Choice of Appropriate Tyres As an Asset to Traffic Safety on Winter Roads – Compilation and Evaluation of Some Winter Tyre Tests in Sweden

International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: 06 1

Choice of Appropriate Tyres as an Asset to Traffic Safety on Winter Roads – Compilation and Evaluation of Some Winter Tyre Tests in Sweden

Adel Abdi 1, Björn Birgisson 2

1 Licentiate Engineer, Highway & Railway Engineering, Royal Institute of Technology – KTH, Department of Transport Science, Division of Highway and Railway Engineering Brinellvägen 23, S-100 44 Stockholm, Sweden, Tel: +46 8 790 8710, E-mail: [email protected]

2 Professor of Highway & Railway Engineering, Royal Institute of Technology – KTH, Department of Transport Science, Division of Highway and Railway Engineering, Brinellvägen 23, S-100 44 Stockholm, Sweden, Tel: +46 8 790 8702, E-mail: [email protected]

Abstract. Winter tyres must be able to cope with the slipperiness which occurs on the surface of the roads during winter. The slipperiness is generated by water vapour in the atmosphere, precipitating and transforming into a solid form i.e. in ice form, either directly or after refrigeration or cooling. Many experimental studies e.g. winter tyre tests have been carried out by many companies and authorities independently from each other to determine if there are any types of tyres which have the best grip on different road surface conditions in the winter. The purpose of this study is to compile, analyse, compare and evaluate the obtained results of some winter tyre tests which have been performed in Sweden during years 2007-2010. The study aims further to find out which types of these tyres are most appropriate as an asset and increase the traffic safety on Swedish winter roads. Both studded tyres and non-studded tyres, so- called friction tyres or all season tyres were used in the tests. The method of the study is partly based on a theoretical study by reviewing some literature in this area and furthermore studying some practical cases i.e. winter tyre tests. The results of the tests are given in tables and figures showing the function of different types of the studded and non- studded tyres on different road conditions. All test-tyres have been supplied by several tyre manufacturers. The study is only limited to compare the grip and effectiveness of different type of winter tyres on different kind of road conditions. The study does not take the impact of the tyres on the environment into consideration. The study shows that currently there are several types of winter tyres on the market but there is no tyre that is ideal or works best under all winter conditions.

Keywords: winter tyre, studded tyre, non-studded tyre, friction tyre, winter weather, winter tyre tests, road grip

1. Introduction measured i.e. the friction coefficient is a system parameter not a material parameter [4]. Sweden as a Nordic Country with its special climate has The tread of a tyre should have such a profile that very tough winters. During the winter, road conditions the water in the contact area between the tyre and the may vary completely between different regions and from road surface can quickly be channelled away thus the snow to ice, slush, wet or dry pavement across the shape and the depth of the tyre profile are two very country. According to a Swedish definition winter road important factors [3]. condition is when there is ice, snow, slush or frost on any New winter tyres are normally manufactured with at part of the road. Winter tyres must be able to cope with least 9-10 mm of tread depth and summer tyres with 7-8 the slipperiness which occurs on the surface of roads mm. On a new road surface, they can stop a car travelling during winter. On the other hand a road surface or a at about 96 km/h in less than 50 meters. However, but the pavement has not a certain friction coefficient because question is what happens with a typical car on a typical friction involves two bodies e.g. tyres and the pavement road and how confident can a driver be when the driver which are extremely variable due to the wetness of the and the cars behind and ahead need to brake quickly in an road surface, temperature and humidity, vehicle speed, emergency? tyre wear and tyre type etc. [1]. Thus friction values are The risks are many times greater if tread depths are only valid for exactly the conditions in which they are

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close to the legal limit and it is an emergency on a wet there is also some partial contact between tyres and worn-out road surface [21]. and road surface. The legal minimum tread depth for summer car-tyres • Phase 3 (marked, blue) : in this phase which is a in the European Union is 1.6 mm throughout a wet interface there is not any contact at all continuous band comprising the central three-quarters of between tyres and road surface. In this phase a the width of tread round the entire outer circumference of completely separation of the tyre tread and the the tyre. In Sweden, the law requires at least 3 mm tread road surface is obvious. The friction coefficient depth in winter road conditions during the period decreases when phase 3 increases and finally December 1 th to March 31 th . results in aquaplaning. This effect will be The purpose of this study is to compile, analyse, intensified with higher vehicle speed. compare and evaluate the obtained results of some winter tyre-tests which have been performed in Sweden during years 2007-2010. The study aims further to find out which types of these tyres are most appropriate as an asset and increase the traffic safety on Swedish winter roads. The method of the study is a review of some international reports and literature in the field of winter tyres that follows by studying of some winter tyre-tests which have been performed by different Swedish companies and authorities during years 2007-2010. In Fig. 1. Illustration of occurrence of aquaplaning this study the impact of winter tyres on the environment In many areas, winter road conditions causes disruption has not been taken into consideration. of road traffic. There are some road conditions that 2. Background reduce traction i.e. the friction between tyres and road surface. In the recent years, the use of studded tyres in some areas On slippery road surfaces, it is more difficult to steer of the bigger cities has been restricted or entirely or brake down a vehicle and it also takes longer to stop a forbidden by some local governments in Sweden. It is car under such conditions. The friction coefficient only stated that the reason behind the restriction is the increases if the speed of the vehicle is decreased. environmental impact of the emissions from such these tyres. But the question is how the restriction influences 4. Tyre wear and inflation the traffic safety during winter specially when the winter Worn tyres contribute to about one in ten accidents in wet maintenance is delayed or neglected. conditions, compared to one in fifty accidents on dry 3. Road surface condition roads. Grip on wet roads is markedly reduced when tread depth is less than 3 mm. When there is a 2 mm film of According to Barnhoorn [3] the influence of rainfall water on the road, even 2 mm of tread may give no better manifests itself as a decrease of the skidding resistance stopping friction than a bald tyre. On wet roads, the risk and risk for aquaplaning (hydroplaning) which is defined of an accident increases three-fold when the tread depth is as skidding on a wet road surface because a layer of water on the road surface causes the tyres to lose contact with at the 1.6 mm legal minimum, and it increases seven-fold when the tread depth is less than 0.5 mm. it. A combination of intensity and duration of rainfall, the According to a British investigation [21] worn tyres amount of water pooled, improper tyre inflation and worn are common and at least one in ten cars has one or more tyres and, car speed can all contribute and increase the tyres with a tread depth at or below the legal limit. Worn risk of aquaplaning. In order to incur aquaplaning (Fig. 1) three phases can be identified as follow: tyres reduce braking capability on wet roads and severely increase the risk of accidents. • Phase 1 (marked, yellow) : is a dry interface and Tyres with less than 1 mm tread depth have a brake there is fully contact between tyres and road friction on a wet road surface that is just one-third of that surface when the water layer is entirely removed. of tyres with the minimum legal tread depth of 1.6 mm. In this phase there are frictional forces between An almost similar Swedish investigation [22] shows two surfaces. that on a wet road surface, when the tyre tread is worn • Phase 2 (marked, white) : in this phase there is still below 3 mm, both braking distance and the risk of a thin layer of water between two surfaces but aquaplaning increase significantly as shown in Figure 2.

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5. Vehicle speed Generally, on a dry road surface, the influence of the speed of a vehicle on the friction coefficient is limited. On a wet road surface, the friction coefficient strongly decreases with increased vehicle speed and increased thickness of the water layer [3]. The friction coefficient will only be greater if the driver slows down or if the thickness of the water layer decreases, e.g. through a greater cross fall on the

pavement. Fig. 2. Relation between tread depth and braking distance on wet road surface [22] 6. Studded or non-studded tyres By preparing a vehicle with appropriate tyres, winter road Different tread depth on front and rear tyres create traffic can be made safer. handling problem. Vehicle handling is also affected when According to Scheibe [12] the performance of the tread depth is significantly greater on the front tyres studded tyres versus non-studded tyres can be measured than the rear tyres. in several ways. Some parameters for measuring can be Improper inflation or tyre imbalance is also a factor as follow: that creates tyre wear, handling problems and increases • Braking in straight line traffic accident risks. Under inflated tyres lead to • Acceleration excessive tyre wear i.e. more wears on the outside than • Driving in the curves on the inside of the tyres. According to the above named • Driving uphill investigations under inflation can also cause vehicle • handling problems and tyre life decreases by up to 10 per Controllability cent for every 10 per cent under inflation. Under inflated As Scheibe [12] shows the measuring results are tyres also reduce the riding comfort in vehicle handling influenced by a number of different variables as follow: and are prone to wearing out due to high temperature • The initial speed for braking tests caused by increased friction. Under inflated tyres cause • Tyre type uneven wear and even more consumption of fuel due to • Number of studded tyres rolling resistance. Over inflated tyres are more prone to • uneven wear and damage from any kind of road hazards. Car type • Over inflation also causes tyres to wear more in the Car with front-wheel drive, rear wheel drive or centre of the tread. Over inflated tyres can lead to poor four wheel drive system • vehicle handling on the road and make the car ride Vehicle weight distribution uneven, irregular and harsh. Tyres with proper inflation • Type of the brake system i.e. Antilock Brake give the best combination of riding comfort and tyre life System (ABS) or non-ABS and increases safety in traffic. Even aquaplaning is more • The structural type and condition of the pavement likely if tyre pressure is low or the tread is worn. Figure 3 • Road condition i.e. dry, wet, loose snow, packed illustrates how a tyre looks like with under inflation, over snow, smooth ice, stud-roughened ice inflation and correct inflation. • Above freezing, near freezing and/or below freezing temperature (both air and road surface)

Scheibe [12] concludes that, studded tyres produce their best traction on snow or ice near the freezing point and lose proportionately more of their traction capability at lower temperatures than the other types of tyres [20]. Non-studded (All-season) tyres are not a substitute for winter tyres. A set of four matched severe snow condition rated winter tyres (studded tyres) are best for winter driving. Winter tyres are specially designed with tread patterns and rubber compounds to deliver superior performance in cold weather or snow. In Richter’s test Fig. 3. Appearance of a tyre with different inflations conditions [8], every brand of studded tyres tested

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performed significantly better than the all-season tyres tram lining, hydroplaning on accumulated water in the [7]. The traction of studded tyres is slightly superior to ruts, excessive road spray, and premature damage to non-studded tyres only under an ever-narrowing set of pavement markings. Studded tyres increase fuel circumstances. With less lightweight studs being consumption by a small amount over non-studded tyres mandated and with the advent of the new non-studded on bare roadways but the other effects of unevenness, tyres the traction benefit for studded tyres is primarily snow and ice are far more significant than this factor and evident on clear ice near the freezing point, a less can increase fuel consumption by 15 Per cent [11]. frequent condition. The precise environmental conditions 7. Winter tyre tests performed in Sweden under which studded tyres provide a traction benefit are relatively rare [12]. A good tyre is a good asset. It is not possible to produce a The maximum frictional gain in comparison to non- tyre with the best grip on every type of road. Therefore, studded is found for new studded tyres on smooth ice there are summer tyres and winter tyres. Winter tyres where they have been shown to provide up to 100 per may be with or without studs [14]. Therefore, choosing cent gain in certain tests. However, the relative frictional winter tyres will not be so easy. Winter tyres are defined gain of studded tyres diminishes or becomes negative on in Sweden as: Tyres especially designed for winter use roughened ice as the temperature drops and as the studs and marked M+S, M.S., M&S, M-S, MS or Mud and wears. Traction performance can be characterized in Snow. The requirement for winter tyres or similar many ways including braking, acceleration, cornering, equipment applies to private cars, motor-caravans with a controllability and grade climbing. Even though all total weight of max 3.5 tonnes, light Lorries, light buses factors are important, the single best indicator of tyre and trailers drawn by these vehicles. Lorries, buses and performance is braking distance and deceleration. motor-caravans with a total weight in excess of 3.5 Studded tyres reduce the difference in the friction factor tonnes must have tyres that have tyre tread depth of at between optimum-slip and locked-wheel braking in least 5 mm. This requirement applies to all tyres on the comparison to non-studded tyres. This may reduce the vehicle, but not to trailers attached to the vehicle risk of drivers misjudging the necessary braking distance (Swedish National Road and Transport Research Institute and may improve the braking potential for antilock – VTI) [18]-[16]. brakes. In another braking distance test in Alaska, Is a studded or non-studded tyre better for winter studded, non-studded and other tyres performed nearly conditions? This question does not have a simple answer. equally on snow when averaged across several vehicles. From a theoretical point of view, studs on winter tyres On ice, the stopping distances for studded tyres were 15 should provide a higher coefficient of wheel cohesion per cent shorter than for non-studded tyres. In another set with the road, which is especially important in icy of tests in Alaska, non-studded tyres offered the best conditions. But sometimes winter tyres without studs act traction performance, especially for braking on both better on slippery roads than studded tyres do. packed snow and ice in comparison to studded tyres According to an investigation by Pecherski [7], the (Scandinavian Tire and Rim Organization – STRO, 2008) cohesion coefficient is heightened in icy conditions with [11]-[12]. The use of two studded tyres on the front of a the help of studs. The more studs there are on a tyre, the vehicle produced stopping traction results on snow and better a car will act i.e. the braking distance decreases and ice that were about halfway between the results for four the probability of thrashing will be less. Studs should not studded tyres. On bare pavement, studded tyres tend to be in line but randomly spread on the tyre surface for have poorer traction performance than other tyre types higher effectiveness. [11] (STRO 2008; Swedish National Road When driving in deep snow, car controllability Administration – SNRA 2003) [16]. This is especially depends not only on the presence of studs but also on the true for concrete. For asphalt, there is little difference in tread pattern of tyres. Usually the most effective are the stopping distance between studded and non-studded winter tyres with a majority of rectangular blocks, set in a tyres. Traction performance of studded tyres is sensitive staggered order. Cohesion with the road is also affected to stud wear. Studded tyres may lose more of their by the presence of small strips on the tread. They improve traction capability over time (from stud wear) than non- the cohesion of studded as well as non-studded tyres. If studded tyres. When stud protrusion decreases to 0.6 mm, the snow on the road is wet or there is no snow at all, the frictional effect from the studs becomes negligible. then non-studded winter tyres can be an advantage [7]. Tyre tread wear on studded tyres has relatively little Year 2007, a winter tyre test was implemented by a frictional effect if stud protrusion is maintained at 1.0-1.1 Swedish company [2] to determine acceleration time, mm. As Scheibe [12] concludes, pavement rutting caused braking distance and road grip. The test was performed by studded tyres can cause such dangerous conditions as on a prepared ice path. The acceleration time indicates

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the longitudinal acceleration capacity of the vehicle and the start as quickly as possible to 60 km/h. When the the braking distance indicates the maximum stopping vehicle reached a speed of just 60 km/h, the driver effectiveness of the vehicle. The length of the test path applied the brake to decelerate as quickly as possible in a was 1 km. In this test, four identical winter tyre types straight path to stop. The result of the test and the with a dimension of 195/65-15 were supplied by different comparison between the two tyre types are as shown in manufactures. The test-tyres were both studded and non- table 1. To avoid the name of the tyre suppliers and studded. All the test-tyres had been test run for 250 km manufacturers, the letter Mn is used in the following on bare ground. In this test, the driver accelerated from tables (n= 1, 2... stands for different manufacturers).

Table 1. Compilation of the studded and non-studded tyre test results, year 2007 [2]

Studded tyres Non-studded tyres Manufacturer Mean Braking Mean Time on acceleration Driving Time on Braking Driving Distance acceleration path time time uphill path distance time uphill m time Sec. Sec. Sec. Sec. m Sec. Sec.

M1 79.61 37.3 11.58 22.43 80.79 36.6 11.48 21.87

M2 80.46 38.6 11.78 22.43 79.58 37.0 11.58 21.84

M3 81.05 36.5 11.85 21.94 82.53 39.0 12.13 21.94

M4 83.09 39.1 12.28 22.89 82.53 38.3 11.88 22.08

Driving time uphill slope in the above table means that supplied by different manufacturers were tested and the the car must drive uphill on a hard-packed snow surface results were compared with each other. The results of the as quickly as possible. test from 2007 are compiled in table 2. The same test was In the same year (2007), a similar but more detailed repeated in 2008 and 2009 with almost similar results. winter tyre test was performed by another Swedish After three years, all the tyre tests were compared company (the test was performed independently from the with each other and when all the test results were first one) to determine the same parameters as the compiled, generally non-studded tyres would be placed in previous test. In this tyre test, seven identical studded and second place after studded tyres [13]. seven identical non-studded winter tyres which were

Table 2. Compilation of the studded and non-studded tyre test results, year 2007 [13]

Studded tyres Non-studded tyres

Manufacturer Surface M1 M2 M3 M4 M5 M6 M7 M1 M2 M3 M4 M5 M6 M7 condition

Ice Braking/stopping distance in m, from 50 to 0 km/h and acceleration time in s, from 5 to 20 km/h Braking distance 54.5 56.5 56.0 52.5 53.0 58.5 56.5 67.0 64.5 64.5 65.5 65.5 68.0 68.5 Acceleration time 5.6 6.0 5.8 5.4 5.6 6.1 5.6 7.0 6.6 6.7 6.8 6.9 6.8 6.9 Snow Braking/stopping distance in m, from 80 to 0 km/h and acceleration time in s, from 5 to 30 km/h Braking distance 63.5 61.5 62.0 60.5 60.5 63.0 63.0 61.5 60.5 60.0 62.5 61.5 62.0 61.5 Acceleration time 5.4 5.4 5.3 5.3 5.1 5.4 5.5 5.3 5.2 5.2 5.4 5.3 5.4 5.4 Slush Grip is estimated through acceleration on slush on a scale from 1 to 10 Snow and mud 9 9 9 9 9 8 8 7 7 7 8 8 7 7 Wet asphalt Braking distance in m, from 80 to 0 km/h Braking distance 38.1 35.6 35.4 35.7 39.4 35.6 34.6 40.2 41.8 42.2 39.2 41.6 44.3 43.7

In another performed winter tyre test that was entirely is just studded tyres that provide an acceptable grip on the independent from the other tests, Rönnblom [9]-[10] road surface. The test results on both studded and non- concludes that on icy roads with temperature near zero, it

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studded tested tyres are compiled in table 3.

Table 3. Compilation of the studded and non-studded tyre test results, year 2010 [9]-[10] Studded tyres Non-studded tyres Manufacturer

M M M M M M M M M M M M M M Surface 1 2 3 4 5 6 7 1 2 3 4 5 6 7 condition Braking distance ( m) 45-5 km/h, acceleration time (s) Braking distance ( m) 40-5 km/h, acceleration time (s) Snow 0-45 km/h, driving duration (s) 10-40 km/h, driving duration (s) Braking distance 42.5 43.0 36.3 39.5 39.9 37.3 36.8 14.3 13.8 13.2 14.0 13.9 14.1 15.3 Acceleration time 7.2 7.0 7.3 7.4 7.2 6.9 7.4 4.2 4.2 4.0 4.1 4.1 4.1 4.7 Driving duration 162.8 161.2 163.5 166.3 163.1 163.1 165.8 84.1 79.4 78.0 80.4 77.0 78.1 79.0 Braking distance (m) 30-5 km/h, acceleration time (s) Braking distance (m) 35-5 km/h, acceleration time (s) Ice 0-30 km/h, driving duration (s) 5-35 km/h, driving duration (s) Braking distance 23.6 24.9 26.7 24.3 23.0 26.7 28.2 28.5 21.2 23.6 24.9 22.9 25.8 23.4 Acceleration time 7.3 7.8 8.9 7.9 7.0 8.2 9.1 9.5 8.5 7.6 8.1 7.3 7.9 6.9 Straight driving 57.4 60.1 62.9 59.7 59.6 59.8 63.2 104.7 89.6 88.9 94.0 86.8 93.0 86.8 Circle driving 49.9 48.6 51.0 49.2 48.5 49.5 11.2 No test was performed in this category Slush The road grip is estimated through acceleration on slush on a scale of 1 to 5 (5=best, 1=worst) Snow and mud 4 4 3 2 4 3 3 4 5 3 2 5 4 4 Wet surface Braking distance (m) 80 -5 km/h, driving duration (s) Braking distance 42.4 40.7 36.8 39.8 38.0 38.8 36.4 34.6 41.6 37.3 40.6 41.9 35.5 39.2 Straight driving 45.6 45.0 44.2 45.0 44.3 44.1 45.0 40.9 45.7 41.6 44.6 42.8 42.4 42.5

To compare the tyre types, a score scale of 1 to 5 has excellent, 4= very good, 3= good, 2= pass, 1= fail, as been used in order to classify the test results. In this score table 4 shows. scale, the higher number means the better tyre i.e. 5=

Table 4. Compilation of the score of studded and non-studded winter tyres after the tests performed year 2010 [9]-[10] Studded tyres Non-studded tyres

Manufacturer

Surface M1 M2 M3 M4 M5 M6 M7 M1 M2 M3 M4 M5 M6 M7 condition

Braking on snow 3 3 5 4 4 5 5 3 4 4 4 4 4 2 Start on snow 5 4 4 4 4 5 4 3 4 4 4 4 4 2 Driving on snow 4 4 4 4 3 3 4 2 3 4 2 5 4 3 Braking on ice 5 4 3 5 5 4 3 2 5 3 2 4 2 4 Start on ice 5 4 3 4 5 4 3 1 2 3 2 4 3 4 Circle on ice 4 5 3 4 5 4 3 No test was performed in this category Driving on ice 5 4 3 4 4 4 2 1 4 4 1 5 3 5 Slush 4 4 3 3 4 3 3 4 5 3 2 3 4 4 Braking on wet 2 3 4 3 4 4 4 5 2 4 3 4 5 4 Driving on wet 3 3 4 3 4 4 4 5 1 5 1 4 5 4 Braking on dry road 3 2 4 3 3 2 4 4 4 4 4 4 4 4 Driving on dry road 2 4 3 3 3 2 2 No test was performed in this category

In one more comparison tyre test which was both the high level of efficiency and the traffic safety performed by another Swedish company in year 2008, the enhancement of the ESP systems. test results show that studded tyres have the best grip on 8. Choice of winter tyre icy road conditions while non-studded tyres do not have the same grip on black ice. The test results also show that When choosing winter tyres there are several options. if some car-users drive with studded tyres on icy roads Winter tyres are an important safety detail. For best tyre the ice on the road surface will be roughened and makes performance it is important to make the choice based on the driving conditions a driver encounters. The choice of the grip better for cars with non-studded tyres [6]. winter tyre has an effect on both traffic safety and the The result of a lately study on winter tyres environment. Summer tyres that are used in the winter are performed by Swedish Transport Administration [19] never an alternative to winter tyres. The rubber shows that studded tyres reduce the risk of fatal accidents compound in summer tyres is adapted to summer by 42%. If studded tyres are combined with Electronic conditions, which makes the rubber harder in cold Stability Programme (ESP) on the cars, these risks are weather. reduced by 71%. It seems very probable with respect to

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In Europe, there are generally three different types studded tyres for middle European conditions are better. of winter tyres: A one more so-called comparison winter tyre-test • Studded tyres which has been performed by VTI, 2008 [18] and has • Non-studded tyres for Nordic conditions (marked N) been almost similar to STRO’s test [11] shows that • Non-studded tyres for middle European conditions (marked E) regardless of road condition studded winter tyres as a group are clearly better than non-studded winter tyres. To help car-users in choosing winter tyres according to The friction difference is also highest on wet ice and the three above named European tyre types, a test has lowest on rough ice. VTI [18] concludes that studs been performed by STRO, 2008 [11] comparing some contribute to the surface wear and it will be visible most studded and non-studded winter tyres. The test results are compiled in the table 5 and figure 4. To compare these clearly in the track formed in the roadway. Stud wear three tyre types, a score scale of 1 to 5 has been used results in the surface being roughened, which gives better (STRO has originally used one to five asterisks [*] to road surface friction and better grip under wet road classify the test results. The author has changed it into conditions. Snow and ice surfaces can also be roughened numbers to simplify the comparison). In this score scale, and even be worn away by the studded tyres. the higher number means the better tyre in winter road In Norway, there have also been studies similar to conditions. Swedish studies that show that the percentage of vehicles Table 5. Comparison between studded and non-studded tyres with studded tyres on the roads should be at least 50 per performed by STRO 2008 cent to improve grip on ice because the road surface will Non- Non- Road Studded be rougher [18]. studded studded condition tyre tyre, N tyre, E 9. Result Wet ice 5 4 2 Rough ice 5 5 3 Based on the compilation and analysis of all the Hard snow 4 4 3 implemented tyre tests, the study shows that currently Soft snow 4 5 3 several types of winter tyres are manufactured but there is Wet asphalt 3 2 5 no tyre that is ideal or works best under all winter Dry asphalt 4 3 5 conditions. Studded tyres (Fig. 5) are categorically or as a group best to have on the cars when there is both snow Studded tyres Non-studded tyres, N and black ice. If it is combined with ESP, the safety in Non-studded tyres, E traffic will increase even more. Non-studded winter tyres for Nordic conditions have comparable performance with 6 5 studded tyres in winter conditions except on ice. 4 All the tyre tests show the similar results year after 3 year that studded tyres are superior to non-studded tyres 2 1 even when applying the brakes on dry roads. On packed 0

ScoreScale snow and ice there is a very big difference between tyres. Those who want the best winter tyres as possible should use tyres on their cars based on their own needs.

Mean Score Road condition 4.2 3.8 3.5 Fig. 4. Comparison between studded and non-studded tyres for different road conditions

STRO (2008) [11] concludes that there is no perfect tyre for all driving conditions. Motorists have to make their own choice based on their own needs. Studded Non-studded Non-studded Studded tyres work best on ice. Non-studded winter tyres tyres, N tyres, E tyres can be an advantage for the car-users who do not mostly drive on roads with winter conditions. The Fig. 5. The mean score of studded and non-studded tyres performance of Non-studded tyres for Nordic conditions can be compared with other studded tyres in winter conditions, except on ice. On wet and dry roads, non-

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10. Conclusion and recommendations 3- Barnhoorn, I. “Road Surface Properties”, Chapter 8, Delft University of Technology, Delft, Holland, 2004, pp 231- Almost all tests which are presented annually agree that 234. the tyres that are manufactured specially to the Nordic 4- Jacobson, S. Hogmark, S. “Tribology: Friction, Lubrication, weather conditions are by far the best. The use of Wear”, Uppsala University, Uppsala, Sweden, 2005. appropriate tyres during the winter is very important. 5- Lacombe, J. “Tire Model for Simulations of Vehicle Motion Currently there are several types of winter tyres available on High and Low Friction Road Surfaces”, U.S. Army but there is no tyre that is ideal or works best under all Engineer Research and Development Centre, Hanover, winter road conditions. Studded tyres are best on ice. USA, 2000. Most drivers probably feel safest with studded tyres, 6- Motormännen Motor Journal.”Take It Cold”, Vol. 8, No. 8, Winter Tyre Test Report 2008, Sweden, 2008, pp. 26-29. which have better road grip. Non-studded winter tyres 7- Pecherski, V. “Which Snow Tyre to Choose”, 2007. that are designed for Nordic conditions have generally the Available: http://newarticlesfree.com/articles/cars/spare- same qualities as studded tyres in winter conditions, parts/which-snow-tyres-to-choose.html except on ice. It is not possible to manufacture a tyre 8- Richter, B. “Equipment Testing and Evaluation Program: that has a perfect grip on all kinds of road conditions. Winter Tire Testing 2004”, Anchorage, Alaska, USA, 2004. Two factors influencing a driver’s safety in winter 9- Rönnblom, E. “Bear Claw Gives the Best Grip in the Ice”, driving are, to know how to drive in adverse conditions Studded Tyre Test Report, October 2010, “Vi Bilägare”, and the safety equipment of the car. The car should be Swedish Car Journal, No.14, Stockholm, Sweden, 2010, pp equipped with ABS system and tyres with sufficient tread 24-29. 10- Rönnblom, E. “Different Road Conditions, Tough depth, which is especially important for cars with non- Challenge”, Non-studded Tyre Test Report, October 2010, studded winter tyres. According to the law, a tyre must “Vi Bilägare”, Swedish Car Journal, No.14, Stockholm, have at least 3 mm tread depth but for a good road grip at Sweden, 2010, pp 22-25. least 4-5 mm tread depth is required. Driving behaviour is 11- Scandinavian Tire and Rim Organization – STRO, “Winter equally important. Driving on winter roads is very Tyre Choice”, Uppsala, Sweden, 2008. different than driving on dry roadways. To increase 12- Scheibe, R. R. “An Overview of Studded and Studless Tire traffic safety on wet roadways drivers have to consider to Traction and Safety”, University of Washington, Seattle, the demands that the car-user, the vehicle and the tyres Washington, USA, 2002, pp x-xii, pp 8-9. have to cope with. A safe driver needs to adjust driving 13- Ström, M. “Grip the Winter”, Winter Tyre Test Report 2007, “Automotor & Sport”, Swedish Motor Journal, speed to the road surface during the winter, and to keep a No.21, Sweden, 2007, pp 51-58. suitable distance to the car ahead. Other factors that have 14- Swedish Information Council for Tyre Industry. to be considered that affect a car’s driving behaviour are “Investigation of Tyre Type and Tread Depth in Sweden”, the quality of the tyres and their air pressure. Stockholm, Sweden, 2002. Finally, technical development in the tyre industry is 15- Swedish Information Council for Tyre Industry. a continuous process. It is clearly visible in the test “Investigation of Tyre Type and Tread Depth in Sweden”, results. Newer winter tyres have much better performance Stockholm, Sweden, 2008. and increase the traffic safety on the winter roads than the 16- Swedish National Road Administration – SNRS. tyres which are at the end of their life cycle regardless if ”Investigation of Tyre Type and Tread Depth in Sweden”, Publication 2009:41, Borlänge, Sweden, 2009. they are studded or non-studded. 17- Swedish National Road Administration. Regulation no. Acknowledgement VVFS 2003:22, 3§ , Borlänge, Sweden, 2003. 18- Swedish National Road and Transport Research Institute – This study which is a part of a research project in the VTI. “Studded or Non-studded?” Linköping, Sweden, 2008. field of “Efficient Winter Road Operation and 19- Swedish Transport Administration. “The Effect of Studded Maintenance” at the Royal Institute of Technology – Tyres in Fatal Accidents and the Benefits of Electronic KTH, Stockholm, Sweden has been financially supported Stability Programme – ESP”, Borlänge, Sweden, 2010. by Swedish Transport Administration. The financial 20- Swedish Transport Agency. "Winter Tyres on Swedish support is gratefully acknowledged. Winter Roads”, Borlänge, Sweden, 2010. 21- The AA Motoring Trust. “Get a Grip: Tyres, Road Surfaces References and Traffic Accidents”, Hampshire, UK, 2005. 1- Abdi, A. “Key Aspects in Winter Highway Operation and 22- The Swedish Police, the Swedish Motor Vehicle Inspection Maintenance”, Technical Report, Royal Institute of Company, Scandinavian Tire and Rim Organization, the Technology – KTH, Stockholm, Sweden, 2010. National Society for Road Safety. “Tyre Raid”, Sweden, 2- Britth, H. “Three Meters Are More Than You Think”, 2006. “Motorföraren” Motor Journal, Vol. 5, No. 5, Winter Tyre Test Report 2007, Sweden, 2007, pp. 34-37.