Tech Focus: Tyres Everything you’ve ever wanted to know about tyres but were afraid to ask… Performance Tyres. As the only contact patch between your car and the road, tyres are the single most important area to upgrade on any car, so let’s find out some more about them and how they work… Words: Jamie King.
Never underestimate the importance of good rubber. There’s absolutely no point in having a performance car if you can’t transfer any of that performance onto the road. As the only part of the car in contact with the road, tyres have a direct impact on all areas of a car’s potential ability. The rubber you choose affects acceleration, braking, and cornering, so you need to choose wisely.
However, life span, excessive road noise, and performance in the wet are also affected by the tyres your car sits on. Therefore choosing the right rubber for your vehicle is a game of compromise, as one characteristic invariably plays off against another. Get it wrong and the car will be all but undriveable in certain situations. Get it right, though, and your car will do exactly what you want it to.
There’s loads of information readily available about tyres, their performance, and why they do what they do, but this can all get a bit confusing – and sometimes even overwhelming. Luckily for you we’ve asked the experts to explain what’s what, and here we bring you the most important factors to be aware of when choosing the next set of boots for your BMW…
How a tyre is made
In order to understand how a tyre works, let’s go back to basics and look at how one is actually put together. Almost all radial road tyres are constructed in more or less the same way, with the major differences being the compounds of rubbers used and the final tread pattern design.
The process starts with steel beads, and the whole tyre is built from the beads upwards. They are encapsulated in rubber which forms the tyre bead as we know it, and provides a starting point for the rest of the tyre to built upon. The next step is the tyre liner, which is a non-porous, nylon-reinforced rubber used to form the basic shape of the tyre. It creates an airtight seal between the beads so the tyres can be inflated, hence needing to be made from a non-porous material to prevent moisture in the air escaping and therefore deflating the tyre.
On top of the liner the tyre body plies are added. These help give the tyre its shape and are made from nylon-reinforced rubbers. The number of body plies depends on the application of the tyre, but a typical road tyre will feature two or three of these nylon plies. Next come the steel belts. These are steel reinforced sheets of rubber that give the tyre its strength and durability. The number and type of steel belts used will affect the overall tyre’s load and speed ratings. Once again the number of steel belts used depends on the application, but typically you will find two or three layers in most road tyres. The blend of plies and steel belts is then capped of with a final layer of nylon-reinforced rubber, before the tread cap is applied. The tread cap is the final, thicker layer of rubber that will eventually form the tread pattern of the tyre. The compounds of rubbers used in the tread caps are closely guarded trade secrets, and will differ from tyre to tyre.
At this stage the tyre is still ‘green’, and the layers of soft, sticky, pliable rubber need to be cured before they can be used on the road. The curing process involves applying both heat and pressure to the tyres in a specific tyre curing press. This causes all the layers to bind together to create what is effectively a rubber laminate, and cures and hardens the rubber to give the desired characteristics. The curing press also indents the specific tread pattern design into the outer tread cap layer, and also indents all the required sidewall markings at the same time.
The specific compounds used for different tyres will vary dramatically. Tyre manufacturers invest fortunes in developing different tyre compounds which meet specific criteria and, as such, the exact ‘recipe’ for each compound is always a closely guarded secret.
There are hundreds, if not thousands, of different tyre compounds in production and there are a vast number of different elements that go into each compound to give its specific characteristics. In fact, a typical road tyre will contain a number of different rubber compounds, each designed to do a specific job. Some will help keep the air within the tyre, others will allow the sidewall to flex, and others will help bind the nylons and steel belts together. However, the compound most people refer to is the tread compound, as this is the part of the tyre in contact with the road. Therefore it is this compound that will have the greatest affect on a tyre’s performance. The tread compound itself is also made of various different elements, including complicated polymers and occasionally trace amounts of silica. These help a tyre’s resistance to abrasion, tears, or cuts, and also prevent the tyre from losing its integrity and going very soft and almost goo-like.
However, the tread compound consists of three main ingredients: natural rubber, carbons, and oils. As a general rule of thumb, the more natural rubber in the blend, the more grip it will offer. However, the trade-off for this is longevity; a softer, grippier tyre will wear much faster than a harder compound. When you hear people talking about ‘hardness’ and ‘softness’ of tyres they are not referring to how hard the tyre is to the touch but about how rubber molecules in the tyre interact with the road.
The rubber used in tyres goes through a process called vulcanising. Basically this is a process that converts natural rubbers and polymers into a more durable material by adding elements such as sulphur. The addition of sulphur creates crosslinks between the polymer chains and rubber molecules, which has the effect of making the material less sticky but more durable.
One of the main differences between harder and softer compounds is due to how the addition of sulphur affects the rubber molecules. In a softer compound (which we already know features a greater percentage of natural rubber) the crosslinks caused by the vulcanisation process are fewer in number. This means the rubber molecules and polymer chains are longer between crosslinks than in a harder compound. The longer the rubber molecules are, the more they can interact with the road surface, thus giving grip. In a harder tyre compound the rubber molecules are more restricted and are unable to interact with the road surface as much, therefore giving less grip.
However, as we have mentioned, a softer tyre will wear more quickly than a harder tyre. It is the aforementioned movement of the rubber molecules and the friction they generate that provides the grip we need to accelerate, brake, and corner. But at the same time this friction, by its very nature, causes the tyre to wear. So ensure you choose a compromise between the levels of grip you desire and the amount of time you want these grip levels for.
Aside from the compounds used, the second biggest difference in tyre design is the tread pattern used. There are thousands of tread pattern designs already available and as tyre research and development continues, manufactures are constantly producing new styles.
In theory, on smooth dry roads a slick tyre will offer the best performance because it has a greater surface area in contact with the road. However, our roads are seldom totally dry and, unlike race circuits, are not what you can really call ‘smooth’. Therefore (as well as the compound needing to be harder than that of race tyres) the tread pattern needs to be different, and must have the ability to clear water from under the tyre in wet conditions while still retaining as much dry performance as possible. The tread pattern designs available for road tyres can be easily split into three main categories: symmetrical; asymmetrical; and directional.
Starting with the most simple, a symmetrical design is exactly as it suggests: the same pattern across the whole tyre. This means that the tyre can be fitted to any of the four wheels, and has no specific fitting instructions. This style is therefore normally only used in lower end, budget tyres where keeping costs low is essential.
An asymmetrical tyre has a tread pattern that differs across the width of the tyre. This means that the tyre has an inside and an outside edge, and needs to be fitted accordingly. Asymmetrical tread patterns offer a couple of advantages over symmetrical designs. The first of which is that the outer edge and shoulder area of the tyre usually features larger, stiffer tread blocks to help with cornering stability. This is because the outer edge of the tyre is subjected to higher loads than the inside when cornering hard. Asymmetrical patterns also tend to have an inner edge which features more tightly packed and more aggressive grooves. This helps with water dispersion and gives better performance and grip in wet conditions. Finally, asymmetrical designs can also feature a fairly wide continuous centre rib that aids high speed straight line stability.
Directional tread patterns differ again from both symmetrical and asymmetrical designs. They are symmetrical across the width of the tyre (meaning they have no specific inside or outside edge) but have a design that only works in one direction of rotation (hence the name). This means the tyres can only be fitted to a pair of wheels (either left or right handed) and need to be turned around before fitting to the opposite pair.
Directional tyres offer different performance advantages to asymmetrical patterns, the main one being wet weather performance. The aggressive nature and number of grooves typically used on a directional pattern means it offers greater water dispersion than an asymmetrical design. These tend to feature a number of deep directional grooves which literally pick the water up from the centre of the tyre and throw it out the side, giving very impressive wet grip. That’s not to say that directional tyres don’t work in the dry, in fact they perform very well in both wet and dry conditions. In the dry the aggressive grooves have no water to disperse but large tread blocks offer a greater contact patch with the road than symmetrical tyres, therefore offering more grip.
The directional grooves used on this type of tyre also help reduce road noise and straight line stability, when compared with an asymmetrical pattern. Asymmetrical and directional tread patterns can also be combined. This means the tyre has both a direction of rotation and an inner and outer face, and so needs to be fitted accordingly. These tread patterns usually feature a stiffer tread block on the outer edge and the pattern often incorporates aggressive grooves. Most performance tyres use this style of tread pattern because it offers the best combination of wet and dry weather performance.
On the subject of tread pattern designs, it is also worth noting the shoulder block area of the tyre. This is the area between the tread pattern and the sidewall. The shoulder blocks bear the brunt of the car’s lateral forces during cornering and, as such, tend to be fairly chunky in their design. This is to maximise the contact patch with the road as the tyre moves around under heavy cornering forces. The grooves between the shoulder blocks are known as sypes, and their job as well as helping to disperse water is to help keep the tyre cool.
Excessive heat (besides neglect) is the biggest killer of tyres, and adding these sypes actually promotes airflow across the tyre helping to keep the temperatures down. Also, now that all tyres are assessed and graded on the volume of road noise they produce, the shoulder blocks are tailored towards keeping the dB rating as low as possible. If the shoulder blocks were all uniform in size and shape the tyre would create an annoying hum as it rotated, increasing road noise. By altering the shape and size of the shoulder blocks, tyre manufacturers can tweak the harmonics of the tyre to help reduce the overall volume.
Many BMWs come factory-fitted with run-flat tyres. Run-flats do exactly as the name suggests and allow the car to drive (for a limited period) on a tyre with zero air pressure thanks to a heavily reinforced sidewall. In a conventional tyre the car is supported by the air pressure within the tyre (which is why tyre pressures are so important); when this air pressure falls too low the tyre will simply fail. However, a run-flat is designed so that the car can still drive, even with no air pressure in the tyres at all.
This is great if you happen to suffer a puncture as you can safely and easily drive the car to garage to get it replaced. However, as always, this comes at a cost – and not just a financial one. Many enthusiasts don’t like the way the car feels when driving on run-flats. The increased stiffness of the sidewall reduces the tyre’s level of compliance, which in turn can result in a crashy and bumpy ride and increased levels of NVH within the cabin. It can also have performance benefits, too. The less rigid sidewalls will actually move around slightly under hard cornering, which results in better grip and performance.
It’s not uncommon for enthusiasts to replace factory-fitted run-flats with conventional tyres as the extra ‘give’ in the sidewall makes for a more pleasant driving experience.
The markings stamped on the tyre sidewall contain a whole load of information about the tyre in question. Far from just showing the size, the sidewall markings contain information about the maximum speed rating, maximum load ratings, date of production, and fitting instructions, amongst many other things.
Most of the sidewall markings are not necessarily applicable to the UK market (for example there is an Inmetreo mark which denotes the tyre is safe for use in Brazil) but we have highlighted the ones that are relevant in this diagram.
MICHELIN Name of tyre manufacturer.
PILOT EXALTO Tyre product name.
205 Width (in mm) of tyre.
45 Aspect ratio in percent. Height of sidewall as a percentage of width. In this case 45 per cent of its 205mm width.
R R denotes tyre is radial.
ZR is a high performance radial. 16 Rim diameter (in inches)
83 Load index. This ranges from 50 to 150 and the figure refers to the maximum load rating of the tyre. 83 indicates a load index of 487kg.
V Speed index. This figure ranges from N to Y and refers to the maximum speed rating of the tyre. V is 149mph.
E2 ECE mark showing tyre meets Economic Commission for Europe standards.
OUTSIDE (not shown) Denotes outward facing sidewall on asymmetrical tyres.
ROTATION (not shown) Shows direction of rotation on directional tyres.
FRV X 1902 Date of manufacture. The first two numbers (19) show the week of the year, and the second two digits (02) refer to the year. In this case the 19th week of 2002.
Nowadays tyres have to be clearly labelled, by law, showing the level of performance they offer. They are assessed on rolling resistance, wet grip, and road noise. This information is then displayed in a labelling system that is visually similar to the ‘energy rating’ labels found on household appliances.
The rolling resistance score will show how economical a tyre is, and the wet weather performance indicates how good the tyre is in wet conditions. Both use a rating system that is graded from A to G, with A being the best performing and G being the worst.
The road noise is expressed as a dB figure, and the icon will show one, two, or three soundwaves. These are significant. Three bars shows that tyre meets current sound limits. Two bars indicates the tyre meets limits that are to come into effect in the future. And one bar shows that the tyre is a further 3dB below future limits.
Tyre health checks
As the only part of a car in direct contact with the road, tyres should be looked after. Yet despite endless warnings and awareness campaigns it is shocking how many tyre failures are a direct result of neglect and poor maintenance. However, if tyres are maintained correctly they should never cause any problems. Here are the top causes of premature tyre failure…
Abnormal tread wear
This is one of the most common causes of tyre failure and is usually caused by incorrect suspension geometry or brake failure. It can also be caused by simply under or over inflating the tyres.
Damage to the tyre bead is one of the rarest forms of tyre damage, but is still one to be aware of. It is usually caused by incorrect fitment, either to the wrong wheel for the tyre or just simply by bad fitting methods. The metal bead can bend or in some cases actually be broken, which can then lead to sidewall damage such as tears or deformity.
Separation of the tyre construction is thankfully not that common but can be a difficult one to spot. Bulges in the sidewall, tread face, or shoulder of the tyre are the usual indications a tyre has started to separate. Typical causes include excessive heat build up caused by under or over inflation or water entering the tyre carcass from a cut or tear already inflicted on the tyre.
Tears, cuts, bubbles, and other damage to the sidewall area of a tyre is potentially very dangerous. If spotted you should certainly replace the tyre immediately. Typical causes of cuts and tears include hazards in the road, while bubbles and scrapes to the sidewall can be caused by impact damage such as simply running over potholes and jumping over kerbs.