Choice of winter tyres has, from mainly being a matter of safety and economic costs for wearing of road pavements, during later years also become a matter of inhalable particles formed during pavement wear from studded tyres and their negative effects on public health. Further, the tyres' effects on environment and noise have been illustrated in several studies. The issue is also complicated by the fact that tyre choice effects on traffic safety have several components, including such diverging parameters as friction and behaviour. Finally all aspects have to be evaluated from a socioeconomic point of view for society to be able to decide which kind of distribution of tyre types that is the most profitable. This report is a summary of current knowledge in this complex research field.

Valet av vinterdäck har, från att i huvudsak varit en fråga om trafiksäkerhet och slitage av vägar under senare år även kommit att handla om negativa effekter av de inandningsbara partiklar som bildas då dubbdäck sliter på vägbanan och de effekter på befolkningens hälsa som dessa partiklar har. Vidare har vägslitagets inverkan på miljön genom spridning av bl.a. tungmetaller belysts liksom däckvalets betydelse för bullerproblematiken. Frågan kompliceras ytterligare av att däckvalets betydelse för trafiksäkerheten har många beståndsdelar inkluderande såväl tekniska parametrar, t.ex. friktion, som beteendevetenskapliga aspekter. I slutändan måste alla dessa aspekter vägas samman samhälls-ekonomiskt för att den för samhället mest lönsamma däckanvändningen skall kunna uppnås. Denna rapport belyser kunskapsläget inom detta komplexa problemområde.

It has been argued that the grip of winter tyres on ice and snow deteriorates with age to such an extent that winter tyres above a certain age e. g. 10 years should be forbidden to use. Little research data do however exist. This investigation has been carried out by VTI in order to get a better background for legal decisions. The results would also be of use for ordinary consumers as a guidance. This investigation that was carried out on wet smooth ice is the third of three part studies. The first was on smooth ice at -3°C and the second on rough ice at -3°C. They have been reported in VTI meddelande 923 and VTI notat 34-2003. The aim of the investigation is to investigate the correlation between ice grip of winter tyres and their age, tread depth and tread rubber hardness and for studded tyres also stud protrusion and stud force. The investigation comprises 23 studded and 42 non studded winter tyres and four summer tyres mainly of the in Sweden popular brands Gislaved, Michelin and Good Year, primarily of size 195/65R15. Most of the tyres had been driven in ordinary traffic and were manufactured from 1981 to 2003. A small number of new winter tyres were also tested. As reference some new summer tyres were tested as well. The steering and braking performance of the tyres has been measured in a special climate controlled indoor high speed flat bed test facility developed by the VTI. Very good correlation has earlier been found between results from the facility and lap time results from ice track circuit tests with cars as well as pure braking and cornering tests with cars on ice tracks. No age related deterioration of the ice grip was found for the fully studded winter tyres with stud protrusion 1 mm and higher. The ice grip deteriorated with increasing age for non studded winter tyres and studded tyres with stud protrusion less than 0,9 mm. The results also show that on wet smooth ice studded tyres with stud protrusion 1 mm and higher are much superior to non studded tyres including the specialised studless winter tyres often called "friction tyres". The differences in friction within the groups are however quite large.

It has been argued that the grip of winter tyres on ice and snow deteriorates with age to such an extent that winter tyres above a certain age e. g 10 years should be forbidden to use. Little research data do however exist. This investigation has been carried out by VTI in order to get a better background for legal decisions. The results would also be of use for ordinary consumers as a guidance. It comprises three part studies on different surfaces, wet ice, smooth ice -3ºC and rough ice -3ºC. The aim of the investigation is to investigate the correlation between ice grip of winter tyres and their age, tread depth and tread rubber hardness and for studded tyres also stud protrusion and stud force. The investigation comprises 20-23 studded and 33-43 non studded winter tyres and four summer tyres mainly of the in Sweden popular brands Gislaved, Michelin and Good Year of size 185/65-15, 195/65/-15, 185/70-14, 165-15. All were radial tyres manufactured between 1981 and 2003. Most of the tyres had been driven in ordinary traffic. A small number of new winter tyres were also tested. As reference some new summer tyres were tested as well. The steering and braking performance of the tyres has been measured in a special climate controlled indoor high speed flat bed test facility developed by the VTI. Very good correlation has earlier been found between results from the facility and lap time results from ice track circuit tests with cars as well as pure braking and cornering tests with cars on ice tracks. The new winter tyres were clearly better than the average of the older tyres. In the age range 5 to 15 years however no significant influence of age on the ice friction of winter tyres could be found regardless of if they were studded or not. The results also show that on this surface studded tyres as a group still are superior to non studded tyres including the specialised studless winter tyres often called "friction tyres". The differences in friction are largest on wet ice and smallest on rough ice. The test results from this investigation do not answer the question whether or how the performance of a stored unused tyre is reduced with time. Results from an earlier investigation by VTI showed no deterioration within three years of storage.

It has been argued that the grip of winter tyres on ice and snow deteriorates with age to such an extent that winter tyres above a certain age e.g. 10 years should be forbidden to use. Little research data do however exist. This investigation has been carried out by the Swedish National Road and Transport Research Institute (VTI) in order to get a better background for legal decisions. The results would also be of use for ordinary consumers as a guidance. This is the first of three part studies. The aim of the investigation is to investigate the correlation between ice grip of winter tyres and their age, tread depth and tread rubber hardness and for studded tyres also stud protrusion and stud force. The investigation comprises 20 studded and 33 non studded winter tyres and 4 summer tyres mainly of the in Sweden popular brands Gislaved, Michelin and Good Year of size 185/65-R15. Most of the tyres had been driven in ordinary traffic and varied in age from three (1998) to fifteen (1986) years. A small number of new winter tyres were also tested. As reference some new summer tyres were tested as well. The steering and braking performance of the tyres has been measured in a special climate controlled indoor high speed flat bed test facility developed by the VTI. The tests in this part investigation were carried out on smooth black ice near zero degrees centigrade (-3°C). Investigations on rough ice and wet ice will follow in the next few years. The new winter tyres were clearly better than the average of the older tyres. In the age range 5 to 15 years however no significant influence of age on the ice friction of winter tyres could be found regardless of if they were studded or not. The results also show that on this surface studded tyres as a group still are superior to non studded tyres including the specialised studless winter tyres often called "friction tyres". The differences in friction within the groups are however quite large. The test results from this investigation do not answer the question whether or how the performance of a stored unused tyre is reduced with time. Results from an earlier investigation by VTI is however presented in this report. The results here showed no deterioration within 3 years of storage.

Syftet med projektet, som bekostats av Vägverkets sektion Fordonsteknik och norska Vegdirektoratet, är att undersöka hur isfriktionen på skrovlig is hos vinterdäck för personbilar påverkas av ålder, slitbanehårdhet och mönsterdjup samt för dubbdäck även inverkan av dubbutstick och statisk dubbkraft. Undersökningen har omfattat 22 dubbade och 42 odubbade vinterdäck samt 4 nya sommardäck, huvudsakligen av de i Sverige vanligaste däckfabrikaten Gislaved, Good Year och Michelin storlek 195/65-15. De slitna däcken hade körts i vanlig trafik. Bland däcken med fullt mönsterdjup ingick däck från 1994, 1988 och 1982 som endast använts vid tidigare av Vägverket bekostade friktionsprov på is och våt asfalt. Ett mindre antal nya vinterdäck har också provats. Som referens har även några nya sommardäck provats. Däcken varierade i ålder från noll (2002) till 21 år (1981). Proven har utförts hösten 2002 på skrovlig is vid -3°C som komplettering till motsvarande prov på slät is hösten 2001.

The aim of this investigation is to investigate the correlation between grip on rough ice of winter tyres and their age, tread depth and tread rubber hardness and for studded tyres also stud protrusion and stud force. The investigation comprises 22 studded and 42 non studded winter tyres and 4 new summer tyres mainly of the in Sweden popular brands Gislaved, Good Year, Michelin of size 195/65 -R15. Most of the tyres had been driven in ordinary traffic and varied in age from zero (2002) to 21 (1981) years. Tyres from 1994, 1988 and 1982 only used in earlier investigations on ice and on wet asphalt were also tested. In addition a small number of new winter tyres and as reference some new summer tyres were tested as well. The tests were carried out in the autumn 2002 on rough ice near zero degrees centigrade (-3°C) as complementary tests to those on smooth ice carried out in the autumn 2001 which are published in VTI Meddelande 923.

The investigation carried out on commission by New Industries, Iceland, comprised comparative tests with a total of 10 new retreaded passenger car tyres classified M+S with the same tread pattern with and without special aggregates mixed into the tread rubber compound. The tests were carried out in the VTI tyre testing facility on smooth and roughened water covered ice at a speed of 30 km/h. Water covered ice can be regarded as one of the most dangerous road conditions. Both braking tests and steering tests were performed. The braking tests measured maximum braking friction corresponding to ideal ABS braking performance and friction with a locked wheel. The steering tests measured maximum lateral friction and friction at a 20-degree side slip angle (skid angle). The latter value represents conditions in skidding of the whole vehicle or excessive steering angle when entering a curve. The results on water covered smooth ice show that a new non studded winter tyre can get about 30% improved braking friction and about 40% improved steering friction by means of mixing the special aggregate composition into the tread rubber compound. Testing this improved tyre on water covered stud roughened ice showed that the friction for normal driving is practically the same as on smooth ice. If the vehicle gets in a 20 degree broadslide, rough ice gave up to 40 per cent improvement compared to smooth ice. Repeated tests however quickly reduced this improvement to 10 per cent. The values obtained on smooth flooded ice with the best tyre aggregate tread compound allows cautious driving while the reference tyre friction is insufficient for keeping a vehicle on the road even for a skilled driver under these very severe conditions.

I utredningen om det i Sverige ska bli ett krav på vinterdäck även för tunga fordon har följande delprojekt ingått. -• Litteraturstudie. -•Kartläggning av tunga fordons däck, dess kvalitet och utnyttjande. -•Isgrepp. -•Köregenskaper. -•Olycksstudie. -•Ekonomiska konsekvenser av ett krav. Litteraturstudien visade hur betydelsefullt det är att ha bra däck både vad avser typ och kvalitet. Projektet fortsatte med att undersöka olika däcks isgrepp. Dubbdäck hade bäst väggrepp. Sajpning (tvärgående snitt i slitbanan) bidrar till goda glidfriktionsegenskaper hos däck utan dubbar. Det var stora skillnader i friktionsegenskaper mellan bästa och sämsta odubbade däck. Stabilitet, styr- och bromsegenskaper undersöktes vid körning på ett isbelagt flygfält. Dubbade vinterdäck på alla hjul gav klart bättre bromsförmåga än odubbade vinterdäck på alla hjul. Nya vinterdäck gav något bättre bromsförmåga än halvslitna sommardäck. Väjningsförmågan vid optimal styrning skilde sig inte tydligt mellan de olika alternativen När fordon var inne på Bilprovningen för besiktning delades enkäter ut till samtliga förare av tunga fordon på vissa utvalda platser spridda över hela landet. Det framgår av enkäterna att det är på drivaxeln som man främst använder vinterdäck (cirka 60 %). Framaxeln och släpen har oftast sommardäck. Dubbdäcksanvändningen är låg. Olycksdata från Vägverket har kompletterats med bl.a. däckdata genom att sända ut enkäter till ägare av tunga fordon som varit inblandade i olyckor under vintern 1998-99. Resultaten ger inget stöd för att dra några slutsatser beträffande bättre trafiksäkerhet om vinterdäck används på is- eller snöväglag. Studierna av isgrepp och köregenskaper visar att om inte beteendet ändrades med däckinnehav skulle det bli en trafiksäkerhetsförbättring på is/snöväglag då man byter från sommardäck till vinterdäck. Detta innebär att det efter denna utredning inte finns argument för att införa ett krav på vinterdäck för tunga fordon. Däck till tunga fordon borde dock förbättras så att de får samma väggrepp som däck till lätta fordon.

The following subprojects were included in the investigation into whether the use of winter tyres on heavy vehicles should be a legal requirement in Sweden: • A literary study. • A survey of heavy vehicle tyres, their quality and use. • Gripping power on ice. • Driving properties. • An accident survey. • Economic consequences of a legal requirement. The literary study showed the importance of having good quality tyres of the right type. The project then went on to examine the gripping power on ice of a range of tyres. Studded tyres had the best gripping power. Sipes (transverse cuts in the tyre tread) contribute to good sliding friction properties in unstudded tyres. There were considerable differences in friction properties of the best and worst quality unstudded tyres. Stability, steering and braking properties were studied in test drives on an ice-covered airfield. Studded winter tyres on all wheels delivered far greater braking capacity than unstudded tyres on all wheels. New winter tyres provided much greater braking capacity than half worn summer tyres. There was no significant difference between the alternatives as regards lateral evasion maneuver capacity during optimal steering. When vehicles were undergoing inspection at the test station, questionnaires were distributed to all heavy vehicle drivers at selected locations across the country. The questionnaires indicate that winter tyres are primarily used on the wheels on the driving axel (approximately 60%). Summer tyres are most frequently used on the wheels on the front axles and trailers. The use of studded tyres is low. Accident data from the National Swedish Road Administration has been supplemented by sending out questionnaires to heavy vehicle owners involved in accidents during the winter of 1998-99. The results do not make it possible to draw any conclusions regarding improved levels of road safety should winter tyres be used in icy and snowy road conditions. Studies of gripping power on ice and driving properties show that if driver behaviour did not change with tyre selection, the change from summer to winter tyres would enhance road safety on icy and snowy roads. This means that after this investigation there can be no argument for introducing a legal requirement regarding the use of winter tyres for heavy vehicles. Heavy vehicle tyres should however be improved so that they have the same gripping power as the tyres of light vehicles.