Grip Grip Société de Technologie Michelin 23, rue Breschet, 63000 Clermont-Ferrand © Société de Technologie Michelin, 2001 Produced by Artice / Japa - Photographs from Michelin picture library Made in France Reproduction, representation, adaptation or translation of any part of this work without the permission of the copyright owner is unlawful. Requests for permission or further information should be addressed to: Manufacture Française des Pneumatiques Michelin Service Groupe Communication / Technique Place des Carmes Déchaux, 63040 Clermont-Ferrand Cedex 09 Contents Grip on road surfaces 4 Introduction 5 Foreword: grip and its double paradox 21 II How road roughness 7 I Rubber and grip affects grip 8 I.1 RUBBER: A VISCO-ELASTIC MATERIAL 22 II.1 CHARACTERISATION OF ROAD SURFACES 8 I What is a visco-elastic material? 23 I Measurement of macroroughness 8 A little more information on… the behaviour of elastic materials 23 I Measurement of microroughness 9 A little more information on… the behaviour of viscous materials 24 G Concepts regarding road surfaces 10 A little more information on… the behaviour of visco-elastic materials 25 I Measurement of the load bearing surface 11 I Where does the visco-elasticity of tyre rubber come from? 26 I Characterisation of the friction coefficient of a rubber-road interface 12 I The modulus of rubber 27 II.2 INFLUENCE OF ROAD SURFACES ON THE 13 I.2 INFLUENCE OF STRESS FREQUENCY AND COEFFICIENT OF FRICTION TEMPERATURE ON THE BEHAVIOUR OF RUBBER 28 I Variation in friction coefficient on a dry surface 13 I Influence of stress frequency 29 I Variation in friction coefficient on damp or wet road surfaces 14 I Influence of temperature 30 I Relative importance of the rubber factor and the road surface 15 I Frequency - temperature equivalence factor in grip 16 A little more information on… the WLF equation 30 I What about snow? 30 I And what about ice? 17 I.3 THE MECHANISMS INVOLVED IN THE RUBBER-ROAD 31 How road roughness affects grip: don’t forget the basics! INTERFACE FRICTION 17 I Road roughness effects 18 I Molecular adhesion 19 Rubber and grip: don’t forget the basics! Page 1 48 I Transversal friction force 33 III Generation of grip forces in 48 I Coefficient of transversal friction τ the contact patch 48 A little more information on... the coefficient of transversal friction 49 I Effect of the bend radius on the maximum cornering speed 49 A little more information on... maximum cornering speed 34 III.1 FRICTION MECHANISMS OF A RUBBER BLOCK 50 I Law of transversal friction Y(δ) 34 I Shear (or pseudo-slippage) 50 A little more information on... the transversal grip coefficient 35 I Slippage 51 I Generation of transversal forces in the contact patch 52 I Analysis of the Y(δ) law 52 A little more information on... the maximum length of shear 36 III.2 LONGITUDINAL GRIP IN BRAKING and slippage 36 I Source of slippage 37 A little more information on... slippage and braking 54 Transversal grip in cornering: don’t forget the basics! 38 I Longitudinal friction force 38 I Longitudinal friction coefficient µ 55 III.4 SHARING THE GRIP POTENTIAL ON THE ROAD 38 A little more information on... the coefficient of longitudinal friction 55 I Combined grip 39 I The longitudinal friction law µ(G) 55 I A potential to be shared 40 I Generation of braking forces in the contact patch 57 I ABS 40 A little more information on... the maximum shear value of the tread block and the beginning of slippage 57 A little more information on... how the ABS works 42 I Examples of slippage and shear as a function of the coefficient of grip and the slippage rate 59 42 A little more information on... maximum length of shear and IV Grip on wet surfaces slippage 43 I Analysis of the µ(G) curve 61 IV.1 THE HYDRODYNAMIC ZONE: DISPERSAL 45 Longitudinal grip in braking: don’t forget the basics! AND DRAINAGE 61 A little more information on... the speed at which aquaplaning occurs 62 I A rounded footprint to reduce the pressure exerted by the bank 46 III.3 TRANSVERSAL GRIP IN CORNERING of water on the tyre 46 I Centrifugal force 62 A little more information on... the rounded contact patch which increases aquaplaning speed 46 I Slip angle Page 2 Contents 63 G Wide tyres and water dispersal 77 VI Testing tyre grip 64 I Angled tread grooves to drain away water to the side 79 VI.1 ANALYTICAL TESTS 65 IV.2 THE VISCODYNAMIC ZONE: STORING WATER IN 79 I Road simulators THE TREAD GROOVES 80 I Laboratory vehicles 65 I Compression of the water by the tread blocks 65 A little more information on... the time for water transfer to the storage zones 81 VI.2 VEHICLE TRACK TESTS 67 G Sipes and grip on wet surfaces 82 I Testing longitudinal grip 83 A little more information on... calculating the coefficient of grip µ 84 I Transversal grip tests 69 IV.3 DAMP TO DRY ZONE: RESTORING DRY CONTACT 69 I Edges to break through the film of water 70 Grip on wet surfaces: don’t forget the basics! 87 VII Grip and rolling resistance 88 I Where does rolling resistance come from? 71 88 I Maximise grip and minimise rolling resistance: a challenge in V Grip and vehicle handling physical science 90 I Two different frequency ranges 72 V. 1 LOAD TRANSFER 72 I Longitudinal load transfer 72 A little more information on... how load transfer affects braking A, B, C… efficiency 73 I Lateral load transfer 91 Index 73 A little more information on... how load transfer affects cornering 74 V. 2 UNDERSTEER AND OVERSTEER Page 3 Grip on road surfaces If there were no such thing as grip, cars just The tyre as a vital link would not be able to move at all. The wheels in the grip system would spin and the driver would not be able to budge the vehicle. Even on a straight road and Pneumatic tyres for automobiles began to be at steady speed, there is no alternative manufactured away back in 1895. They very quickly to grip. This is because a moving vehicle replaced the solid tyre, which inflicted increasingly has to deal with natural forces, such as severe punishment on vehicle mechanics and was a the banking, the slope or the unevenness source of discomfort for passengers as the drive of the road, or rolling resistance, which power and speeds became greater. Greater are constantly trying to slow the vehicle comfort however was not the only improvement, down or push it off its path. since the grip ensured by pneumatic tyres also However it is only during cornering proved to be vastly superior to that of solid tyres. or braking that a driver or passenger is really Part of the kinetic energy developed by a vehicle aware of grip, because the vehicle has to be has to be absorbed by the suspension system, steered or speed has to be reduced without the brakes and the tyres during cornering skidding, even on a wet road. In all circumstances, and braking. Where the car meets the road, there grip and safety go together. As the only contact are only the vehicle’s tyres to ensure the ultimate point between the vehicle and the road, the tyre contact patch. The mechanics of grip are to be ensures two fundamental functions. It gives explained by the astonishing visco-elastic properties the vehicle its directional stability, which of the tyre’s rubber which within the contact patch the driver needs to steer it. The tyre acts produce a host of physical phenomena that strive as a transmission component for brake to counteract any untimely skidding over the road and drive torque. surface. Page 4 Grip Grip and its double paradox Two paradoxes are contained in the ability of the Furthermore, even though the flattening of tyre to move and yet to grip at the same time. the contact area constantly produces micro- movements between the tread blocks and the road surface, the contact area does not move – it changes, 2.Vvehicle Motionless yet moving! Path described as one contact area continuously replaces by a point on the previous one. The tyre contact patch of a vehicle travelling the wheel It is only when the vehicle brakes, accelerates or at constant speed does not move in relation to the Vvehicle corners that the contact area and the road begins road surface! to move in relation to each other: this relative In order to fully apprehend this first paradox, movement is known as slippage. it must be remembered that before the wheel In contact with the road surface was invented, men used sleighs and the load V = 0 instantaneous horizontal speed is zero Slippage means no skidding! The above two features apply to any wheel, Slippage in the contact patch is produced when although the pneumatic tyre has transformed braking, acceleration or cornering occurs. the old wooden wheel or the metal one, because it Here lies the second paradox, which is every deflects and so flattens out on the road surface. bit as surprising as the first: a tyre slips in order Traction force There is no longer just a contact line across not to skid! Resistance to forward movement the width of the tyre since the tyre spreads out into a contact patch where the tread blocks Generating grip involves generating friction forces was dragged along the ground. There was a great are laid down and lifted off like caterpillar tracks.