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112 ELECTRIC/ELECTRONIC SYSTEM D AILY Base - May 2004 The braking of a vehicle in motion and the consequent deceleration and stopping space mainly depend on the grip between the tyre surface and the type of road surface. improvement of braking can be obtained only acting on the tyre friction characteristics or on the quality of the road surface. is not however guaranteed when needing to cope with particular critical situations, such as low grip due to the conditions of the wet or icy road surface: this compels the driver to moderate the braking action to prevent one or more wheels from partially locking, with the possibility of dangerous skidding. correspond to the friction between rigid bodies but to the skidding (or slipping) between the tyre and the road in the contact area. The figure shows the indicative trend of the longitudinal µl" and transversal µt" rip coefficients in relation to the percentage of slipping "S". The diagram reveals that the maximum value of the longitudinal grip coefficient is not when the wheel is locked, but for a much lower slipping value. Therefore, the road-tyre grip can be exploited to optimise longitudinal or transversal control of the vehicle. Namely, it is possible to utilise the grip trying to make the braking distance as short as possible or to ensure the best possible handling. An excellent compromise is obtained using slip rates near the point S* in which there is a high µl value which ensures optimum braking and a µt value that offers good lateral roadholding. TREND OF WHEEL LONGITUDINAL AND TRANSVERSAL GRIP IN RELATION TO SLIPPING WHEEL VEHICLE TRANSVERSAL GRIP LONGITUDINAL GRIP STABLE AREA OPTIMUM AREA UNSTABLE AREA Figure 111 |