Data Sheet

#15 Tyres and Tyre Care

#15 #15 Tyres and Tyre Care

Choosing and maintaining your tyres

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The only contact your vehicle has with the road is through its tyres. The total tyre contact area for an ordinary car is often little more than the size of an A4 sheet of paper. It is therefore essential you have the correct tyres fitted and they are inflated to the correct pressure. They must also be roadworthy and regularly inspected for damage and wear as it is through the tyres that you are able to accelerate, brake, corner and steer safely.

The type and size of tyres fitted will have been carefully specified by the vehicle manufacturer to take account of the vehicle and its loadings. The tyres should be able to grip the road surface in most weather conditions and dissipate the heat produced as they flex when moving at speed. Needless to say, heavy demands are placed on them and they are a key safety component of your vehicle.


Structure

Modern tyres typically feature a radial type structure, although the much less common cross-ply version still exists. Both terms refer to the tyre's internal structure and the two types of tyre are quite different and they behave differently under load. It is an offence to mix radial and cross-ply on the same axle.

Cross-ply tyres are no longer widely available so when a replacement is required radials may have to be fitted. However, you can only use tubeless radial-ply tyres if the wheel is one of the safety rim type, which have a hump or flat ledge on the outer bead seat.

Older caravans, over 25 years old, may not have safety rims. In these cases it may be possible to fit radial tyres with an inner tube. Not all radial-ply tyres are suitable so consult a tyre specialist.


Tread

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Most tyres supplied in the UK are a summer tyre, which are optimised for use in warm months and will perform well enough in most conditions. For those wishing to travel further afield or live in cooler parts of the UK there are also all-season and winter tyres to choose from.

An all-season tyre tries to achieve everything - offering reasonable performance in cold, snowy, warm or wet conditions - a tall order for any tyre. They have a place in the market but applications can be quite limited.

To cope with snow covered roads, the pattern is optimised to grip on the soft surface by providing open gripping edges and also to trap small amounts of the snow within the pattern. This snow then adheres to the snow on the road surface – snow grips to snow. Think of rolling a snowball over a snow covered lawn, the ball gets bigger and bigger.

The tread rubber properties are modified so that they are less sensitive to temperature change. Standard summer tyres become harder as the ambient temperature plummets, whereas a winter tyre tread rubber remains compliant at temperatures below 7C and hence keys into the road surface generating better grip.

Tyres marked M+S (but not carrying the snowflake motif) are not classed as full winter tyres. The M+S stands for mud and snow or slush, which indicates they have a traction type tread pattern and although they may give benefits when fitted on the driven axle of a car, they will offer little benefit if fitted to caravans. Note: All winter tyres that carry the snowflake motif are also marked M+S, but not all tyres marked M+S are marked with the snowflake.

In many Alpine and Scandinavian countries, tyres with at least a M+S rating are compulsory through the winter months even for temporary foreign visitors. This is also becoming an increasing trend across central Europe and there may be local laws dictating a minimum tread depth for winter tyres. The latter varies between countries but it is worth noting that a winter tyre cannot perform properly on snow once the tread is below about 4mm, so check with the local tourist board what the limits are and also whether snow chains are compulsory equipment. In simple terms the rule is that tyres must be suitable for the road and weather conditions.

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A worn out winter tyre

When you wear out a pair of tyres on a car or campervan, particularly where the powertrain is front wheel drive, consider having the new tyres fitted to the back axle and bring the older slightly worn rears to the front. This will then have your newest and better pair of tyres on the rear axle where they can control the vehicle better. This advice is now given by many tyre manufacturers and driving organisations and it is considered good practice to fit your best tyres to the back axle. Always consult the vehicle handbook though, as some manufacturer recommendations may differ from that described above, for example some all-wheel drive vehicles need the tyres to be rotated periodically to keep the wear even.

What is described as a winter tyre is one that provides higher levels of grip on ice and also on cold, wet or damp road surfaces. As a result they provide higher levels of grip on snow. Winter tyres are marked on their sidewall with a snowflake motif inside the outline of a mountain peak (pictured above). They achieve their performance by a combination of a dedicated tread pattern and specialised tread rubber type.

The tread pattern is a key part of a tyre and can be optimised in many different ways, for example to disperse water, assist in reducing rolling resistance for fuel economy or to grip on slippery or loose surfaces.

 

Load and speed

All tyres are marked with a series of codes to tell you the maximum load they can carry and their maximum speed rating.

Many vehicles are equipped with tyres with a speed rating considerably higher than the legal limits - if a speed rating is lower than the legal limit then the tyre's limit takes precedent. Note: A tyre's speed symbol is also an indicator of its performance under cornering, acceleration and braking.

Vehicle tyres are often supplied with a load carrying capacity in excess of the maximum permitted for the chassis. Rather like having a high speed rating, this is a useful surplus capacity to have. When you are looking for replacement tyres seek advice from the suppliers, and avoid going to a lower speed or load index unless you are sure the tyres can still cope with your demands. Taking a tyre beyond its weight limit is overloading which renders the vehicle illegal and potentially unsafe.

Caravans do not generally use special trailer tyres, they are often the same as those used on light commercial vehicles.

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Load and speed (continued)

The highest speed allowed for towing in this country is 60mph but can be higher in some other countries. The weight of a motorhome dictates the speed limits that should be adhered to.
Older caravans may have been originally supplied with tyres that on paper were under-rated, which is due to something known as the bonus load, where a tyre's load capacity may be increased by 10 per cent when the speed was restricted to 62mph.

If the bonus load is taken into consideration then a tyre with a load index of 80 (450kg per tyre) can carry 900kg plus ten per cent bonus load making a maximum total of 990kg - subject to a strict maximum speed of 62mph. For safety reasons the caravan industry has moved away from this practise and it's quite common to find a modern tourer running higher load capacity tyres where the maximum technically permitted laden mass (MTPLM) of the caravan does not exceed 90 per cent of the combined tyres - load capacity.