Ageing of the new CPX reference tyres during a measurement season

Ageing of the new CPX reference tyres during a measurement season

Ageing of the new CPX reference tyres
during a measurement season
Erik Bühlmann1, Sebastian Schulze2, and Toni Ziegler3
1,2,3
Grolimund & Partner AG – Environmental Engineering
101A Thunstrasse, Bern CH-3006, Switzerland
ABSTRACT
Recently, new reference test tyres have been specified by the ISO to be used for tyre/road noise
measurements with the close-proximity (CPX) method. Various studies have provided evidence that tyre
ageing is accompanied by significant and continuous changes in the noise emission properties. It is therefore
essential to consider the changing state of reference tyres when carrying out tyre/road noise measurements
using the CPX method. A reliable quantification of these ageing effects and their influence on noise emission
levels requires that individual reference tyre sets are monitored over time.
This study aims at investigating the ageing process of the new reference tyres SRTT and Avon AV4 during
one measurement season. Measurements, which test the indentation resistance of tyre rubber with the type A
durometer, were repeated on a monthly basis. This revealed substantial increases in rubber hardness during
the 2012 measurement season, exceeding 3 units Shore A for the SRTT tyre and 6 units Shore A for the Avon
AV4 tyre. This corresponded with a considerable rise in noise levels, suggesting that tyre ageing is a primary
influencing factor when carrying out tyre/road noise measurements using the CPX method. The study
provides a simple tyre specific model for estimating rubber hardness changes based on the number of
measurement days. The evaluation of temperature data suggested that, due to the physical and environmental
strain on in-service tyres, usage influences tyre ageing to a larger extent than standardised operational storage
conditions. The data implies, moreover, that individual corrections for the CPX reference tyres SRTT and
Avon AV4 are needed.
KEYWORDS: Tyre/Road Noise, Tyre Aging, CPX (Close-Proximity) Measurement Method
1. INTRODUCTION
Recently, the International Organisation for Standardisation (ISO) specified new reference test
tyres for tyre/road noise measurements with the close-proximity (CPX) method [1]. Various studies
have provided evidence that tyre ageing is accompanied by significant and continuous changes in the
noise emission properties due to mechanical abrasion and chemical oxidation [2–4]. It is therefore
essential to consider the changing state of reference tyres when carrying out tyre/road noise
measurements using the CPX method.
1
2
3
[email protected]
[email protected]
[email protected]
1
Some studies carried out laboratory experiments to assess chemical and mechanical ageing
separately, but tend to exclude synergized ageing effects, resulting from a combination of the physical
strain during intensive usage and frequently changing environmental conditions (ambient ↔ storage)
while in service. Since both mechanical and chemical ageing strongly depend on the material
composition of the tread [2], they need to be assessed for different tyre types individually. A reliable
quantification of these ageing effects and their influence on noise emission levels requires, therefore,
that individual reference tyre sets are monitored over time. Simply comparing different tyre sets of
varying ages for the quantification of noise emission changes may lead to incorrect conclusions due to
acoustic non-conformity, which has been observed particularly for tyres from different production
batches [4]. Considering the necessity of continuous improvements in standardisation, it is of great
importance to assess to what extent the material and acoustic properties of reference tyres change
while in-service, in order to specify critical limits regarding their usage duration and, if necessary, to
establish procedures for correction.
In this context, the main objectives of this study are the three following: firstly, to assess the ageing
process of the new CPX reference tyres SRTT and Avon AV4 by measurements of rubber hardness in
the course of one measurement season; secondly, to establish a straightforward model for estimating
rubber hardness changes as a function of tyre usage; and thirdly, to investigate the impact of rubber
hardness changes on the measured noise levels.
2. MATERIALS & METHODS
2.1 Tyres under Consideration (CPX Reference Tyres)
The present study focuses on the changes of rubber hardness and their acoustic influence of the
CPX reference tyres during one measurement season. The tyres under consideration, therefore, consist
of two sets of new (i.e. run-in) reference tyres recommended for carrying out tyre/road noise
measurements with the CPX method in the draft standards ISO/DIS 11819-2 [1] and the ISO/TS
11819-3 [5]. The first set, the Standard Reference Tyre Type (SRTT) with the dimensional code
225/60R16, is typically used for representing noise levels of passenger cars. The second set, the "Avon
Supervan AV4" or Avon AV4, is used for measuring tyre/road noise levels of light trucks and vans and
is specified by the dimensional code 195R14C. Details on the reference tyres under consideration are
presented in Table 1.
Table 1 – Tyres under consideration (new reference tyres CPX)
Dimensions
Manufacturing date
Rubber hardness (at
start of meas. season)
SRTT
(for passenger cars)
Avon AV4
(for heavy vehicles)
225/60R16
Week 18, 2011
62.4 Shore A (left)
62.0 Shore A (right)
195R14C
Week 45, 2009
63.7 Shore A (left)
63.9 Shore A (right)
2
M ATERIAL PROPERTIES
PostRun-in
*En v iron mental Cond ition s
New
Operational
Pre-operational
storage
Operational storage
Standardised
(cool, dry, darkend)
Exposure to varying ambient and storage conditions
USAGE
ENV CON* STORAGE AGE
2.2 Ageing of CPX Reference Tyres
Reference tyres have to be replaced at regular intervals. In this context, the draft standard on
reference tyres ISO/TS 11819-3 [5] defines margins for maximum rubber hardness and maximum tread
abrasion. However, the time between the first usage of a tyre and its replacement can vary greatly. This
depends upon the tyre's intrinsic material properties [6–9], the storage duration and conditions [3], as
well as upon the usage duration, intensity and corresponding environmental conditions as will be
shown in this study. A schematic overview of the determinants that influence the lifecycle of CPX
reference tyres is given in Figure 1.
(temperature, humidity, solar radiation, etc.)
In-service
Operational
Run-in
(days, kilometers)
Chemical ageing & mechanical wear
Tyre type
Production batch
Aged
Material
property
changes
Tread
profile depth
Rubber
hardness
Replacement
Unsuitable
for
producing
representative measurement
values
Figure 1 – Schematic overview of the lifecycle of CPX reference tyres and its main factors of influence
Generally, tyres are required to be stored indoors in subdued light, under cool and dry conditions in
order to minimize chemical ageing of the tyre [3]. Considering the degree of physical and
environmental strain on in-service tyres, it may be that usage duration, intensity and conditions,
influence tyre ageing to a larger extent than standardised operational storage conditions. Indeed,
several authors have measured continuous changes in tread profile depth due to mechanical wear
[2,10] and an increase in the rubber hardness due to chemical ageing [7,11–13]. In an aged state, at the
end of a tyre's lifetime, these changes will make the tyre unsuitable for producing representative values
for tyre/road noise measurements. Measurable tread abrasion within the lifecycle of a reference tyre
only occurs with self-powered measurement systems (on a powered or steered axis). Hence, increasing
rubber hardness is likely to be the critical factor for limiting the in-service time of CPX reference tyres,
and, therefore, constitutes the main focus of this study.
2.3 Monitoring Measurements of Rubber Hardness
Throughout the 2012 CPX measurement season (April to October), tyre rubber hardness was
monitored on a monthly basis. Prior to the rubber hardness measurements, the tyres were kept in a
heated room overnight to be conditioned to a temperature of approximately 20 °C. Tyre rubber
hardness was measured with a Shore A durometer according to ISO 7619-1 [14] and ASTM F2493-08
[15]. Each tyre was assessed individually by a series of ten consecutive hardness measurements on the
inner rib (n=3), on the outer rib (n=3) and on in the centre rib (n=4) of the tyre tread. The ten
measurement results corresponding to one tyre were averaged arithmetically, resulting in a robust
estimate of the actual Shore A hardness per reference tyre.
2.4 Measurement of Tyre/Road Noise
To determine the acoustic effect of rubber hardness changes, rubber hardness measurements were
accompanied with tyre/road noise measurements using the CPX method at the start and the end of the
2012 measurement season.
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Measurement System & Corrections
The rubber hardness monitored reference tyres were mounted in a closed two-wheeled trailer. The
CPX measurement system fulfilled all performance requirements specified by the draft standard
ISO/DIS 11819-2 [1] with respect to disturbing influences from sound reflections and background
noise. Noise levels were recorded at the “mandatory” microphone positions of 20 cm from the inner
tyre sidewalls (height = 10 cm) to the front and the rear of the tyre-road contact area. Measurements
were corrected for trailer influences and fluctuations in measurement speed as specified in the same
draft standard. The variation of speed (reference speed 50 km/h) was kept to a minimum using
automatic speed control. Tyre/road noise levels were obtained by averaging four measurement runs for
each tyre set.
Measurement Set-up & Procedure
The measurements were carried out according to the draft standard ISO/DIS 11819-2 [1]. Since the
focus of this study has been targeted at isolating and quantifying the effect of tyre ageing on tyre/road
noise measurements, a measurement set-up and procedure was chosen allowing control of the principal
influencing parameters. The two CPX measurement series were carried out with the same
measurement system, trailer, location, road surface, reference speed, measurement set-up and
procedure, and under almost the same environmental conditions. Date and time of the second
measurements (at the end of the measurement season) were chosen so that they matched the weather
conditions (cloudy, limited sun radiation) and temperature (T ≈ 5 °C) of the first measurement series.
Due to full cloud cover, temperature gradients over the day were small and air and road surface
temperatures remained nearly constant during the measurements.
Warming up (6 km)
Measurement runs:
- Tyres: SRTT and Avon AV4
Road surface DAC 0/16 (length: 500m)
- Speed: 50 km/h
Section of measurements
Figure 2 – Schematic overview of the measurement set-up
A schematic overview of the measurement set-up is given in Figure 2. Both measurements where
conducted on an aged, but well conserved and undamaged DAC 0/16 road surface, whose acoustic
properties could reliably be assumed to have remained unchanged during the
eight-month-measurement-season. The tyres were warmed up as follows: (A) At the beginning of a
measurement series, the tyres were brought to operating temperature by driving for at least 15 minutes.
(B) Prior to each measurement run, the tyres were brought to a stable temperature state over a constant
distance of 6 km. Details on the two measurement series at the start and the end of the measurement
season are presented in Table 2.
Table 2 – Details on the tyre/road noise reference measurements with the CPX method.
Date & Time
Location
Road Surface
Reference Speed
Device
Temperature
Weather
Warm-up
Tyre usage
Tyre usage duration
Start of the season
March 6, 2012 (6pm - 8pm)
Deitingen, CH
DAC 0/16
50 km/h
M+P two-wheeled trailer
5.5 °C
cloudy
~ 6 km
200 km Run-in
1 measurement day
4
End of the season
October 13, 2012 (11am - 3pm)
Deitingen, CH
DAC 0/16
50 km/h
M+P two-wheeled trailer
4.9 °C
cloudy
~ 6 km
~3000 km
58 measurement day
3. RESULTS & DISCUSSION
3.1 Measurement Season & Environmental Conditions
The 2012 measurement season was of 58 days and can be considered an average measurement
season for CPX measurement systems in commercial use. Considering that the mid-latitudinal CPX
measurement season lasts typically from April to October (approx. 210 days), the usage of the
reference tyres – out of its controlled storage conditions – accounts for roughly one quarter of this
period. Details on the 2012 CPX measurement season are given in Table 3.
Table 3 – Details on the 2012 CPX measurement season.
2012 measurement season
PERIOD
Start
End
Duration
TYRE USAGE
No. of measurement days
Tyre usage distance (per tyre set)
Air temperature during measurements (mean)
Air temperature during measurements (range)
Road surface temperature during measurements (mean)
Road surface temperature during measurements (range)
Tyre temperature during measurements (mean)
Tyre temperature during measurements (range)
TYRE STORAGE (OPERATIONAL STORAGE)
No. of days tyres in storage
Av. air temperature operational tyre storage
6 th of March
13th of October
221 d
58 d
~3000 km
20.3 °C
5 to 32 °C
42.6 °C
6 to 60 °C
34.1 °C
15 to 46 °C
163 d
18.6 °C
Temperature has been shown to constitute an important – if not the most important – influence
during rubber ageing [11,16]. In this context, oven ageing has extensively been used in order to
investigate tyre ageing within considerably shorter time periods than tyre ageing under conventional
environmental conditions [7,11]. Figure 3 gives an impression on the variation of the operational
ambient air (blue), tyre (red) and road surface (brown) temperatures compared to the storage
temperature (grey) during the 2012 CPX measurement season.
T [°C]
70
Air ambient
Tyre
Road surface
Air storage
60
50
40
30
10
07.10.12
27.09.12
17.09.12
07.09.12
28.08.12
18.08.12
08.08.12
29.07.12
19.07.12
09.07.12
29.06.12
19.06.12
09.06.12
30.05.12
20.05.12
10.05.12
30.04.12
20.04.12
10.04.12
31.03.12
21.03.12
11.03.12
01.03.12
0
Date [dd.mm.yy]
20
Figure 3 – Mean operational ambient air, tyre and road surface temperatures and the storage air temperature
over the 2012 CPX measurement season
Operational tyre temperatures are significantly higher than ambient air temperatures. Considering
mean temperatures of 20.3°C for operational air temperature and 34.1°C for operational tyre
5
temperature, the difference amounts to almost 14°C on average. While it appears trivial that high road
surface temperatures trigger elevated tyre temperatures, tyre temperatures have also been measured to
surpass road surface temperatures on several cold measurement days in the early and late measurement
season. This indicates that elevated tyre temperatures, due to both high road surface temperatures and
generated heat from the frictional force between pavement and tyre, may constitute an important factor
when investigating tyre ageing.
In summary, CPX reference tyres experience consistently higher tyre temperatures when compared
to ambient air and storage temperatures. In-service tyres were measured to reach average in-service
temperatures of 46°C, which closely compare to temperatures used for laboratory accelerated tyre
aging [7,11,16,17]. With reference to the major influence of high temperatures on tyre ageing, these
findings confirm the importance of the in-service time for investigating the ageing process of CPX
reference tyres.
tyre rubber hardness [units Shore A]
3.2 Rubber Hardness Changes during the Measurement Season
In order to analyse changes of rubber hardness of in-service CPX reference tyres during the 2012
measurement season, tyre rubber hardness Shore A was monitored on a monthly basis using the
durometer method. The obtained rubber hardness changes are displayed in Figure 4.
72
y AVON = 0.0975x + 64.936
R² = 0.9409
SRTT
70
AVON AV4
70.1
68
66
65.3
64
63.8
62
y SRTT = 0.0525x + 62.213
R² = 0.9715
62.2
60
50
40
30
20
10
0
60
No. of measurement days (season 2012)
6th of March
12th of October
Figure 4 – Development of tyre rubber hardness of in-service CPX reference tyre sets (SRTT and Avon AV4)
as a function of measurement days during the 2012 measurement season
As presented in Figure 4, substantial increases in Shore A rubber hardness were determined for the
2012 measurement season. Rubber hardness increased by 3.1 units Shore A for the SRTT tyres and by
6.3 units Shore A for the Avon AV4 tyres. Strong linear relationships between rubber hardness and the
number of measurement days were observed for both tyre sets within the assessed period.
Facing the end of a tyre's lifetime Shore A rubber hardness is likely to show an asymptotic
behaviour against a tyre specific saturation value and would therefore require a logarithmic rather than
a linear model (see Sandberg & Glaeser 2008 [2]). However, linear fits for discrete segments of
logarithmic functions represent well-suited approximations. This is confirmed by the high coefficients
of determination for the examples presented here (R2 > 0.94 and Radjusted 2 > 0.92). The regression
coefficients for both models are statistically significant (p < 0.001).
The results clearly indicate that the rubber hardness of SRTT and Avon AV4 tyres change at
∂y AVON
individual gradients of ∂y SRTT ≈ 0.05 and
≈ 0.1 for the same time in service. Considering the
∂x
∂x
propositions for tyre replacements in the ISO draft standard [5], the Avon AV4 reached the critical
rubber hardness of 70 Shore A after roughly 60 days of CPX measurements or one typical CPX
measurement season.
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3.3 Influence of Rubber Hardness Noise Levels
In order to gain an insight into the effect of tyre ageing on tyre/road noise levels, the changes of tyre
rubber hardness presented in section 3.2 were compared to the CPX measurements that were carried
out at the start and the end of the 2012 measurement season on a DAC 0/16 road surface. The average
noise levels were subtracted from each other. Since the acoustic properties of the road surface are
presumed to have remained constant over the season 2012, the resulting difference gives a good
indication of the change in the noise level attributed to the tyre ageing effect. Table 4 shows the
individual rates of increase in noise levels for the SRTT and Avon AV4 separately. Since the acoustic
effect of tyre rubber hardness increase is likely to be road surface dependent [4], the values presented
for DAC 0/16 in Table 4 are of indicative nature and may differ for other road surface types.
Table 4 – Effect of tyre rubber hardness increase on tyre/road noise (road surface: DAC 0/16)
CPX REFERENCE TYRES
SRTT
Avon AV4
(for passenger cars)
(for heavy vehicles)
Noise level increase
(season 2012)
Increase rate per
unit Shore A (approx.)
Increase-rate per
meas. day (approx.)
+0.9 dB(A)
+0.9 dB(A)
+0.3 dB(A) /unit Shore A
+0.15 dB(A) /unit Shore A
+0.015 dB(A) /meas. day
+0.015 dB(A) /meas. day
Tyre/road noise levels increased with a rate of +0.3 dB(A)/unit Shore A for the SRTT tyre and +0.15
dB(A)/unit Shore A for the Avon AV4 tyre on the reference track with the DAC 0/16 road surface. Such
differences can be explained by the significantly different layout of the tread pattern and probable
divergences in their material compounds. The data implies that an individual approach for the CPX
reference tyres SRTT and Avon AV4 is needed, when correcting tyre/road noise levels for shore
hardness.
4. SUMMARY & CONCLUSION
While assessing ageing of CPX reference tyres during the 2012 measurement season, we measured
strong increases in rubber hardness exceeding 3 units Shore A for the SRTT tyre and 6 units Shore A
for the Avon AV4 tyre. Relating rubber hardness changes to usage resulted in rates of increase of 0.05
units Shore A/measurement day for SRTT and 0.1 for Avon AV4. Since the critical rubber hardness of
70 Shore A was reached within merely 60 measurement days (Avon AV4), limiting in-service duration
of reference tyres should be considered. The evaluation of temperature data suggested that, due to the
physical and environmental strain on in-service tyres, usage influences tyre ageing to a larger extent
than standardised operational storage conditions. Tyre/road noise measurements revealed that tyre
ageing accounted for approximately 1 dB(A) increase in noise levels during 2012 measurement season.
This emphasises the need for correcting tyre/road noise levels for rubber hardness. The data implies,
moreover, that individual corrections for the CPX reference tyres SRTT and Avon AV4 are needed.
ACKNOWLEDGEMENTS
We are grateful to Patricio Lerena for the valuable inputs and to David Murton for language editing.
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