Tire Performance

Just think for a moment about what goes on when one drives a car. Each tire, and there are only four on cars; a few more on trucks, touches the ground on an area not much larger than the average person’s footprint. First of all the tire will be called upon to support the load of the vehicle. The tire must then transmit the driving, braking and cornering forces applied by the vehicle as it accelerates, brakes and goes round corners over a wide range of speeds in dry and wet conditions, even in atrocious storms, snow and ice. It must carry this out without generating excessive noise within the environment. Roads may be covered in potholes, ramps, sharp objects, debris and all other manner of obstacles. The tire must be capable of passing these obstacles without detracting from the ride comfort of the occupants or sustaining any damage that may cause it to fail. Tires must reliably get their users from the start to the finish of their journey for tens of thousands of miles, hundreds of thousands in the case of truck tires. Finally there is the vexed issue of what to do with the tire at the end of its life: retread, burn, bury or reclaim it.

There are several performance criteria the designer and compounder must consider; not all are mutually exclusive or even helpful to each other. In addition the difficult global economic climate often demands year on year relative cost reductions for the product.

The most important parameters are:

Traction

Tire test engineers refer to traction as the ability of a tire to transmit torque from the engine and drive train through the tire contact patch to generate motion in a straight line or through controlled curves. In the dry a maximum area of contact will enable the maximum frictional force to come into play hence the reason why some racing tires have slick, not worn out or bald, tread surfaces. Friction for rubber does not follow the classical laws of force being proportional to the vertical load through the coefficient of friction; the friction mechanism involves several components including adhesion, deformation, tearing and a viscous component. These in turn are related to the sliding speed, the temperature in the interface and time.

Under more adverse and unfortunately in many countries, more typical conditions, roads are not always dry. The idea of wet traction or winter traction becomes the key parameter. Tires have several fairly large grooves moulded along and across the surface. These typically occupy around 25 percent of the surface area within the contact patch, a term known as the sea to land ratio of a tread pattern. Once the pattern’s macrotexture has done its job of removing bulk water it is then the work of micro-slots in the tread pattern and the compound itself to squeeze out any residual
water, snow or ice and so form an intimate contact between the rubber and the road. Road surface microtexture too plays a vital role in wet traction, as it does for many other performance properties, but road surface technology is such a large topic it could occupy
a whole review.

Objectively the tire’s wet grip and handling performance can be measured with instruments that record the lateral and longitudinal forces at various cornering speeds. A measure of the tire’s subjective performance may also be given through a rating of how confident a driver feels when making certain manoeuvres or how fast he can perform a manoeuvre.

On moving in a straight line or through a curve, if the tires run through deep standing water, any loss of grip is a phenomenon known as aquaplaning. If the tire is unable to remove excess water from under the full contact patch quickly enough then, once a critical area is reached , the tire will begin to lift off the ground and be supported by hydrostatic pressure. The tire will then have almost no grip or traction and the vehicle could easily go out of control.

Under cornering conditions the tire contact patch is distorted away from the centre line. The force generated by the tread rubber and the belt package attempting to recover from this distortion provides the lateral cornering power needed to drive the vehicle around a curve. Since the action of this lateral force is displaced a little behind the centre line of the tire (known as the pneumatic trail) it offers the driver an opposing force or feedback sensation through the steering wheel known as self-aligning torque, that a force is still being generated by the tires at the contact patch. When water is present under the contact patch the driver may sense a decline in the lateral aquaplaning resistance and wet handling performance. A tire’s internal structure, tread pattern and compound recipe all work together to provide the maximum lateral force and self-aligning torque for a given steering angle, such that the driver feels confident that the vehicle is in control.

The opposite of a tractive force involves braking. One critical tire performance characteristic related directly to vehicle safety is the minimum distance that is required by a vehicle to come to rest from a given speed. Here again a simple parameter becomes a grey subject when we ask some basic questions. What is the depth of the water? What is the texture of the road surface? What are the environmental conditions such as humidity or air temperature? Does the vehicle have an anti-lock braking system? Are there any influences from different vehicles, such as suspension types or weight distribution? All are important and valid factors, but how does the compounder address these effects when developing a single formulation in a laboratory to suit many vehicles in many operating environments?

To brake a vehicle in the wet the tread pattern must first remove the bulk water then, as the speed reduces, the compound must provide the grip to the road surface.
Again the texture of the road will dictate the balance between pattern and compound influences on the total stopping distance.

Winter tire performance can be viewed in a similar way by assessing the ability to maintain traction and control in snow and ice conditions. Here again there are problems of definition and consistency since snow and ice conditions are variable and differ significantly with global location. For example, in Europe the conditions which prevail in northern Scandinavia differ greatly from those in the central alpine regions. There are basically four states for snow
including: new, powdered, and compacted through to ice - although the Eskimos have a whole language to describe it. The tire and compound designers have to take many environmental conditions into consideration when developing tires for each of these territories. Very low operating temperatures will critically influence the choice of polymers. Also, since snow packing and ice grip are very different from wet grip, winter tires have many knife blade shapes or sipes moulded into the tread pattern to offer extra edges for grip. In some locations studs or spikes may be fitted to gain extra grip. In this case compound strength to resist tearing may be an
important consideration.

Coefficient of Rolling Friction
(Rolling Resistance)

The second critical factor arising from friction is better known as rolling resistance, or the energy required to roll a tire along a road under a given vehicle load. The rolling resistance coefficient is calculated by dividing the drag force by the applied load. When a vehicle moves, energy is lost in the engine, transmission and vehicle aerodynamics. A small but significant portion of the energy is also lost through deflection of the tire. Tires, hence tire compounds and structures, have
therefore become a strong focus for the reduction of fuel consumption in vehicles. The rolling resistance force is usually quoted as a percentage of the vertical load and for most tires on smooth roads it is typically of the order of 1%. Following the launch by Michelin in 1992 of their silica filled ‘green tire’ offering a potential fuel saving of around 0.5 litres per 100 km of travel , there has been a steady move towards tires known variously as ‘Green’, ‘Energy’, ‘Fuel Saver’,
etc. Rolling resistance values for car tires have now begun to fall below 1%, although truck tires with natural rubber based treads have always been down at around 0.5%. In the passenger car field the carmakers are striving to produce vehicles with lower levels of fuel consumption. This has set up a technology pull for original equipment (OE) tires to be more fuel-efficient , whilst meeting all the other demanding technical requirements. Modern electric or hybrid
vehicles may need even greater reductions in drag forces to maximise the running times and distances between battery recharging.

Various attempts have been made to determine the contribution which each tire component makes to the energy loss, but all studies show that the tread compound is the dominant factor contributing between 30 to 50% towards the total energy loss within the tire. The apex, however, may be the strongest contributor on a basis of energy loss per unit volume. Much research has investigated the relationship between rolling resistance and fuel consumption with various figures quoted that energy saving tires with a certain reduction in rolling resistance can offer a pro rata reduction in fuel usage. Generally the ratio is quoted as being around 5 to 1 between rolling resistance and fuel consumption.

Many factors can influence the actual rolling resistance. Some are in the operator’s control such as speed of travel (at average speeds 1 kph relates to 1% on fuel), tire inflation pressure (a loss of 0.3 bar relates to 1% on fuel) and the severity of driving, braking and cornering forces applied. Others are in the hands of the tire designer. Factors include the tire’s deflection under a given load; the recommended operating pressure; the stiffness of the tire structure; the volume of rubber used in components; the weight of the tire (50 g relates to 1% on fuel although this depends very much upon where in the tire the weight is reduced) and the level of hysteretic loss for each compound. From a compounder’s viewpoint this comes down to a balance of energies. Under deformation, the viscoelastic nature of rubber means that the tire absorbs energy. That is what gives tires their ride comfort, their low noise and their ability to grip the road. If the bulk energy stored is not efficiently released once the rubber leaves the contact patch then there will be a resultant energy loss. This loss of energy results in an unnecessary use of fuel. Coupled to the energy loss, a corresponding level of heat generation may also be detrimental to the integrity of the tire structure.

For motorsport tires, rolling resistance is an important parameter since it may limit the maximum speed out of corners and on long straights. The compounder must therefore make the compound as elastic as possible whilst considering other, often conflicting, tire requirements.

A measurement of rolling resistance is commonly made with a tire running under equilibrium operating conditions against the surface of a smooth or slightly textured steel drum. This is somewhat artificial since it does not represent real road surface conditions. It has been possible to determine the effects of road texture by testing against simulated surfaces. Rolling resistance coefficients here are somewhat higher. Some companies prefer to look at vehicle coast-down
measurements, i.e., the distance covered as a free rolling vehicle is allowed to coast to a halt. Alternatively instruments can be used to accurately monitor the fuel consumption under defined driving conditions. This aspect is particularly pertinent for owners of large fleets of trucks where fuel cost is the major operating expenditure. Again a compounder has the task to look at laboratory properties to predict the likely effects of hysteresis on rolling resistance. To a slightly lesser degree, other tire components like the sidewall, apex or casing will, through their hysteresis, contribute to the overall rolling resistance and so must also be considered by the compounder. Many compounding options have been developed to offer improved rolling resistance, these will be discussed later.

Finally the inner liner plays its part in the fuel consumption through its ability to retain air over long periods. Checking of tire pressures by users is not always as regular as it should be. If, during the intervening period, pressure is lost, then rolling drag hence the fuel consumption will increase. Pressure maintenance is as important to fuel consumption as it is to safety. Seasonal changes in ambient temperature can lead to significant shifts in tire pressure hence fuel consumption. The option to design tires which run at higher inflation pressure may give benefits of reduced rolling resistance, but here again there is a down side in poorer ride comfort.

A parameter closely coupled with rolling resistance is tire weight. The use of less material will ensure there is a reduced level of heat build-up due to hysteretic energy loss. Lighter tires also mean less energy is required to start and maintain them in a rolling state. Many attempts have been made to produce lighter tires through reduced component gauges, thinner, stronger cord materials and the use of alternative lightweight materials, such as aramids in place of steel.

Treadwear and Durability

Of vital importance to many users is the question, ‘how long will my tires last?’ That becomes even more important to a truck fleet operator and the tire service engineer for an airline. Tire life may be defined in many ways. Firstly, a tire should never fail structurally. Secondly, for truck and aircraft tires, the casing should be structurally sound at the end of its first life and capable of being retreaded. The concept of retreading for car tires is a feasible but moot subject, one that will grow in importance as tire disposal problems and costs gain a higher profile. Perhaps retreads may find their way to become standard as spare tires. Thirdly, the tire tread
pattern should not wear out at an excessive rate.




Tire wear itself is again a complex topic to define. There are many factors impacting on treadwear including the environment, the harshness and condition of the road surface, longitudinal and lateral movements of the tire while rolling, tread pattern design, vehicle drive axle configurations and driving habits. In addition, the minimum tread depth limits set for safety lead to a demand for even wear across the tread surface. Excessive wear around certain parts of the tire, possibly due to poor contact pressure distribution, may cause customer dissatisfaction that some value for money has been lost. Towards the end of a tire’s pattern life there are increasing dangers from loss of control or from penetration of sharp objects into
the belt structure. It has often been considered that current minimum tread depth limits should be increased on the grounds of safety in the wet (3 mm has been suggested). This may be a challenge to compounders in the future to achieve greater wear resistance to compensate for the reduced usable pattern depth, or perhaps to aim for a constant level of tire performance as the tread depth reduces.

For the compounder, many factors come into play when developing good wearing compounds including the correct selection of filler type, the state of cure of the compound through the whole tread pattern depth and, in conjunction with other properties, the grade of polymer. It is generally believed that polymers with high glass transition temperatures (Tg) offering improved grip are more prone to rapid wear. Firstly though, how does one assess wear? On a road the effect is a relevant measure, but the time and expense involved in carrying out controlled tests, usually convoy tests, may be prohibitive.

Thus people seek laboratory tests to indicate likely levels of abrasion. These tests may indicate a compound’s potential abrasion rate but they are not always totally reliable predictors of treadwear under all circumstances. Placing tires with large fleet operations where driving routes and conditions offer some degree of repeatability is another very suitable way to assess wear of tire compounds.

The tire’s structural durability as a measure of life is affected by many factors including the compounding ingredients and methods used in manufacture as well as the type and conditions of use. Tire components may be exposed to temperature cycles from –40 °C to +60 °C or greater, with continual flexing over perhaps 50 million cycles. Appropriate antioxidants must be used to protect compounds from thermal degradation over the lifetime of the tire. Low hysteretic compounds are also desirable to aid in low heat build-up with a resulting improvement in durability. Oxygen, moisture, ozone in the air and UV light can also degrade rubber, and antiozonants are employed to minimise this effect.






Improper assembly of tire components, contamination, lack of building tack and poor
adhesion of components can also cause premature failure in the field. Operating conditions such as under- or over-inflation, overloading the vehicle and improper wheel alignment can also contribute to early failure of a tire. For off-road tires, cut and tear resistance is important and special fillers, additives or cure mechanisms may be used to improve this.

Changes to tire legislation in the wake of claims against failed tires could mean far more focus being paid to the testing of tires and components for structural integrity under many conceivable conditions. Quality and integrity must therefore be paramount right through the supply chain and manufacturing process.

Noise

Noise is another form of environmental pollution that is gaining a growing interest from lobby groups and legislative bodies. Final agreement was reached in 2001 through a European Parliament directive (2001/ 43/EC) to set limits for coast-by noise generated by tires on a so called smooth (ISO10844, 1994) road
surface. The mechanisms for noise generation and transmission have been widely studied, but generally the sound pressure level depends upon the way in which the tire tread pattern, there to remove water, impinges on the road surface. The process starts with the rubber impacting on the road, air pumping from any sealed chambers between the pattern and road surface texture, followed by the elastic recovery of tread blocks as they spring out of the contact area.
All these contribute to the airborne and structural noise generation. Randomised, open tire tread pattern designs, the structural stiffness of the tire and compound modulus or damping must all be considered when designing tires with reduced noise in mind. However, the designer must not reduce noise at the detriment of tire safety, a factor also under consideration in the new noise directive.

On the other side of the argument the road surfaces themselves play an even greater role in the generation of noise as they do likewise on grip and rolling resistance. Roads vary widely in their texture, roughness and general state of repair giving differences in pass-by noise greater that those between different tire patterns. The concept of porous road surfaces not only offers benefits of reduced noise but also of improved water removal, which can be beneficial both for wet grip improvements and also for spray suppression. As yet these road surfaces have seen only
marginal acceptance in many countries.