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Société de Technologie Michelin
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C o n t e n t s
Grip on road surfaces
4 Introduction
5 Foreword:grip and its double paradox II How road roughness
21
I Rubber and grip affects grip
7
8 I.1 RUBBER:A VISCO-ELASTIC MATERIAL 22 II.1 CHARACTERISATION OF ROAD SURFACES
8 (cid:2) What is a visco-elastic material? 23 (cid:2) Measurement of macroroughness
A little more information on…
8 the behaviour of elastic materials 23 (cid:2) Measurement of microroughness
9 A little more information on…the behaviour of viscous materials 24 (cid:3) Concepts regarding road surfaces
10 A little more information on…the behaviour of visco-elastic materials 25 (cid:2) Measurement of the load bearing surface
11 (cid:2) Where does the visco-elasticity of tyre rubber come from? 26 (cid:2) Characterisation of the friction coefficient
of a rubber-road interface
12 (cid:2) 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 (cid:2) Variation in friction coefficient on a dry surface
13 (cid:2) Influence of stress frequency 29 (cid:2) Variation in friction coefficient on damp or wet road surfaces
14 (cid:2) Influence of temperature 30 (cid:2) Relative importance of the rubber factor and the road surface
15 (cid:2) Frequency - temperature equivalence factor in grip
16 A little more information on…the WLF equation 30 (cid:2) What about snow?
30 (cid:2) 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 (cid:2) Road roughness effects
18 (cid:2) Molecular adhesion
19 Rubber and grip: don’t forget the basics!
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48 (cid:2) Transversal friction force
III Generation of grip forces in
33
48 (cid:2) Coefficient of transversal frictionτ
A little more information on...
the contact patch 48 the coefficient of transversal friction
49 (cid:2) Effect of the bend radius on the maximum corneringspeed
A little more information on...
49 maximum corneringspeed
34 III.1 FRICTION MECHANISMS OF A RUBBER BLOCK 50 (cid:2) Law of transversal friction Y(δ)
A little more information on...
34 (cid:2) Shear (or pseudo-slippage) 50 the transversal grip coefficient
35 (cid:2) Slippage 51 (cid:2) Generation of transversal forces in the contact patch
52 (cid:2) Analysis of the Y(δ) law
A little more information on...
52 the maximum length of shear
36 III.2 LONGITUDINAL GRIP IN BRAKING and slippage
36 (cid:2) Source of slippage
A little more information on... 54 Transversal grip in cornering: don’t forget the basics!
37 slippage and braking
38 (cid:2) Longitudinal friction force
38 (cid:2) Longitudinal friction coefficientµ
55 III.4 SHARING THE GRIP POTENTIAL ON THE ROAD
A little more information on...
38 the coefficient of longitudinal friction
55 (cid:2) Combined grip
39 (cid:2) The longitudinal friction law µ(G)
55 (cid:2) A potential to be shared
40 (cid:2) Generation of braking forces in the contact patch
A little more information on... 57 (cid:2) ABS
40 the maximum shear value of the
A little more information on...
tread block and the beginning of slippage 57 how the ABS works
42 (cid:2) Examples of slippage and shear as a function of the coefficient of
grip and the slippage rate
IV Grip on wet surfaces
A little more information on... 59
42 maximum length of shear and
slippage
43 (cid:2) 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
A little more information on...
61 the speed at which aquaplaning occurs
62 (cid:2) A rounded footprint to reduce the pressure exerted by the bank
46 III.3 TRANSVERSAL GRIP IN CORNERING of water on the tyre
A little more information on...
46 (cid:2) Centrifugal force 62 the rounded contact patch which
increases aquaplaning speed
46 (cid:2) Slip angle
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C o n t e n t s
VI Testing tyre grip
63 (cid:3) Wide tyres and water dispersal 77
64 (cid:2) 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 (cid:2) Road simulators
THE TREAD GROOVES
80 (cid:2) Laboratory vehicles
65 (cid:2) Compression of the water by the tread blocks
A little more information on...
65 the time for water transfer
to the storage zones 81 VI.2 VEHICLE TRACK TESTS
67 (cid:3) Sipes and grip on wet surfaces 82 (cid:2) Testing longitudinal grip
A little more information on...
83 calculating the coefficientof grip µ
84 (cid:2) Transversal grip tests
69 IV.3 DAMP TO DRY ZONE:RESTORING
DRY CONTACT
69 (cid:2) Edges to break through the film of water
VII Grip and rolling resistance
70 Grip on wet surfaces: don’t forget the basics! 87
88 (cid:2) Where does rolling resistance come from?
V Grip and vehicle handling 88 (cid:2) Maximise grip and minimise rolling resistance: a challenge in
71
physical science
90 (cid:2) Two different frequency ranges
72 V.1 LOAD TRANSFER
72 (cid:2) Longitudinal load transfer
72 A little more information on...how load transfer affects braking A, B, C…
efficiency
Index
73 (cid:2) Lateral load transfer 91
A little more information on...
73 how load transfer affects
cornering
74 V.2 UNDERSTEER AND OVERSTEER
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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.
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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.V
Motionless yet moving! vehicle as one contact area continuously replaces
Path described
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
V
at constant speed does not move in relation to the vehicle 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!
the width of the tyre since the tyre spreads out into
Resistance to forward movement
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. which counteract the vehicle’s skidding off
deal of resistance to forward movement unless the the road. However, it must be borne in mind that
sleigh was able to slide easily over snow, ice it is slippage which produces the friction forces
2.V
or wooden rollers. Friction was high because vehicle of grip. In fact there are two forms of relative
the movement over the ground was equal to the movement in the contact patch, micro-movement,
speed at which the mass travelled. commonly known as slippage, which counteracts
macro-movement, commonly known as skidding.
V
The wheel produced a technological revolution: vehicle
- Movement was no longer related to a load being
dragged directly over the ground, but only to an
axle hub.
- At any given time the contact point on the wheel
Ground Flattening in the contact patch
was motionless with respect to the ground, since
V = 0
instantaneous horizontal speed was zero.
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I Rubber
& grip
Tyres are made from rubber,
that is elastomeric materials
to which they owe a large part of
their grip capacity. To begin with,
we shall examine the special
characteristics of these materials.
Then, we shall take a detailed look
at the phenomena involved in friction
which generates grip.
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