Friction measurement methods and the correlation between road - VTI

Where FI is the friction index (at 60 km/h slip speed), A, B and C are devicespecific constants, F is the measured friction value with the specific device usingslip speed S in km/h. S p is the predicted so called golden value speed number, Txis a measure of the macrotexture (for example MPD in mm), a and b are constantsdepending on the macrotexture measurement device. For smooth tyres theconstant C is equal to zero [54].Using one friction measurement at one slip speed and a measure of the surfacetexture the friction at any slip speed, for that device, can be estimated. Thisrelation between friction and slip speed is illustrated in figure 3.13 where also themethod of friction index calculation is shown.FrictionFR60⎛⎜⎜⎜⎜⎝S−60S= ⋅pFRSe⎞⎠IFI = A+B*FR60BV11Reference (golden curve)12{17%}60{86%}Slip speed (km/h){slip rate at 70 km/h}Figure 3.13 Demonstration of the calculation of the International FrictionIndexIn the report from the PIARC experiment [54] the friction index was calculatedat a slip speed of 60 km/h, also demonstrated in figure 3.13. This slip speed wasby some researchers considered to be too high and a revised version waspresented, also found in the draft to an European standard (prEN13036), using areference slip speed of 30 km/h:FI=A + B * F * e⎛ ⎞⎜S −30⎟⎜ ⎟⎝ S p ⎠S p= 57 + 56* MPDWhere FI is the friction index (now at 30 km/h slip speed), A and B are devicespecific constants, F is the measured friction value with the specific device usingslip speed S in km/h. S p is the predicted so called golden value speed numbercalculated from the macrotexture given as the MPD (Mean Profile Depth) value.24 VTI meddelande 911A