Noise and Rolling Resistance Properties of Various Types of Winter Tyres Compared to Normal Car Tyres Ulf SANDBERG 1; Piotr MIODUSZEWSKI 2, Jerzy EJSMONT2, Tiago VIEIRA1 1
Swedish National Road and Transport Research Institute (VTI), Sweden 2
Technical University of Gdansk (TUG), Poland
ABSTRACT To cope with winter weather conditions, potentially including snow and ice, it is common to use winter tyres, or “all-seasons” tyres assumed to be safe both in summer and winter. In some northern countries, winter tyres are mandatory. It is commonly assumed that winter tyres are noisier than normal tyres (here called summer tyres) and winter tyres equipped with studs are assumed to be extra noisy. This paper presents a study where noise and rolling resistance properties of tyres for winter conditions are compared to summer and all-season tyres. The winter tyres include types optimized for central European climate, tyres optimized for Nordic climate and tyres with studs. In total, approx. 50 car tyres have been tested. Noise properties have been measured with the CPX method and rolling resistance has been measured with a trailer method; all measurements made on two road surfaces; SMA 8 and DAC 16. Results indicate that winter tyres are not noisier than summer tyres; except for studded tyres. Despite high differences among individual tyres, rolling resistance is approximately similar among the main types; surprisingly including also studded tyres. An alarming result was that there was no correlation between measured values and values on the tyre labels. Keywords: tyre, winter, climate, stud, noise, rolling resistance, tyre label I-INCE Classification of Subjects Number(s): 11.7.1, 13.2.1, 89
1. INTRODUCTION Until recently, it has been a general view that tyres for winter service are noisier than regular ("summer") car tyres; this being applicable to both interior and exterior noise emission. In some cases in past decades, this could be backed by test results measured on smooth-textured surfaces . The reason was that in order to provide good traction and friction in winter conditions; especially in snow and slush, tyres were designed with a high air/rubber ratio in the tread pattern, which mostly meant that tread patterns contained relatively large blocks with relatively wide grooves in-between. Rubber compounds were far from today's quality. The technical progress, however, has been very successful with respect to winter tyres, and today's winter tyres are made with very advanced, modern, soft rubber compounds, and they no longer have large solid blocks; instead they have tread elements which are cut by sipes into rows of rather narrow "ridges". This makes such tread patterns very flexible. Wide grooves only exist in the longitudinal direction. In northern countries with severe winter climates, such as Sweden, Norway, Finland, Iceland, Russia, Canada and some states in the USA, studs mounted in the tyre tread are used in order to provide the best possible friction on icy roads. These studs, which may amount to 80-200 per tyre, have the disadvantages that they dramatically increase the wear on the road surface and subsequent spread of fine particles in the air, and also that they increase noise emission. Consequently, the safety benefits are compromised by environmental, health and economic effects, which makes the acceptance of studded winter tyres arguable and often questioned by authorities and environmental interest groups. Rolling resistance is another relevant environmental parameter which is affected by the choice of tyres, as it causes energy consumption and this will influence not only the vehicle operating cost but 1 2
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also causes exhaust gases, for example CO 2 , and consequently have effect on climate change. Rolling resistance is primarily affected by the tyre deformations and hysteretic losses as it rolls on the pavement surface, causing deflections on the tyre curvature to form the contact patch. The knowledge about noise and rolling resistance for winter tyres is still limited. Not many scientific publications are available, and when considering studded tyres, there is even less information available. This is most probably because testing studded tyres poses special operational difficulties and since such tyres are allowed only in a few and rather sparsely populated regions in the world. For instance, testing studded tyres in a drum would damage the drum equipment itself. The study reported here has been performed by the Swedish National Road and Transport Research Institute (VTI) in cooperation with the Technical University of Gdansk (TUG) in Poland, where TUG made all the measurements on commission from VTI.
2. PURPOSE OF THE STUDY The purpose of the study is to provide information related to the environmental performance of winter tyres in new condition (and a few in used condition) intended for use on passenger cars, and how these differ for various categories of tyres, for the Swedish Transport Administration (Trafikverket). This is intended to provide a better basis for decisions on national winter tyre regulations, and also to see how the EU tyre labelling works for winter tyres. This study is part of a larger research project by VTI for the Swedish Transport Administration including also the frictional properties of winter tyres on icy and snowy surfaces. However, those tests and their results will be reported elsewhere. There is also considerable consumer interest in the performance of these tyres under real road conditions.
3. PREVIOUS WORK BY VTI AND TUG REGARDING NOISE OF WINTER TYRES The first major testing of winter tyres was within a VTI/TUG project run in 1995-2000, in which approximately 100 car tyres of rather similar dimensions were tested for noise, rolling resistance and wet friction properties [1, 2]. The test program included 41 winter tyres, of which 7 were studded, compared to approximately 60 “summer tyres”. Noise was measured with an early version of the CPX method, nowadays specified in , and rolling resistance with an early version of today’s drum method . The latter could not include the studded tyres since they would destroy the drum surfaces. The results can be summarized as follows, as tested on a dense asphalt concrete (DAC) with 16 mm maximum aggregate size. With the summer tyres as a reference, the winter tyres were on the average 0.9 dB (A-weighted) quieter, while the studded winter tyres were 2.3 dB noisier. The rolling resistance values were approximately 4 % lower for the winter tyres than for the summer tyres (studded tyres were not tested). One should note that these tyres were of a design typical of the market tyres in the 1990’s and may not necessarily be typical of today’s tyres. The next study was made in 2011/12. This only compared noise and friction properties of tyres , but the tested tyres were similar to today’s designs. The results regarding the noise levels (measured with the coast-by method on an ISO 10844:1994 surface) at 50 and 80 km/h are shown in Figure 1. Each bar represents one tested tyre. With regard to the two types of winter tyre, refer to text below.
Figure 1 – Noise levels measured at 50 km/h (left part of the figure) and 80 km/h (right part), on an ISO 10844:1994 test track, using the coast-by method of ISO 13325:2003.
4. TESTED TYRES 4.1 Different types of winter tyres There are basically three different types of winter tyres for passenger cars: winter tyres optimized for central-European or similar climates, winter tyres optimized for north-European or similar climates and studded winter tyres. In this paper they are referred to as “European winter tyres”, “Nordic winter tyres” and “Studded winter tyres”, respectively. The latter are generally used only in northern winter climates. Additionally, there is a type called “all-season” tyres, which used to be a compromise between normal (“summer”) tyres and winter tyres, but recent models are more advanced. Furthermore, there is a special winter tyre design which is essentially optimized for northern climates, but having silicium carbide granules evenly mixed into the rubber compound of the tyre tread. This is intended to provide a frictional performance somewhat similar to that of studded tyres. It is an invention from Iceland, using a retreaded tyre, offered on the market in the Nordic countries and North America. In this paper they are referred to as “Green Diamond” tyres, referring to their brand name. This study has tested tyres of the following types: x x x x x
All-season tyres (Central) European winter tyres Nordic winter tyres Green Diamond (winter) tyres Studded winter tyres
with two additional types included as references: x x
Summer tyres (often referred to as ”normal tyres”) SRTT (standard reference test tyre according to ASTM 2493:14)
4.2 The tested tyres The measurements have been made in two time periods: first in February-April 2015, then in March-April 2016. Table 1 shows the tyres tested in 2015 and Table 2 the tyres tested in 2016. All tyres tested in 2015 were in new condition (but run-in). The Green Diamond tyres are always retreaded and the ones used here were tested with new treads. In 2016, a few more tyres were tested in new condition, but some of the testing applied to tyres in used condition. “Used” means that the tyres had been used during at least two winter seasons. A few of the tyres were not tested for noise and a few were not tested for rolling resistance, but the vast majority were tested for both parameters. 4.3 Tyre load and inflation pressure The tyres were loaded essentially in accordance with ECE regulation R117 (406 kg for noise, 408 kg for rolling resistance). Inflation pressure was also adjusted in accordance with the specifications in ECE R117 (fixed 180 kPa in cold condition for noise, regulated 200 kPa for rolling resistance). An exception was the SRTT; a much larger tyre but yet it was tested with similar load and inflation as the other tyres. The reason was that the SRTT served only as a control tyre during each day of rolling resistance measurements, to check that there was no drift in results from day to day, apart from that corresponding to varying temperature. Air, road and tyre surface temperatures were measured simultaneously with the noise and rolling resistance measurements. They were similar to typical Swedish winter and early spring temperatures.
5. ROAD SURFACES USED IN THE TESTS Two test surfaces were used for the measurement, designated S1 and S2 in this paper; see Table 3. Test surface S1 is an SMA 8 pavement on an in-VHUYLFHURDGQHDU*GDĔVNLQ3RODQG7KLVVXUIDFHZDV selected in order to serve as a proxy for the surface used when testing tyres according to EU regulations on tyre noise limits and noise labelling (ISO 10844:1994). The surface texture was characterized by its mean profile depth (MPD) according to ISO 13473-1 , which was evaluated with the VTI laser RST in June 2015. Test surface S2 is a DAC 16, also on an in-service road near *GDĔVN7KLVVXUIDFHZDVVHOHFWHGDVLWVWH[WXUHDSSHDUHGWREHVLPLODUWRWKH60$URDGVXUIDFH which is the dominating wearing course on Swedish highways. This surface was also evaluated by VTI Laser RST in June 2015. The tested surfaces and MPD values are presented in Table 3.
Table 1 - Tested tyres in the 2015 winter season (all in new condition). DESIGNATION T1077 T1122 T1123 T1124 T1126 T1127 T1128 T1129 T1130 T1131 T1132 T1133 T1134 T1135 T1136 T1137 T1138 T1139 T1140 T1141 T1142 T1143 T1145 T1146 T1151 T1152 T1153 T1154 T1155 T1156 T1157 T1158 T1159 T1160 T1161 T1162 T1163 T1164
MANUFACTURER UNIROYAL USA FULDA FULDA FALKEN CONTINENTAL CONTINENTAL HANKOOK HANKOOK MICHELIN MICHELIN NOKIAN SAVA SAJLUN SAJLUN PIRELLI PIRELLI SAVA AGI SAVA GREEN DIAMOND GREEN DIAMOND NOKIAN NOKIAN VREDESTAIN UNIROYAL GOODYEAR NANKANG NANKANG MICHELIN MICHELIN MICHELIN WEST LAKE GOODYEAR NOKIAN NOKIAN
MODEL TigerPaw ECOCONTROL HP ECOCONTROL HS 439 EUROWINTER TS850 ContiWinterContact TS850 ContiWinterContact W442 Winter i*cept RS W442 Winter i*cept RS ALPIN A4 ALPIN A4 W+ Eskimo Stud ICEBLAZER WST1 ICEBLAZER WST1 WinterCarving Edge WinterCarving Edge Eskimo S3+ SAREK2 SUBB (retreaded) Eskimo S3+ (retreaded) (retreaded) HAKAPELIITTA R2 Studless HAKAPELIITTA R2 Studless QUATRAC 3 (all season) ALL SEASON EXPERT ULTRA GRIP ICE 2 SNOW VIVA SV-1 SNOW VIVA SV-1 X-ICE ENERGY SAVER + ENERGY SAVER + SP06 EFFICIENT GRIP PERFORMANCE HAKAPELIITTA 8 XL HAKAPELIITTA 8 XL
SIZE P225/60R16 195/65R15 195/65R15 195/60R15 195/60R15 195/60R15 195/60R15 195/60R15 195/60R15 195/60R15 195/60R15 185/65R15 185/65R15 185/65R15 185/60R15 185/60R15 185/60R15 185/65R15 185/60R15 185/65R15 185/65R15 185/65R15 185/65R15 195/60R15 195/60R15 195/65R15 195/60R15 195/60R15 185/60R15 195/60R15 195/60R15 195/60R15 195/60R15 195/60R15 195/60R15
LI 98 91 91 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88 88
TYPE M+S S S M+S M+S M+S M+S M+S M+S M+S M+S M+S M+S M+S M+S M+S M+S M+S M+S M+S M+S M+S M+S AS AS M+S M+S M+S M+S S S AS S M+S M+S
STUDS? EU or Nordic? Type code Reference SU SU E WI E WI E WI E WI E WI E WI E WI E WI St WIST St WIST St WIST St WIST St WIST E WI St WIST E WI SiC N WIGD SiC N WIGD N WI N WI AS AS N WI N WI N WI N WI SU SU AS SU St WIST (N) WI
Table 2 - Tested tyres in the 2016 winter season. The last four (in Italics) were tested in used condition, the other tyres in new condition. DESIGNATION T1077 T1184 T1185 T1190 T1191 T1192 T1193 T1194 T1195 T1196 T1197 T1198 T1199 T1200
MANUFACTURER UNIROYAL MICHELIN CONTINENTAL NOKIAN CONTINENTAL NOKIAN MICHELIN CONTINENTAL GOODYEAR NOKIAN NOKIAN NOKIAN NOKIAN CONTINENTAL
MODEL TigerPaw CrossCLIMATE CONTI.ECONTACT BLUECO HAKKAPELIITTA R CONTI VIKING CONTACT 5 WR G2 X-ICE NORTH 3 X-GREEN CONTI PREMIUM CONTACT 5 ULTRAGRIP ICE ARCTIC HAKKAPELIITTA 7 HAKKAPELIITTA 7 HAKKAPELIITTA R WR G2 CONTI VIKING CONTACT 5
SIZE P225/60R16 195/60R15 205/55R16 205/55R16 205/55R16 205/55R16 185/65R15 195/60R15 185/65R15 195/60R15 195/60R15 205/55R16 205/55R16 205/55R16
LI 98 92 91 94 94 91 92 88 88 92 92 94 94 94
TYPE M+S AS S M+S M+S M+S M+S S M+S M+S M+S M+S M+S M+S
STUDS? EU or Nordic? Type code Reference AS SU N WI N WI E WI St WIST SU St WIST St WIST St WIST N WI E WI N WI
With the two tested surfaces, the results are representative of a vast majority of the Swedish roads and streets. The rolling resistance tests were performed on actual roads, despite the EU regulations stating that rolling resistance should be performed on drums, as testing on road surfaces is expected to result in more realistic values than the ones obtained with drums, which would either have a smooth
steel surface or a sandpaper-like surface. It is also noteworthy that studded tyres cannot be tested on drums, as they would damage the equipment. Table 3 - Tested pavement surfaces and their average MPD values Road surface designation S1 S2
Pavement type in European terminology SMA 8 (proxy for ISO 10844) '$&§60$
Corresponding Swedish pavement type ABS 8 ABS 16
Average MPD [mm] 0.78 1.09
6. MEASUREMENT METHODS AND EQUIPMENT 6.1 Noise Noise measurements were performed by the GdaĔsk University of Technology (TUG) with the CPX method, according to the procedures given in ISO/FDIS 11819-2 , using a dedicated CPX trailer (Figure 1). The selected test speeds were 30, 50 and 80 km/h, and a minimum of two (usually four) runs were made for each test condition.
Figure 1 - The CPX trailer by TUG (in operation in Sweden in another project). 6.2 Rolling resistance Rolling resistance measurements were performed with the trailer owned and operated by the GdaĔsk University of Technology (TUG), see Figure 2, following a method proposed in project ROSANNE (http://rosanne-project.eu/). The selected test speeds were 50 and 80 km/h, and a minimum of three runs were made in each test condition.
Figure 2 - Rolling resistance measurement trailer from TUG (in operation in Sweden in another project).
7. NORMALIZATION TO REFERENCE TEMPERATURES All measurement results have been normalized to a reference air temperature of 10 °C, in order to minimize that varying temperatures during the measurements affect the results. Although air, road and tyre temperatures were measured, only air temperatures were used for normalization. This reference temperature 10 o C was chosen, contrary to the 20 o C which is often used in other cases, since it is more representative to winter conditions and is rather similar to the average temperature during the measurements; thus minimizing the corrections. For noise, the correction procedure described in  was used (but with 10 o C as reference temperature instead of 20). The maximum corrections were -1.4 and +1.0 dB. For rolling resistance, not yet published procedures in the ROSANNE project (http://rosanne-project.eu/) were used. The maximum corrections were -0.0001 and +0.0003 in rolling resistance coefficient RRC (equal to less than 3 % of the measured RRC values). The reason for this small correction is that temperatures were within 8 and 11 o C during these measurements. In the noise measurements, temperatures varied much more.
8. NOISE RESULTS: COMPARISON OF INDIVIDUAL TYRES The A-weighted CPX noise levels for different tyre types and the three test speeds are presented in Figure 3. In this case the levels measured on both road surfaces were averaged, and only the averaged values are shown here, since there was almost the same ranking of the tyres on both surfaces. Figure 4 shows the average levels within each tyre category. The following features are noteworthy: x The different speeds do not create any remarkable change in tyre ranking; but tyre differences are larger at the low speed than at the high speed. x The only clear difference between the tyre categories is that the studded tyres are much noisier, seen as an average, but there is considerable overlap x Within each category there is a large difference between the noisiest and quietest tyres (4-6 dB) x The Green Diamond tyres seem to be the quietest tyres at 80 km/h and among the quietest at the lower speeds.
Figure 3 - CPX noise levels for all the tyres; average values measured on the SMA 8 and DAC 16 surfaces. The tyre categories are distinguished by different colours. The order of the tyres within each category is essentially random (although the 2016 measurements are always at the right end).
Figure 4 - CPX noise levels: average for each tyre category. Average values measured on the SMA 8 and DAC 16 surfaces. A distinction between the two winter tyre categories optimized for central Europe and northern Europe is made in Figure 5. It seems that the northern Europe category is marginally (ca 1 dB) quieter than the central Europe category. It is worth noting that the studded winter tyres, on the average, are 6-10 dB noisier than the non-studded winter tyres, with the highest differences at the lowest speed.
Figure 5 - CPX noise levels for the winter tyres without studs; distinguishing between the tyres optimized for central European use (left) and for north European use (right). Average values measured on the SMA 8 and DAC 16 surfaces.
9. NOISE RESULTS: COMPARISON OF USED AND NEW TYRES The differences between the noise levels measured for the four tyre pairs that were tested both in new and used condition are shown in Figure 6. Each tyre type is shown as a pair of bars – one red (tyre in new condition) and one black (tyre in used condition). The highest pair of data (the pair at the right) are for the studded tyre; the other three pairs are for non-studded winter tyres. It appears that the used tyres are significantly and consistently quieter than the new ones, by 2 to 7 dB, depending on speed and tyre. The studded tyre had a tread depth of 10 mm in new condition and 8 mm in the used condition. Stud protrusion was approximately 1.5 mm in new and 0.5 mm in used condition. In general, the tread depth was 1-3 mm lower for the used than the new tyres. This is a surprisingly large difference in noise levels. However, something similar was shown in Figure 18 of , where the winter tyre in that tyre selection showed the highest drop of 2 dB from new (8 mm) to 6 mm tread depth. Maybe this is something quite unique for winter tyres? The tyres tested here in used condition are only four and many more would be needed to be able to make robust estimations. This is worth further studies.
Figure 6 – Comparison of noise levels for tyres in new condition and similar tyres in used condition.
10. NOISE RESULTS: LABELLING For those of our tested tyres that have a tyre label according to , the relationship between the measured CPX noise levels and the label values are shown in Figure 7. The diagram is for the measurements made at 80 km/h on the SMA 8 surface, since this is most similar to the conditions for which the noise levels for the labels are determined. However, one must note that the measurements in this project have been made with the CPX method, while measurements for the label (by the manufacturers) are made with the coast-by method; thus the values are not directly comparable. The results, showing no correlation at all, must be considered as quite alarming. The reason should be identified.
Figure 7 - Measured CPX noise levels versus the tyre noise label values
11. ROLLING RESISTANCE RESULTS: COMPARISON OF INDIVIDUAL TYRES The rolling resistance coefficients (RRC) for tested tyres grouped into four tyre types at the speed of 80 km/h are presented in Figure 8. In this case the coefficients measured on both road surfaces were averaged, since there was almost the same ranking of the tyres on both surfaces. Speed is usually not an important factor for the RRC. Therefore, only some tyres were measured also at 50 km/h. These are indicated in the same diagram (Figure 8) as green points in circles. Figure 9 shows the average levels within each tyre category. The two SRTT symbols show measurements in 2015 and in 2016, respectively, but those values cannot be compared with the others due to different load conditions.
Figure 8 - Rolling resistance coefficients (RRC) for the tested tyres at 80 km/h, averaged for the two road surfaces. The data measured for some tyres at 50 km/h are indicated in the same diagram as green points.
Figure 9 - Rolling resistance coefficients (RRC) at 80 km/h, averaged for the two tested road surfaces. The RRC values for tyres within each category were averaged to give a value for each category.
The following features are noteworthy: x The two speeds (50 and 80 km/h) give very similar values, albeit marginally lower at 50 km/h. x There is no clear difference between the tyre categories, perhaps except that the all seasons tyres have higher RRC than the other categories, but there is considerable overlap x There is a remarkably large range of RRC, both seen over the entire tyre selection and within each tyre category. For example, tyre WI-T1156 has almost double as high RRC as tyre WI-T1191. x With regard to tyres optimized for central Europe versus tyres optimized for northern Europe, there is no significant difference x The studded tyres as a group give about 5 % higher rolling resistance then the non-studded winter tyres, but there is such an overlap that this is not a statistically significant difference x The Green Diamond tyres are, again, among the best tyres also with regard to rolling resistance.
12. ROLLING RESISTANCE: COMPARISON OF USED AND NEW TYRES The differences between the rolling resistance coefficients measured for the four tyre pairs that were tested both in new and used condition are shown in Figure 10. Each tyre type is shown as a pair of bars – one red (tyre in new condition) and one black (tyre in used condition). The tyre pair at the right are for a studded tyre; the other three pairs are for non-studded winter tyres. It appears that two of the used tyres have approx. 10 % higher RRC than the corresponding new ones, and for the studded tyre pair, there is an increase of 13 %. These increases are high enough to justify further studies.
Figure 10 - Comparison of RRC values for tyres in new condition and similar tyres in used condition.
13. ROLLING RESISTANCE: LABELLING As for noise, the measured rolling resistance coefficient (RRC) values were also compared to the tyre label values; see Figure 11. The label values were taken from the labels attached to the tyres; in this case the energy label class (which is indicated as a certain code A, B, C….). These classes refer to a certain range of RRC . Then the medium RRC in that class for each tyre were used on the abscissa in Figure 11. The measured RRC values in this project are those that were measured at 80 km/h on the two surfaces and averaged. It must be noted that the measurements in this project have been made with a non-standardized trailer method operated on real road surfaces, while measurements for the label (by the manufacturers) were made with the laboratory drum method specified in ; thus the values are not directly comparable. The results, showing no correlation at all, must be considered as quite alarming. In this project, measurements of rolling resistance were also made on a drum facility at TUG, according to the method of ECE R117 , but these were not fully analyzed when this paper was written. The purpose was to check whether the results obtained by TUG, in full accordance with the method used for the energy label, would correspond reasonably with the actual mark on the tyre labels. The outcome of this study will be reported in a forthcoming paper or article.
Figure 11 - Measured rolling resistance coefficient (RRC) and tyre energy label values
14. DISCUSSION Compared to earlier results, it appears that the studded tyres tested here have higher noise levels relative to regular winter tyres and summer tyres than the earlier results show . Regarding the difference in noise between the central European and the Nordic designs, the results in this project confirm that the latter are somewhat quieter, but not as much as the 2 dB indicated in reference . The winter tyres with baked-in SiC granules, the so called “Green Diamond” tyres, show good performance for both noise and rolling resistance, being among the best for both parameters. Figure 12 shows a Green Diamond tyre which has been used in Sweden for two seasons. This tyre is a retreaded tyre and a relatively low cost tyre invented in Iceland. Friction measurements have not yet been made in this project, but earlier tests by VTI have shown good winter friction for this tyre.
Figure 3 - A “Green Diamond” tyre which has been used in Sweden for two seasons. The SiC granules baked into the tread rubber can be seen as lighter spots. Note that some of the granules have been lost (the darker spots).
15. CONCLUSIONS This investigation on noise and rolling resistance of various types of winter tyres, including studded tyres, is probably the most comprehensive one made so far for such tyres, allowing a quantification of noise emission and rolling resistance of winter tyres. Based on the tyre selection made in this study, the major conclusions from the results are the following: Studded tyres as a group emit substantially more noise than other tyres (summer and non-studded winter tyres); more than previously thought. However, studded tyres have approximately the same rolling resistance as non-studded winter tyres. This is somewhat surprising, as it has been assumed that the studs would increase rolling resistance. Except for the studded tyres, winter tyres are not noisier than summer tyres. The few tyres tested here in both new and used condition, all show remarkably lower noise levels for the used tyres than for the same tyres in new condition. For rolling resistance, the used tyres showed higher values; especially the studded tyre pair. Yet, these used tyres were not near the end of their lifetime. These unexpectedly high differences between new and used tyres justify specific studies of this matter, using more test tyres and exploring the reasons for the changes. A retreaded tyre – Green Diamond - with baked-in silicon carbide granules show very good performance in the tests performed here. It is also an inexpensive tyre. Therefore, the performance of this tyre should be studied more, and also for winter friction properties, vehicle dynamics and wear. There are substantial noise differences both between all the tested tyres and within each tyre category. Thus, there is a potential for reduction of road traffic noise if consumers would choose the quietest ones. For rolling resistance, the differences between tyres are amazingly large. Thus, the potential for fuel and CO 2 emission savings is considerable if consumers would choose the best tyres. However, there is no correlation at all between measured values and values indicated on the tyre labels; both for noise and rolling resistance. Consequently, it is very important and urgent to investigate what makes this correlation so poor and to take actions to correct this problem.
ACKNOWLEDGEMENTS This research is made within a project at VTI concerning winter tyres, entirely sponsored by the Swedish Transport Administration. Mr Mattias Hjort is project manager at VTI. The authors are very grateful for this financial support.
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