CIRCUIT RACING, TRACK TEXTURE, TEMPERATURE AND RUBBER FRICTION RACIN… · CIRCUIT RACING, TRACK...
Transcript of CIRCUIT RACING, TRACK TEXTURE, TEMPERATURE AND RUBBER FRICTION RACIN… · CIRCUIT RACING, TRACK...
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CIRCUIT RACING, TRACK
TEXTURE, TEMPERATURE AND
RUBBER FRICTION
Robin Sharp, Patrick Gruber and
Ernesto Fina
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Outline
• General observations
• Grosch's experiments
• Interpretation of Grosch’s results
• Rubber properties
• Persson's hysteresis-loss theory
• Persson's theory versus Grosch's results
• Conclusions
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General observations
• Importance of tyre shear forces
• Forces depend on friction between rubber and road
• Racing demands the maximum possible forces
• Forces are functions of -• normal load
• surface nature and texture
• rubber compound
• rubber temperature
• surface temperature
• sliding speed
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Observations from motor racing
• Track surfaces not all the same
• “Green” tracks get faster with usage
• Rubber “B” often grips rubber “A” poorly
• Rain on a used track affects the racing line
• New tyres grip well for a short time
• Higher friction tyres have shorter lives
• Rubber and road temperatures are vital
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The focus
• Now - how does rubber friction
work?
• Later – how does rubber friction
relate to tyre/road interactions?
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Grosch’s experiments
• Flat rubber blocks loaded against smooth
(wavy glass) and rough (silicon carbide)
surfaces – 4 compounds
• Sliding under constant normal load
• Low velocities to avoid heating
• Temperature control -50°C to 100°C
• Sliding speed and friction force
measurements
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Fz
V
friction force
loading by weights
moving surface
stationary
rubber block
temperature
regulated box
Grosch’s experiments
4 compounds:
INR, ABR,
SBR, Butyl
energy dissipation by adhesion and/or deformation
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Grosch measurements; 4 compounds;
INR, ABR, SBR, Butyl
emery cloth
temperature controlled enclosure
force
measurement
speed-controlled motor
loading on test
rubber block
liquid flow
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Lorenz experiments (2011)
Equivalence of energy dissipation and friction force
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Grosch results for INR on silicon carbide
(left) and for ABR on glass (right)
T = 90 to -350 C
T = -40 to -580 C
T = 85 to 200 C
T = 10 to -150 C
Log(V/Vref) Vref = 1 cm/s
Friction coefficient
Log(V/Vref) Vref = 1 cm/s
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Temperature – frequency / sliding
speed equivalence
• Rubber state depends on temperature
relative to glass-transition temperature, Tg
• Standard temperature, Ts ≈ Tg+500 C
• Williams Landel Ferry (WLF) normalisation
to Ts; plot aTω or aTV (not ω or V), where
S
ST
TT
TTa
5.101
86.8log10
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Grosch master curves
• Combining temperature and sliding velocity by WLF
transform gives master curve for ABR on glass;
S
ST
TT
TTa
5.101
86.8log10
WLF transform
T range: -15°C to 80°CResults for
different
temperatures, T
T-compensated
results
1
2
0
1
2
0
200 C
re (1 cm/s)re (1 cm/s)
-4 -2 0 -4 -2 0 -4 0 4 8-8
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Grosch master curves for SBR at
200 C on glass and silicon carbide
Log[aTV/Vref] Vref = 1 cm/s
on glasson silicon
carbide
on powdered silicon carbide
adhesion deformationfriction
coefficient
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Grosch master curves for ABR at
200 C on glass and silicon carbide
on polished
stainless steelon glass
on silicon
carbide
on powdered
silicon carbide
Log[aTV/Vref] Vref = 1 cm/s
friction
coefficient
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Grosch master curves for Butyl at
200 C on glass and silicon carbide
Log[aTV/Vref] Vref = 1 cm/s
on glass
on silicon
carbide
on powdered
silicon carbide
friction
coefficient
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Rubber vibration testing
commercial
analyserclose-up
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ωLMP
ωLTP
G(ω) = G’(ω)+jG’’(ω)
tan(δ) = G’’(ω)/G’(ω)
Rubber vibration properties
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SBR elasticity at constant temperature
maximum loss modulus at ωLMP
maximum ratio at ωLTP
18
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Non-linearity (Lorenz)
Amplitude
dependence of
storage (upper)
and loss
(lower) moduli
large strain
large strain
small strain
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Non-linearity (Westermann)
carbon
black
filler
storage modulus
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Adhesion mechanism
• Smooth surface peak due to adhesion
• Rubber bonds to road; bonds stretch and
break
• All 4 rubbers, VSP≈ 6e-9 ωLMP /(2π) m/s
• Characteristic length, 6e-9 m - molecular
• If bonds break at this stretch, rubber is
forced at ωLMP when V=VSP
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Deformation mechanism
• Rough surface peak due to deformation
• All 4 rubbers, VRP≈ 1.5e-4 ωLTP /(2π) m/s
• Characteristic length, 1.5e-4 m, close to mean particle spacing in the surface
• If wavelength is 1.5e-4 m, rubber is forced at ωLTP when V=VRP
• VSP/VRP=6e-9 ωLMP /1.5e-4 ωLTP
• If ωLMP and ωLTP are wide apart, adhesion and deformation peaks are close
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Persson’s deformation ideas - simple
(1) sinusoidal surface; waves normal to sliding
(2) rubber deformation from linear elastic theory
(3) calculate energy dissipation for given sliding speed
• wavelength and speed give ω
• temperature gives rubber visco-elastic properties
• expect maximal energy loss at ω = ωLTP
stationary rubber
sliding
speed, V
simple surface
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Persson’s deformation ideas - complex
(1) isotropic surface
(2) conformity to short waves depends on long waves
(3) accounting for (1) and (2), integrate energy-loss
contributions from all wavenumbers from qL to q1
• qL non-critical, q1 needs estimating
• divide power by V to get shear force; hence μ
complex surface λ 0
stationary rubber
sliding
speed, V
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Persson’s deformation theory
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Persson’s notation
• μ, friction coefficient
• C(q), road spectral density function
• P(q), contact area ratio – actual/nominal
• qL, q1, wavenumbers for longest and shortest waves
• Tq, temperature
• E, rubber complex elastic modulus; , Poisson’s ratio
• v, sliding velocity
• σ0, nominal normal stress
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Silicon carbide 180 mesh measured
displacement spectrum
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SBRubber properties at 200 C
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Simulated friction master curves
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Reconstructed rubber properties
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Simulated friction master curve
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Summary and conclusion (1)
• Smooth surface friction - adhesion, not
understood, wide open
• Rough surface friction - deformation
• Persson’s hysteresis mechanics plausible
• Rubber treated as linear viscoelastic
• Amplitude dependence
• Which properties to use?
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Summary and conclusion (2)
• Surface represented by displacement
spectrum in range qL to q1
• qL non-critical, q1 uncertain, influenced by
cleanliness and debris
• Which q1 to use?
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Summary and conclusion (3)
• With favourable treatment, rough-surface
friction peak realistic with respect to
Grosch
• Below peak, adhesion can account for
differences
• Above peak, predicted friction falls too
much as sliding speed increases
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Summary and conclusion (4)
• In racing, “rubbering-in” involves transfer of
rubber to road
• Surface on racing line becomes smoother and
chemistry changes
• Contact area will increase and adhesion will
increase for “same” compounds
• Deformation friction will reduce
• Racing line friction is enhanced but if it rains,
adhesion is impeded - best line changes
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Reference
• E. Fina, P. Gruber and R. S. Sharp,
Hysteretic rubber friction: Application of
Persson’s theories to Grosch’s
experimental results,
• ASME Journal of Applied Mechanics
• Vol. 81, No 12, December 2014.