Winter Tyres

This article looks to expound the available data, explore some of the oft’ quoted myths and to establish if winter tyres really are of benefit to the average motorist.

Lotus Evora in the snow

The first and obvious thing to point out is that the tyres are the only points of contact between the vehicle and the road surface. As such they have the final, crucial influence on the dynamic performance of the vehicle: no matter how expertly tuned the anti-lock braking (ABS) and stability control (ESC/ESP) systems may be, they are only as effective as the level of grip provided by the black, round pieces of rubber! Hence the safety and mobility of the vehicle, and the occupants inside, is ultimately down to the performance of the tyres.

The primary functions of the tyre include the requirement to both transmit and react the forces generated by acceleration, braking and cornering. This can be simplified as the level of grip provided by the tyre. Often the attributes and features of the tyre required to maximise grip can be in conflict with the requirements to meet other functions including reduced rolling resistance (improved fuel economy), improved NVH (reduced noise levels), reduced mass and vehicle styling aids. In addition there is the need to meet these diverse requirements over a wide range of both climatic and road conditions, covering a wide variety of vehicle classes. So, as with many aspects of Driving Dynamics, the performance and design of the modern tyre is all about ‘managing the compromises’.Wide range of tyres

If you are in the market for replacement tyres on your car – as well as the large selection of manufacturers to choose from, you will be presented with the option of a number of different types of tyre, based on their application: all season, summer and winter. One popular misconception, especially here in the UK, is that winter tyres only provide performance benefits in snow and/or ice, and hence are often mistakenly referred to as snow tyres.   Winter tyres are also often confused with studded tyres. These misconceptions are not helped by what may be considered both confusing labelling and inadequate standardisation (more of which later). This may help to explain why there was such a low take-up of winter tyres in the UK (<1% of motorists), as the number of days where snow covers the ground is very low and hence can’t possibly justify the investment in a second set of wheels and tyres for those rare occurrences. Can it? As we will see from the data presented below, perhaps it would be more accurate and representative to term them ‘cold weather tyres’.

Map of winter tyre requirements in Europe

2WD and 4WD

Another common misconception is from drivers of the increasingly popular compact and crossover SUVs plus other 4WD vehicles, who mistakenly believe that 4 wheel drive will provide them with added safety measures when driving in adverse weather conditions and on snow and ice, hence negating the need for winter tyres. Whilst it is true that the 4WD system will provide additional traction to get you moving, and provide some benefit during cornering, it will be no better than a 2WD vehicle in braking and stopping performance.

As previously reference, it is the tyre grip that we are interested in. This grip, and hence the tyres ability to generate traction, is in particular a function of the tyre’s tread design, which in turn is defined by the chemical composition and mechanical construction. The winter tyre is actually designed to work optimally in cold, wet conditions, with or without snow or ice on the ground. In fact the deciding factor on when to change to your winter tyres should be based solely on the ambient temperature, with the threshold set at 7°C (44°F). This is why the legislation that is in place generally applies over a set period, typically November through til April, when the average day time temperatures are at or below this level. So how do the manufacturers optimise the winter tyre design to work in these conditions?

Tyre Composition

Looking first at the composition then there are some important changes required to the chemical make up of the winter tyre to enable it to provide improved performance over summer and all season designs in cold and wet conditions. The improvement comes from the winter tyre’s ability to maintain the elasticity and flexibility of the tread, at low and even extreme sub-zero temperatures. This flexibility enables the tread to adapt to the contours of the road surface, maintaining the contact and hence grip. Although manufacturers are not keen on disclosing their unique recipes it is possible to simplify down to a combination of rubbers (natural and synthetic) with additional fillers (including carbon black and silica) and additives. To maintain flexibility at low temperatures, the manufacturers increase the ratio of natural to synthetic rubber and replace carbon black with increased amounts of silica. The increased silica content also has the added benefit of both lowering the rolling resistance and improving the wet grip of the tyre. The compromise for the tyre manufacturer is to optimise the rubber flexibility at low temperatures, and hence maximise the grip, whilst minimising the increase in wear rate that may be experienced when the tyres are used in conditions above the threshold temperature.

Stopping distances of winter and summer tyres

The changes in mechanical construction of the winter tyre tread are rather more obvious to see, when compared to your normal, summer tyre (see images below). Typically the tyres have a directional pattern (making them uni-directional), with more blocks and with deeper grooves. This improves the pumping capacity of the tread, and hence reduces the chances of aqua-planing. The increase in the number of tread blocks also increases the number, and hence length of leading edges presented to the road surface. This feature not only improves the grip and traction, but has particular benefit in improving the braking performance. This is further increased by the addition of multiple ‘slits’ in the tread blocks. These so called sipes (named after their inventor John Sipe, who back in the 1920’s introduced them on the rubber soles of his shoes to improve the grip on wet floors) can increase the total length of leading edges on a winter tyre to in excess of 80 metres (even for a 16” tyre). The sipes also aid in the dispersion of water between the tyre contact patch and the road surface, further improving the grip.

An additional benefit of the increased number of tread blocks and depth is that the movement of the blocks under load generates temperature in the rubber, which helps to maintain the flexibility of the tread.  This movement also helps remove debris from the grooves.

A compromise that then has to be managed is the balance between improving traction though the increased number of tread blocks and sipes, against the loss of steering feel due to the movement of the tread relative to the sidewall of the tyre. This can give a disconcerting and disconnected feel between the steering wheel and the road surface.

To overcome this issue the manufacturers are introducing designs where the tread blocks, and sipes interlock with increasing lateral load and hence improve the steering feel especially when cornering on a dry road.

So what do these advances in composition and construction mean for the safety and stability of your vehicle when fitted with the latest winter tyre? This is probably best demonstrated with a graphical representation of the comparison in the stopping distance performance between summer and winter tyres in a variety of different conditions:

There are numerous published results for winter tyre testing available, many including handling as well as braking tests in the various conditions, and all show the same trend: as soon as the temperature drops below 7°C, on anything other than a dry road, the winter tyres offer a distinct advantage in performance.

Tyre Labelling

So, now that you’ve been convinced by the performance gains achievable, what should you be looking out for to identify the winter tyres when you go to buy them? Unfortunately this is where it gets a little less precise! As referenced earlier both current legislation and tyre labelling are not well defined, in the European Union (EU) at least, although this is about to change. Sidewall markings for winter tyres have historically shown ‘M+S’ (Mud and Snow) but although this defines what a winter tyre should look like (’tyres with a tread and structure designed to provide better handling characteristics in slush and fresh or melting snow than normal tyres. The tread of M+S tyres is in general characterised by larger tread grooves and/or lugs that are separated by larger gaps in the tread than is the case on normal tyres’) and currently defines the legal requirement of a winter tyre in Europe, it is based on a Manufacturer’s declaration with no form of testing. It also takes no consideration for the change in tread compound required to ensure operation at reduced temperatures.

New EU Legislation

To address this issue the EU will shortly implement, from November 2012, a new regulation updating the definition of a winter tyre. This is based on the current North American standard and requires the tyre to be tested using an ASTM testing procedure, and achieve a traction index equal to or greater than 110 (where 100 is a standard reference tyre performance). On achieving this rating the tyre is able to carry the ‘Snowflake-on-the-Mountain’ symbol (shown below) on the sidewall. So in summary whilst tyres marked only with the ‘M+S’ symbol may have improved performance over a summer tyre in snow and slush, they should not be taken as the definition of a winter tyre, which is indicated by the ‘Snowflake-on-the-Mountain’ symbol. Tyres bearing this symbol will provide superior winter (and snow) performance. Currently most, if not all, winter tyres are marked with both symbols.

Now that you have obtained the correct specification of winter tyre you can head off to mainland Europe where there are a variety of both regulations and recommendations governing the use of winter tyres, specific to each country. These can best be summarised in the graphic below. Mandatory use of winter tyres is also applicable in other parts of the world, notably in Canada.

Conclusion

So in conclusion it can be stated that winter tyres enhance both the safety and mobility of drivers and their vehicle in a variety of winter conditions, and not just in snow! In fact it is the prevailing ambient temperature that is the deciding factor on which is the most appropriate type of tyre to have fitted to your car. At temperatures less than 7°C, in either wet, snow or icy conditions, then winter tyres have been proven to show a marked benefit in performance over your regular summer tyre. The tyre manufacturers have achieved this through developments both in the chemical composition of the materials used and in the mechanical construction of the tyre, primarily in the design of the tread.

So now that it has been demonstrated that the tyre manufacturers have been able to manage the compromises in the design and construction of winter tyres it is down to the end user to manage the compromises of having two sets of tyres, summer and winter, but maybe that’s an article for another issue…..

Lotus Evora in the snow

 Author: Peter Studer