Analysis of tire wet grip mechanisms and their respective weightings in a wet braking test

Abstract

Car drivers constantly demand a higher level of road safety, whether in terms of road infrastructure, vehicle and/or tires. They also ask for more information about the level of safety they have when driving. To meet this need, the tire label – which has existed since 2012 in Europe – gives them information about the tire’s wet grip performance as well as its noise and fuel consumption impact. However, this information is worth for a new tire, and it is known that wet grip performance is one of the few tire performances that reduces as a tire wears.

In this study, the analysis of wet grip mechanisms is conducted through an innovative method, which is based on a detailed analysis of the regulatory wet braking test. This method enables a decomposition of the respective weighting of each mechanism, both for new and worn tires. It has been observed that there are two main mechanisms of wet grip: rubber friction and hydroplaning. While rubber friction enables the generation of a braking or steering force by the tire through the rubber-road contact, hydroplaning reduces its efficiency by diminishing the contact surface area.

This study demonstrates that the relative importance of the involved mechanisms depends on the tire state: while rubber grip is the predominant mechanism of a new tire, both hydroplaning and rubber friction are important for worn tires. This decomposition study also shows that the reduction of wet grip from a new to a worn tire is tire dependent as decrease may be linked to rubber friction, or to a reduced hydroplaning capability.