Friction measurement methods and the correlation between road - VTI

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2 Friction vocabularyA brief description of the friction related terminology used in this report.FrictionThe resistance an object encounters in moving over another object. Often the forceneeded to move the object, the frictional force.Friction value, friction numberThe numerical value of the friction given by a specific measuring device. Forsome devices it corresponds to the friction coefficient or the friction coefficientmultiplied by 100. Important to notice is that a certain friction value is alwaysconnected with a specific measuring device, a specific measuring tyre and specificoperating conditions.Friction coefficientNormalised friction. The frictional force divided by the normal force (the load).SkidSliding on slippery ground. In road circumstances skidding is the sliding of thelocked wheel on the pavement.Skid resistanceThe resistance to skidding or the friction for locked wheel tests. Also used todescribe results from tests using the SRT pendulum, se figure 3.11. According toISO 8855 the longitudinal friction coefficient obtained on a locked wheel is calledsliding braking force coefficient.Skid numberThe friction value as measured according to ASTM 274 which describes thefriction measurement using a locked, smooth or ribbed standard test tyre. The skidnumber is the measured friction coefficient multiplied by 100. It is designated asSN test speed R for ribbed tyre (ASTM E501) and SN test speed S for smooth tyre(ASTM E524), e.g. SN65S = 55. The test speed is normally 65 km/h (40 mph).Slip speedThe relative speed between the tyre and the travelled surface at the centre of thecontact area.Longitudinal slip, longitudinal slip ratioThe quotient of the slip speed by the operating speed.Slip angleThe angle between the wheel and direction of travel of the centre of tyre contact.Slip resistanceThe resistance to slip (to lose one’s footing) for pedestrian surfaces. Can be thefriction coefficient of that surface measured with special slip resistance devices,the British pendulum or the VTI PFT.8 VTI meddelande 911A
3 Road friction measurementFriction is an important road parameter that unfortunately is very difficult tomeasure. The devices used for friction measurement are not very complicated butthe friction forces they try to measure are very sensitive to a number of parametersthat are difficult to control. Meyer et al. [32] have summarised investigations ofseveral influencing parameters on the locked wheel friction value, frictionmeasurements according to the ASTM 1 E274. In the UK the SCRIM (Side forceCoefficient Road Inventory Machine) is used and the factors influencing theSCRIM friction value has been reported in a series of investigations [24-26]. It isnot surprising to have a difference in friction of about 5% between twoconsecutive measurements of the same road surface, using the same device. Thatdifference may increase when using another friction device or with increasingtime between measurements.In a friction measurement there are often three bodies involved, the measuringtyre, the road surface and some kind of contaminant interacting with both tyre androad like for example water (wet friction), dust or wear particles etc. The frictionvalues measured depend to a great extent on all three bodies, their materialproperties, the local contact pressures, relative velocities etc. A summary of theimportant factors influencing the road surface friction is given in table 3.1.Table 3.1 Factors influencing road surface friction (extracted from Kummer[29] and Sandberg [46]).Road Contaminant (fluid) TyreMacrotexture Chemical structure Tread pattern designMicrotexture Viscosity Rubber compositionUnevenness/Megatexture Density Inflation pressureChemistry of materials Temperature Rubber hardnessTemperature Thermal conductivity LoadThermal conductivity Specific heat Sliding velocitySpecific heat Film thickness TemperatureThermal conductivitySpecific heatOf the road parameters, the texture is the most important and the influence ofmacrotexture has also gained most interest in road friction research. A very goodoverview of road surface texture measurements can be found in a report bySandberg [46]. The differences between micro, macro and megatexture is definedin table 3.2, from Sandberg [46] and ISO 13473-1.1 American Society for Testing and MaterialsVTI meddelande 911A 9
Page 1 and 2: VTI meddelande 911A • 2001Frictio
Page 3 and 4: Publisher:Publication:VTI meddeland
Page 5 and 6: ContentsAcknowledgementsSummary 51
Page 7: 1 IntroductionRoad friction, its me
Page 11 and 12: 0,70,60,50,40,30,20,10Dense asphalt
Page 13 and 14: 3.1.1 Friction measurement devicesW
Page 15 and 16: BV11 is a trailer with the water ta
Page 17 and 18: Figure 3.7 The VTI Saab Friction Te
Page 19 and 20: Some devices use a fixed slip, like
Page 21 and 22: In Sweden mainly three friction mea
Page 23 and 24: 10 cmFigure 3.12VTI Portable Fricti
Page 25 and 26: The concept of international fricti
Page 27 and 28: Öberg [60] has compiled measuremen
Page 29 and 30: homogeneity of the surface conditio
Page 31 and 32: Figure 4.5 Accident rate (mean and
Page 33 and 34: Figure 4.7 Histograms of locked whe
Page 35 and 36: RoadwayappearanceDriverbehaviourAcc
Page 37 and 38: A number of surveys of friction num
Page 39 and 40: Figure 4.14 Multiplier for locked-w
Page 41 and 42: Table 4.2 Accident rates at differe
Page 43 and 44: 5 References1. Instructions for usi
Page 45 and 46: slitlagerbeläggning - inverkan på
Page 47: 50. Sävenhed H: Relation Between W

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