Measuring Tyre Performance in Esc Intervention Situations

Fatal accidents during winter time in Sweden are to a large extent due to vehicle spinning. Therefore, it is crucial to analyse and compare vehicle/tyre performance on slippery roads with respect to oversteering. We have performed experimental tests to compare the stability performance of various winter tyres on ice, with and without an electronic stability system (ESC). Two different test methods are presented, one closed loop and one open loop test. Results from both studies are discussed, and it is clear that different tyres may have a large impact on vehicle stability even with ESC activated. 1. Backgr ound Active safety systems are becoming increasingly common in today’s vehicles. Electronic Stability Control (ESC) systems were introduced during the end of the 1990s, and accident statistics show that they have had a huge impact on traffic safety. Early tests on track have indicated that ESC systems have a great potential at reducing accidents where the driver suddenly lose control over the vehicle (Yamamoto and Kimura 1996). Many studies based on accident statistics confirm that ESC systems have been very effective in reducing the number of single accidents among passenger cars and SUVs. It is estimated that the introduction of ESC has decreased the number of fatal single accidents with 30-50% for passenger cars and between 50-70% for SUVS [1]. Some studies have also found that ESC systems are increasingly effective for wet or icy road conditions [2-4]. The ESC system aims at helping the driver to maintain vehicle stability. This means that the vehicle should follow the drivers intended path without spinning out (over steering) or ploughing straight ahead (under steering). Research has shown that most drivers have problem controlling the vehicle at the limit of its handling capability. This is discussed in [5]. The main design philosophy for the ESC system is that the system should help the driver to keep the vehicle controllable i.e. to avoid excessive vehicle side slip angles. This is achieved by using individual wheel brakes to control the vehicles yaw motion. Since the ESC system is dependent of sufficient available braking forces there is a risk that its performance is strongly coupled to the tyre properties on surfaces with low friction, like ice and snow. Moreover, it is on these surfaces that the difference in road grip is the largest for different types of tyres. In Sweden the tyre manufacturers distinguish between three different kinds of winter tyres. Studded tyres have always been the tyre offering the least compromises for Nordic conditions with respect to traffic safety, and in general offer very good grip in winter road conditions, paired with good grip on bare roads. If one wants to drive with an unstudded winter tyre there is a choice between those constructed for Nordic conditions, which offer good winter road grip, while the bare road performance is worse, or the unstudded winter tyre of central European type, which in general are worse on ice and snow, but offers very good bare road performance. VTI has conducted two separate studies comparing tyre performance and ESC intervention: one closed loop and one open loop test. The first was a double lane change manoeuvre with 6 different drivers, comparing the performance of 4 different winter tires on ice. The latter study used predefined steering manoeuvres with a steering robot in order to obtain as much oversteering as possible in different icy conditions. This paper will briefly describe the methodology that was used and the main results. 2. Double lane change test For a closed loop test, which involves the action of a driver, we chose the modified ISO 3888-2 manoeuvre [6] as a starting point. This manoeuvre consists of a double lane change. By driving through the manoeuvre with