Asymmetric tyres have differing tread patterns on the inner and outer edges of the tread to improve performance.
On the outside edge, large stiff tread blocks help with cornering. On the inside edge, smaller tread blocks are designed to shift water and improve wet grip. The middle of the tread often has a continuous rib to help straight line stability.
The words ‘inside’ and ‘outside’ on the tyre sidewall indicate which way round the tyre should be fitted.
Directional tyres are designed to rotate in one direction only which is indicated by an arrow on the tyre sidewall.
These tyres are better at dispersing water which builds up in front of the tyre. They also reduce road noise and improve directional stability.
At temperatures below 7°C, the rubber in normal tyres begins to harden, reducing grip on cold, wet roads, ice and snow.
In contrast, winter tyres stay soft and pliable at low temperatures. They retain grip, and with the help of numerous special grooves in the tread called sipes, they provide improved braking, handling and traction in all winter driving conditions. A typical summer tyre may have around 200 sipes compared to a winter tyre with around 1500.
According to Tyresafe, a winter tyre can give up to 60% better grip in cold conditions and can reduce braking distance by as much as six whole car lengths. That could make a big difference in terms of road safety.
Across Europe it is normal practice, even a legal requirement, for drivers to keep a set of summer tyres and a set of specialist winter tyres. (see the TRAVELLING ABROAD page)
In this country, winter tyres are not compulsory. In many parts of the country their use could rarely be justified. In more remote areas, where winter really takes a grip, they would make sense.
Whether to fit winter tyres is very much a personal decision depending on several factors - risk of bad weather, driving ability and confidence on snow and ice.
Winter tyres are not suitable for all year round use. Summer tyres give better performance when temperatures are higher and roads are dry, so you'll need two sets of tyres.
Winter tyres must be fitted in sets of four. Fitting only one pair will affect the balance and stability of the car.
An alternative to winter tyres are All Season Tyres. These have a high silica content for low temperature flexibility and a tread pattern somewhere between a normal summer tyre and a winter tyre.
They are rarely as good as a specialist tyre but do perform better than a summer tyre on wintry roads and do avoid the hassle and cost of swapping wheels and tyres twice a year.
When replacing tyres, the safest option is obviously to choose new tyres. Part worn tyres can be fitted but various criteria have to be met. The Motor Vehicle Tyres (Safety) Regulations 1994 set out minimum safety standards for the supply of part-worn tyres.
Under the Regulations, part-worn tyres (except retreads) should have an EC approval mark and a speed and load capacity index moulded into the sidewall at the time of manufacture. In addition, all types of part-worn tyres must be marked 'PART-WORN' in upper case letters at least 4mm high.
It is illegal to have unsafe tyres in possession for sale.
Many vehicle manufacturers today supply their new cars with a non-standard spare wheel and tyre. This is lighter and slimmer than normal to save weight and space. For that reason, there is a maximum speed limit when using it - usually 50mph.
A space-saver wheel is intended for emergency purposes and should only be used as a temporary measure. Motorists should be advised to get the original car tyre repaired or replaced as soon as possible.