Measuring Performance Differences
By TA News Bureau:
At VTI, the internationally renowned Swedish National Road and Transport Research Institute, Mattias Hjort is a top researcher who is recognised as a world leader in many groundbreaking works on vehicle-road interaction. He is responsible for some of the world’s most advanced equipment for tyre friction measurements. Currently he is working closely with VTI´s world-class driving simulator, leading projects that range from vehicle dynamics assessments to pure behavioural sciences. In this interview to Tyre Asia, he elaborates on some of the cutting-edge technologies and new methodologies for carrying out and analysing a large number of tyre-related measurements. He has written several applications, and been a project leader for both small and large projects on tyre measurements where there is involvement of researchers with different skills and areas of expertise. Excerpts from the interview with Hjort
What are the major findings of your study on how the asphalt grip during winter conditions for different types of winter tyres degrade by age?
Just to clarify, our study has not specifically looked at age degradation. The purpose of the study was to investigate whether there were any performance differences between different types of winter tyres when they have become worn and aged, and then compare the performance of the used tyres to the performance of new tyres. In total, 50 used and 27 new winter tyres were tested with respect to ice, snow and wet asphalt grip. For the asphalt tests, four new and four used summer tyres were also included for comparison. The used winter tyres were all carefully selected to be representative for a worn winter tyre with a tread depth between 5 and 7 mm, and were 5 years old on average.
The winter tyres in Sweden can be divided into three main types: studded winter tyres, unstudded winter tyres made for Nordic conditions, and unstudded winter tyres made for central European conditions. The results show that with respect to grip on smooth ice, the studded tyres have a larger performance decrease when worn than the unstudded winter tyres. However, the ice grip of worn studded tyres was still clearly superior to that of worn unstudded tyres. The worn unstudded tyres of European type had in general a very poor ice grip. On snow, the unstudded winter tyres of Nordic type in general had a slightly better grip than the studded tyres, while the unstudded tyres of European type performed worse. The snow grip for the worn European tyres was significantly worse than worn tyres of the other two types.
The asphalt tests were conducted on wet, salted asphalt road at freezing temperatures. This is a condition where European winter tyres generally perform better than the two other types of winter tyres. While the worn European tyres had lower performance compared to the new ones, they still had a performance advantage compared to Nordic or studded winter tyres, both new and worn. The Nordic and studded tyres had in general the same road grip on the wet, salted asphalt surface, and the differences between new and worn tyres for these two tyre types were for most of the investigated performance measured negligible.
Interestingly, although the asphalt tests were performed in temperatures around 0°C, the tested summer tyres as a group performed as good as or slightly better, than the European winter tyres. This, of course, does not suggest that summer tyres should be used in these conditions, as the risk of ice is apparent at these temperatures.
What are the differences that you have noticed in the use of studded and unstudded winter tyres with regard to their age?
To get a picture of the condition of the winter tyres used in Sweden, we visited and collected data from a few ‘tyre hotels’. Tyre hotels are facilities where people can store their winter tyres during summer, and are used by many Swedes. The data we got indicates that the studded winter tyres in service on average seems to be slightly older than the unstudded ones, 4.35 years compared to 3.75 years.
How was the performance of studded budget tyres with regard to braking and steering?
In general the premium tyres in our tests had better ice and snow performance compared to the budget tyres, for both new and used tyres. Looking at a specific tyre type it is important to bear in mind that dividing the tyres into
smaller subgroups always make the statistics more uncertain. That being said, comparing the results of the studded budget and premium tyres in our tests, the premium tyres had a performance advantage on both ice and snow. On ice, this was true for both braking and steering, while on snow it was mostly true for steering. On the wet asphalt we could not see any differences.
Can you elaborate on your observation that the summer tyres perform as good as or better than the European winter tyres?
This was quite surprising to us, considering the test conditions. The tests were performed during a week when both air and road surface temperature was in the range from a few minus to a few plus degrees Celsius. It has often been said that summer tyres perform considerably worse when the temperature gets below +7 degrees Celsius, due to rubber beginning to harden. We do not want to draw any general conclusions from this test, since the number of summer tyres were quite small (only eight) and not as carefully selected as the winter tyres. It is also possible that the relative performance of different type of tyres may depend on the asphalt type of the test track. The asphalt on the used track is representative for a typical Swedish rural road.