Guide for Pavement Friction (2009) / Chapter Skim
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E-1 INTRODUCTION This primer is based in part on a 1999 report written for the Joint Winter Runway Friction Measurement Program and the American Society for Testing and Materials (ASTM) (Andresen and Wambold)
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E-2 The Friction Measurement Devices A number of different types of devices have been invented and deployed at different highway agencies to provide information about the road surface frictional characteristics. Few devices have been designed specifically for predicting ground vehicle braking performance.
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E-3 Several other resistive forces can be important to the stopping of a vehicle. Aerodynamic drag and impingement drag are examples of non-frictional stopping forces.
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E-4 Figure E-2 depicts typical compositions of the braking slip friction mechanisms for two different surfaces interacting with the same tire. The pie chart on the left depicts a rigid surface, such as a dry, bare pavement.
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E-5 coefficient of friction, μ. In figure E-3, the perpendicular contact force is the weight of the block, FW.
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E-6 Figure E-4. Two wheels of different size and type on the same surface.
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E-7 Friction as Function of Travel Speed and Slip Speed Figure E-5 illustrates how braking slip friction can vary with travel speed and degree of braking, in terms of slip ratio. This figure suggests that a simplified, universal friction model for tire–surface object pairs can be expressed with a speed variable and a degree of braking called slip speed.
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E-8 There are circumstances in which the friction has negligible influences of traveling speed and degree of braking, but for a universal friction model those circumstances are special cases. To circumvent this, a frequent tactic is to fix the measuring speed and slip speed and compare the device-tire configurations at those speeds.
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E-9 groups of devices are highlighted. Intermittent or spot friction measuring devices are not fully addressed in this primer.
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E-10 A small longitudinal tire slip force in the footprint supports the deflection work. As a result, the normal pressure distribution becomes uneven, such that the resultant normal force (center of pressure)
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E-11 Since the nature of the tire rolling resistance involves slip in the tire–surface contact area, a friction coefficient can be defined as follows: r a F F F F W Wr a W R R = ⋅==μ Eq. E-17 The tire rolling resistance is geometrically defined.
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E-12 A brake moment causes the wheel rotation to slow down and creates a slip resistive force, FB, in the tire–surface contact area. An increased pulling force, FX, is required to uphold the tribometer at a constant speed of travel.
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E-13 Friction Forces from Contaminant Dynamic Planing2 Planing occurs when the fluid3 contaminant material is trapped under the rolling tire in sufficient quantities at a high enough traveling speed to detach some or the entire tire tread from the base surface. Some, or all, of the tire rides on the trapped fluid contaminant, which acts like a lubricant.
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E-14 r Direction of travel ω r ω a b FM Free body diagram, steady state Base surface (pavement, ice, compacted snow) Simplified normal pressure distribution when rolling Fluid cover (water, slush, loose snow)
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E-15 Substituting for FDG and for FLG using equation E-26, and simplifying, RLDM FFr bF r tF −⋅+⋅⋅= 2 Eq. E-29 Since GR Fr aF ⋅= and LWG FFF −= , then ( )
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E-16 predominate. The other is thin film lubrication, where viscous properties of the fluid predominate.
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E-17 when disregarding fluid displacement drag. The tire-rolling resistance coefficient of friction is as follows: G R R F F=μ Eq.
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E-19 Effectively, the fluid gets trapped in the center where the tire is weaker. If it is present in sufficient amounts, it can escape to the sides in texture voids of the surface.
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E-20 It can be argued that an automotive tire will tend to keep the line of attack closer to a vertical plane. Viscosity is a measure of the shear forces that a fluid can sustain when interspersed between opposing surfaces.
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E-21 For viscous planning of a very high pressure tire, the area AD is a constant: twAD ⋅= Eq. E-50 where w is the gross width of the tire footprint and t is the contaminant fluid layer thickness.
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E-22 0 10 20 30 40 50 0 200 400 600 800 1000 Speed, V (m/s)
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E-23 The fluid forces are closely related to tire geometry, tire carcass design, inflation pressure, and weight carried by the wheel. Different tire types exhibit different behaviour with planing.
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E-24 In the derived PIARC model, the zero intercept of this equation is replaced by a constant friction value at an arbitrarily chosen reference slip speed of 37 mi/hr (60 km/hr) and another exponential term.
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E-25 This equation is valid for wet pavement only. It has successfully captured the commonly observed influences of texture and slip speed for a device tire configuration-surface pair.
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E-26 Slip Speed, S (km/h)
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E-27 The PIARC model and its IFI are primarily intended for long-term monitoring of the pavement for budgeting renewal of the surface when polished or worn to unacceptable levels. The Rado model is intended for the prediction of braking performance.
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E-28 Figure E-16. Series of variable-slip measurements with an automotive tire at different measuring speeds on dry concrete pavement [6]
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E-29 Committee C1 on Surface Characteristics decided to conduct an experiment to see whether harmonization could be achieved. The data collected and analyzed enabled an international scale of friction values called IFI to be defined.
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E-30 where S is the slip speed of the measurement and μ(S) device is the measured friction value by the device.
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E-31 μ(60) device μ(60)
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E-32 LIST OF SYMBOLS AND ABBREVIATIONS A Calibration constant for the International Friction Index AD Area in the vertical plane associated with contaminant deposit displacement drag AL Contact area between a tire and a fluid AR Real contact area between a tire and a surface AS Area of shearing contact between a tire and a surface B Calibration constant for the International Friction Index CD Coefficient of displacement drag CV Coefficient of variation Ĉ Shape factor in the Rado friction model (log normal) E A force, or a sum of forces, that constitute an error term in a measured braking slip force F Force FB Force due to braking slip friction FD Force resulting from positive displacement of fluid or plastic material in the frontal area of a tire FDG Reaction force in the tire–surface area due to contaminant displacement drag FE Resultant dynamic contaminant deposit force FG Reaction force from the ground FL Lift force due to dynamic fluid viscous resistance (Petroff's equation)
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E-33 TM The moment measured by a friction measuring device about the wheel axis TX Texture measurement, generic V Travel speed VB Tangential speed of a braked wheel in the tire–surface contact area VC Critical planing speed V0 Speed constant a 1) Horizontal distance between a point of application of the vertical ground reaction force and vertical line through the wheel axis 2)
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E-34 μR Tire-rolling friction coefficient, FR/FW ρ Contaminant fluid or particle mass density σ Normal stress or contact pressure σS Normal stress in the shear area of a tire–surface contact patch τult Ultimate shear stress of a surface material ν Dynamic viscosity ω Angular velocity ωB Angular velocity of a braked wheel ABS Antilock Braking System ASTM ASTM International IFI International Friction Index ISO International Organization for Standardization JBI James Brake Index JWRFMP Joint Winter Runway Friction Measurement Program NASA National Aeronautics and Space Administration PIARC Permanent International Association of Road Congresses (The organization has changed its name to World Road Association (PIARC)
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