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.