Road surface estimation

A traction control method for a vehicle having a pair of wheels driven through a differential, the wheels traveling over a road surface having a coefficient characteristic where a tractive force, related to the amount of slip between each wheel and the road surface, exists between each wheel and the road surface, the method comprising the steps of:

• measuring an amount of slip between each of the driven wheels and the road surface;
• measuring wheel acceleration of each driven wheel;
• measuring vehicle acceleration;
• determining, for each driven wheel, a coefficient of friction between the wheel and the road surface as a predetermined function of (i) the measured vehicle acceleration and (ii) the measured acceleration of the wheel decreased by the measured acceleration of the other driven wheel of the driven wheel pair;
• determining, for each driven wheel, a critical value of slip where the rate of change in the ratio of the determined coefficient of friction to the measured wheel slip is minimum, the critical value of slip corresponding to the value of measured wheel slip where the tractive force is maximized;
• detecting, for each driven wheel, an incipient spin condition where the measured wheel slip is greater in magnitude than the determined critical slip value;
• and
• limiting, for each driven wheel for which an incipient spin condition has been detected, the speed of the wheel to limit the slip of the wheel to the determined critical slip value.