Tire Friction During Locked Wheel Braking

Accurate values of tire-roadway friction are an essential requirement for an accurate collision reconstruction. This paper presents updated tire friction data for three grades (economy, touring, and performance) of commercially-available tires under both wet and dry road conditions. Differences between tires and road conditions were tested using 540 locked wheel braking tests with a single passenger vehicle on a single road surface over six consecutive days. The vehicle was braked from about 60 km/h to a stop using a mechanical brake actuator to minimize variations in brake pedal application. These results showed differences between the friction measured with economy, touring and performance tires under wet and dry road conditions. Dry road friction values were higher than those reported previously in the literature using older model tires and these dry road friction values were normally distributed. These data also showed that vehicle speed calculated using skid distance, even with improved estimates of skidding friction, underestimated pre-braking speed. To account for the pre-skid braking, adjustments to both the pre-skidding vehicle speed and onset of the driver's perception/response times appear to be warranted.