no means of artificially controlling variations in air pressure are permitted. Meanwhile, in rallying, the positive benefits of nitrogen - an inert gas employed increasingly in road tyres - would be cancelled out by the lubricant that serves to ease the fitting of the ATS mousse inside the covers. Drivers and co-drivers are however authorised to adjust the pressure of their tyres between stages.

Fortified molecules

The chemists at Michelin have consequently worked on the molecules of the compound themselves with a view to making them more versatile. In addition to the grip they offer, these molecules become 'fortified' between 15and 30 km into the stage in order to supply an even bigger effort. The way they do this is obviously a very closely guarded secret!

On damp road surfaces, the situation is different. The tyres are submitted to fewer constraints, the temperature of the rubber does not climb so high, nor so quickly, and the size of the contact patch between the tyre and the ground is more constant. The tyres do not degrade so much which means that delivering consistent performance is no longer as problematic. Accordingly, rain tends to level out the importance of the car/tyre package and places the focus more on driver skill.


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Series WRC
Drivers Sébastien Loeb