THE advertising hoarding is well placed, overlooking the site where one of London鈥檚 main commuter arteries inexplicably narrows from three lanes to one. On a good morning the traffic creeps slowly by. On other days, drivers sit and stare at the slogan that proclaims: 鈥淪ave up to 5 per cent on every tank of fuel.鈥� But this is not an advert for cheap four-star. Instead, the poster is advertising the benefits of tyres that somehow save petrol.
So, marketing trick or motoring treat?
The benefits are real, says Fraser Kyle, who is in charge of new product design at the French tyre manufacturer Michelin, whose ad this is. And tyres of this type are certainly a commercial success story 鈥� around 14 million were manufactured last year alone. Kyle says that they reduce rolling resistance by 20 per cent. This translates into an average 5 per cent saving in fuel, he adds, although figures vary according to driving conditions. 鈥淲e have measured figures as low as 2 per cent and as high as 8 per cent,鈥� he says.
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Rolling resistance arises because tyres deform as they roll along the road, and this requires energy. If a tyre was perfectly elastic, all the mechanical energy that went into deforming it would be returned as it relaxed back into its original shape. But tyres are not perfectly elastic, and a proportion of this energy is lost as heat. Just as a rubber band heats up when it is repeatedly stretched and allowed to relax, so tyres heat up as they continually flex.
A car tyre undergoes two main types of deformation. The first flattens the tyre tread against the road, creating a large 鈥渇ootprint鈥�. A proportion of the mechanical energy required to deform the tyre at the front of the footprint is not returned when the material at the rear returns to its original shape. Instead it is converted to heat. At 100 kilometres per hour, a tyre rotates about 20 times a second, with every part of the tyre deforming and reforming once during each revolution.
The cause of rolling resistance lies deep in the chemical make-up of the tyres, where a number of processes are at work. Tyre compounds are made of complex materials in which long, chain-like polymer molecules bind together particles of soot, a form of carbon. The carbon helps make the tyre hard wearing, while the polymer molecules act as a network of tiny springs. As the tyre on a moving car deforms, the bonds joining the atoms of the polymer chains reorient themselves to accommodate the change. The material returns to its original form when these bonds spring back into shape. But they don鈥檛 respond instantly: there is a short delay before they return to their original shape, and this gives the material a property known as viscoelasticity.
This property is desirable to cope with a second kind of deformation that occurs as the tyre meets the road. Tiny irregularities in the road, perhaps no more than a few thousandths of a millimetre deep, create matching indentations in the tyre. In effect, the rough road surface creates a momentary imprint of itself in the tyre tread. The two surfaces fit together perfectly, helping the tyre to grip the road. This type of small-scale deformation occurs at a much higher frequency than the gross deformations of the tyre as it rotates 鈥� in a skid, portions of the tyre in contact with the road undergo up to a million deformations a second. The viscoelasticity of the tyre means that it stays in contact with the surface of the road, maintaining the imprint of the surface on the tread and improving its grip.
As the tyre flexes, the strong atomic bonds along the polymer chains remain intact, even though the molecule flexes. However, the much weaker van der Waals forces that link the polymer molecules to the soot particles are easily broken during deformation. The polymer chains and soot particles then slide over each other, until the polymer鈥檚 atomic bonds pull the tyre back into shape. 鈥淚t is this process that causes the material to heat up,鈥� explains Peter Roch, chief chemist at the American tyre manufacturer Goodyear, which also makes tyres with low rolling resistance.
The challenge to tyre manufacturers is to reduce rolling resistance in the material, without reducing its grip on the road 鈥� qualities that are both linked to high viscoelasticity. One way to do this is to make different parts of the tyre from different materials. Only the tread has to be viscoelastic. The rest can be made from an elastic material that springs back into shape without generating heat. 鈥淭yres are more complicated than most people think,鈥� says Kyle. 鈥淪ome are made from more than 10 different components.鈥�
In theory, the rolling resistance of the tyre could be reduced further by creating a material that is elastic at the low frequencies created by tyre rotation, but behaves viscoelastically at the high frequencies produced in skids. Low-frequency deformations would bounce back into shape without a loss of energy and without increasing the rolling resistance. The delay in responding to high-frequency deformations, on the other hand, would maintain the tyre鈥檚 grip.
Strong and durable
In the late 1970s, tyre manufacturers realised that polymer chains mixed with silica (silicon dioxide) instead of carbon have just this property. Because silica is strong and durable, it can do the same job in the tyre compound that carbon conventionally does. But unlike carbon, silica forms chemical bonds with the polymer chains and therefore holds them firmly in place during deformation. During high-frequency deformations, the atomic bonds cannot orient themselves at the same rate that the force is changing, so the material acts viscoelastically, just as in conventional tyres. At low frequencies there is plenty of time for the bonds to adapt and the material acts elastically, but because the polymer chains are firmly bound relative to the silica particles, little energy is lost as heat.
The problem lies in making a material in which the silica and polymer are combined. Silica is a polar substance that does not mix easily with the organic polymers used in tyres. 鈥淚t鈥檚 just like oil and water,鈥� explains Roch. 鈥淲hen the silica is added it tends to stick together rather than disperse.鈥� During the mixing process, particles of silica tend to form themselves into chains, which prevents them dispersing.
In 1983, Michelin approached the French chemicals company Rh么ne-Poulenc with this problem. The firm already had wide experience of creating emulsions 鈥� mixtures of oily and polar substances that would otherwise stay apart. Emulsions are found in a variety of products, from organic pigments in water-based paint to the oil and vinegar used in mayonnaise and salad dressing. Now Rh么ne-Poulenc has added tyres to this list, and is releasing its technology to other tyre manufacturers, as well as Michelin.
The secret lies in making tiny silica particles form into clumps, akin to the lumps of carbon in soot. These larger particles are much easier to disperse, says Jean-Yves Derrien, a research manager at Rh么ne-Poulenc. The new process uses a detergent-like agent to help mix the silica with the molten polymer in the same way that soap helps to mix oily substances in water. In addition, the temperature of the mixture is controlled to make sure that the silica does not react with the polymer until it has been dispersed. The details of the process remain a closely guarded secret.
Costly materials
One disadvantage is that the resulting compound is expensive. Conventional tyre materials are made simply by heating a mixture of rubber polymer with carbon. The close control needed when silica is used makes the manufacturing process more complex, and silica is more expensive than carbon. But the most expensive ingredient is the coupling agent, which costs ten times as much as any of the others. In future, motorists may have to pay more for their high-tech tyres.
Silica creates another problem. The carbon in conventional tyres forms a route by which static electricity can discharge to the ground. But silica does not conduct electricity, and while it is known that tyres can help discharge static, nobody is certain whether they contribute to its build-up.
When static electricity builds up on a car, anyone touching the bodywork may feel an uncomfortable electric shock as the charge runs to earth through their body. Static can also interfere with the microelectronics that control the engine, or produce sparks that can create a serious fire hazard. In February, Vauxhall recalled almost 500 000 Astra cars built since 1992 after fears that sparks resulting from the discharge of static could cause a fire while the cars were being refuelled. Vauxhall says the problem is not connected with the tyres, and is adapting the design by making an electrical connection from the neck of the petrol tank to the vehicle body to carry any static away from the fuel.
Nevertheless, tyre manufacturers have continued to develop ways to make the new tyres more conductive. One approach is to coat them with a conductive substance. Another is to design the tyre so that part of the sidewall, which is still made with carbon, touches the road. Alternatively, mixtures of carbon and silica can be used in the tread material. Kyle is reluctant to reveal the route that Michelin has taken, but he says that conductivity is not a problem. However, others in the industry are concerned that the solutions are not yet perfect, and tyre conductivity is still the subject of investigation.
The eventual result of all this research should be tyres with even lower rolling resistance. Derrien claims to have developed a material that produces 40 per cent less rolling resistance than equivalent carbon-based types by further improving the way silica mixes with rubber polymers. For the moment, the details are confidential. Michelin hopes to use such formulations in its next generation of tyres, but will not say when they are likely to appear. 鈥淲atch this space,鈥� says Kyle (see Diagram).
