Grip – European tire label

aquaplaning
aquaplaning

In the previous post we have looked at some aspects related to tire rolling resistance, strictly related to fuel consumption.

The same phenomena are also strictly related to the tire ability to generate grip, although their influence takes two opposite directions in these two cases. So, the more is the tire hysteresis, the more is the grip, but at the same time the fuel consumption of the car is higher.

Just to clarify: the grip class on the European tire grip is an information about the grip on wet surface.

How grip is created?

As already described, in the post about the influence of the tire on fuel consumption hysteresis was described. Because it is the main reason of grip generation, it can be useful revise the concept avoiding repeating it now.

Grip is composed by three phenomena:

  • Local deformation;
  • Adhesion;
  • Wear.
grip tire

Contributes

First contribute: let’s imagine a rubber block sliding on a surface perfectly lubricated with some roughness. If we could see the interaction between rubber and surface we would see the picture highlighted as “deformation” known also as “indentation”. Due to hysteresis the pressure distriubution around the obstacle is not symmetrical, so a force opposite the motion direction is created. The relation between grip and rolling resistance is direct: the greater is the hysteresis, the greater are grip and rolling resistance.

Second contribute: let’s imagine now the same rubber block sliding on a surface perfectly dry without roughness. In this case the rate to grip is given by adhesion, i.e. the molecular interactions between rubber and the surface, better known as Van der Waals. The nature of these forces is the same of that ones that maintain a solid body as such but with a very lower intensity, giving however a significant contribution to grip generation.

The last contribute not always is highlighted. Maybe it is significant when we talk about racing tires, where a locally excessive compound deformation generate its laceration and so an increased energy dissipation whith an increasing on grip.

What is the grip influenced by?

Many factors affect tire grip, so there is no a single friction value. The following parameters affect mainly the grip:

Temperature

The molecular structure of tire tread can assume two configurations: glassy if molecules behaves like the glass (stiff but fragile); or amorphous (the compound is soft and flexible). We can guess that at low temperatures the compound behaves like the glossy state, at high temperatures it behaves as amorphous. There is a big difference between these two configurations in terms of mechanical characteristics, and in the transition region (very narrow) the grip is maximum (and the maximum hysteresis). From that we can guess why is not recommended to use a summer tire in winter, because the tire wear would be excessive, and the vice-versa, because the grip would be too low than the one generated by a summer tire.

Frequency

The understanding of the influence of this phenomenon is less immediate. Fortunately there is a mathematical relationship between it and temperature, in fact at the same temperature if the frequency is higher the molecular structure of the compound moves toward the glossy state and vice versa.

Type of road surface

The road roughness, divided in macro and micro, modify the way in which the compound moves between unevenness, modifying the grip level. The adhesion level is influenced by the road condition, id it is dry or wet, as already mentioned.

What happen on wet road?

The ability of the tire to maintain a good grip on wet road is due to the capability of the tread to drain the wates as well as possible in order to offer a drain contact between the road and thread blocks, allowing the adhesion forces to work properly.

When we drive on a wet road the tires of the car push forward the water on the road. At first a small “wall” of water is created that counteract the motion of the tire. A certain amount of over pressure is generated proportional of the vehicle speed. If that pressure is the equal o greater than tire inflation pressure, the last one tend to be lifted, the phenomenon is known as aquaplaning.

How increase tire wet grip

Reducing the aquaplaning risk means increase the vehicle speed at which it starts, behalf some design method of tire tread.

The first point is that is useful to have an oval contact patch instead a rectangular, in order to drain the “wall” of water in a better way. In the second phase tread sipes (transversal channels) are useful to drain the water outward.

The residual water is finally removed by tread blocks and grooves that work in synergy: blocks push the water to go inside the grooves. Tread blocks dimensions should be the correct compromise between the ability to drain water and maintain enough stiffness. Furthermore, the role of the edges of the blocks is important because destroy the surface tension of the drops, ensuring the contact with the road as dry as possible.

In the reference legislation the tire grip on wet road is divided in 7 classes, from G (the worst) to A (the best). Just to quantify the difference, from the last to the first class the braking distance is reduced up to 30%.

Conclusions

Should be better to have a look at the European tire label ad think about what the classes mean, avoiding buying too cheap tires…first of all safety! Keep in mind that all the forces that act on your car are applied also on the road through tires. Sometimes few centimeters on braking distance are enough to avoid a crash.

Tire noise – European tire label

Tire noise - European tire label

Buying a new tire set you may have seen a label like this.

It is just a summary of informations about the characteristics of the tire we are buying, in terms of fuel consumption, wet grip and sound emission.
This label is subjected to European regulations, and tire manufacturer must declare  tire class on three fields, defined behalf standard tests.
The difference between the last and the first class can be summarized as follows:

  • Fuel consumption: tests define tire rolling resistance coefficient. From G class to A the consumption is reduced by 7,5%;
  • Wet grip: From G class to A the braking distance is reduced by 30%;
  • Sound emission: one black wave corresponds to a silent tire, 3 waves indicate a noise one, and there is always the value in dB.

Let’s explore the three categories, starting with the sound emission.

Tire noise

Who have driven an electric car, will have noticed that when the car stops there is no sound…is very difficult understand if it is on or off.
The “sound” changes when the speed increase, the driver is able to listen the aerodynamic and tire noise.
Why tires are noisy? Which are the main reasons?

Sidewall vibrations

Tire noise from sidewalls pumping

During wheel rotation, the tire portion that touch the ground entering the contact patch causes tire deformation.
One of the main parts involved in this deformation process are the sidewalls, due to their lower stiffness compared to the other parts of the tire.
When this region leaves the contact patch, the corresponding portion of the sidewall return to its undeformed shape.
This cyclic deformation create pressure waves in the air with a variable frequency in the range from 500 Hz and 800 Hz.

Horn effect

Tire noise horn effect

Let’s look the tire from the side, as if it were bidimensional. When the tire region enters the contact patch, pressure waves are generated and their propagation is toward the vehicle running direction, and due to the shape of the cavity between the tire and the ground, like a horn, the amplitude of wave pressure is increased.
The same effect is produced when the tire leaves the contact patch.

Tread pumping

Every tread has is own sculpture, created mainly to improve handling on every type of ground, dry, slippery or to drain in the best way the water behalf circumferential and lateral channels.
When the tire region touches the ground, tread blocks are deformed and as consequence also grooves. The air that fills the grooves is pushed out the tire and noise is generated. In proximity to the trailing edge of the contact patch the tire returns to its original shape and the air comes back to fill the grooves.
The amplitude of this phenomena is a function of:

Grooves dimensions, the bigger are, the more is the noise;
– The angle between lateral grooves and vehicle running direction, proportional to the noise generated;
Vehicle speed, also proportional to the noise generated.

Cavity noise

Tire noise - cavity

When the tire is radially deformed pressure waves propagate also inside the tire, in the area between the tread and the wheel rim, and noise is generated.

Stick-slip

Tread blocks in contact with the road are subjected to cyclic stick and slip. In some conditions these vibrations enter in the hearing frequency range, so we can hear the typical tire screech. The frequency of these vibrations depends on blocks dimensions and stiffness.

Tire noise is also influenced by wheel torque, not as source itself but as a parameter that can modify the amplitude of the phenomena; the greater is the torque, the noisier is the tire.
Furthermore, it is influenced proportional to inflation pressure. For low wheel torque the trend can be reversed, so a tire with higher inflation pressure can be less noisy.

Tire noise reduction

Tire noise reduction

In order to reduce the noise produced by tread is useful optimize grooves dimensions and its angles, finding a compromise between noise and the need to have a good grip, also on wet.
It is possible also create a circumferential offset between internal and external tread blocks.
The internal noise can be reduced using foams (in the following image Contisilent) to damp carcass vibrations; it can be mounted also on the rim internal channel. The disadvantage of the first option is the increased moment of inertia of the wheel, so it needs more power to be accelerated or slowed, but foams are very light.

Another option to reduce sidewalls noise is change their stiffness using a different compound. Anyway, each of these solutions is a compromise between low noise level, a good grip and a low fuel consumption.

TPMS

TPMS

TPMS is the acronym of Tire Pressure Monitoring System, and nowadays is widely used in production cars. How does this system work? How many types exist?

Tires pressure and handling

Before talking about the system we need to understand why is important monitoring tire pressure. The answer seems to be simple, the reason is security. But from which phenomena does security depend?

Starting from a reference condition, the contact patch of tire with an excessive inflation pressure is reduced. In this condition main effects are a reduced grip and an irregular tire wear, more in the center and less on the external surface. Due to the increased tire vertical stiffness the ride comfort is worse. Furthermore, the steering wheel is “lighter” due to the reduced self-aligning moment of the tire, i.e. the moment created by the tire lateral force that in normal conditions is in opposition to the steering moment.

At the opposite a too low tire pressure leads anyway to a reduced contact patch and an irregular tire wear, more on the outer and less in the center. Vertical stiffness is reduced because vertical stiffness is lower, but at the same time strain amplitude is increased and more heat is generated inside the tire, increasing the possibility to damage.

In both cases tire cornering stiffness changes, in this last case decreases, so tire tends to generate less lateral force.

Tire deflation

Pressure can be lost mainly in two ways:

  • Tire damage or valve failure: this two cases lead to a fast deflation, and are the most dangerous;
  • Air diffusion: it consists in air leakage through tire compound due to its permeability. It is a slow process, but for this reason car manufacturers advice to check tire pressure regularly. In order to reduce air diffusion, tire manufacturers introduce on the inner part of the compound a special layer, called inner layer, with a low permeability.

Types of TPMS

There are two types of systems: direct and indirect. The main differences are the measurement accuracy and the cost, higher for the first one.

 1. Direct system

Is a stand-alone system. Is composed by pressure (and sometime also temperature) sensors mounted instead of normal inflation valves that send the informations as a signal to one or more antennas. The last ones send the signal to the Electronic Control Unit and are processed.

TPMS valve

This kind of TPMS is able to measure not only id the tire pressure is too high or too low, but indicates which wheel is affected to the problem and shows the values of pressure and temperature for every tire.

The software used to process the signals must take in account all the different conditions in which tire pressure varies but falls in the normal range, for example the increased or reduced vertical load on the tire  due to load transfer on braking or turning.

In order to have always the correct reference pressure, the system bust be initialized correctly. When tire pressure is changed manually, or when tires are changed, the reset button must be used. From this moment learning phase starts: sensors start to record the pressure, using that values as reference. Being a very sensitive system it should be initialized correctly to avoid false alarms.

Among advantages there are of course measurement accuracy and the possibility to read pressure and temperature values.

Some weaknesses are the higher cost of the system than the indirect one due to necessity to design a stand-alone system; the possibility of failure of one or more sensor and the presence of a battery with its own life cycle.

 2. Indirect system

This TPMS does not need additional components but are used speed sensors already mounted on the wheels and used by ABS system.

This is the first advantage, i.e. a cheaper system because no additional components must be installed, programmed and calibrated. The disadvantage can be found on the title of the paragraph: because it is an indirect system, tire pressure is not measured but estimated…even better the difference between a reverence value and a limit one is estimated.

How does it work?

Let’s imagine that we are driving our car with all four wheel with the same size on a straight line, If tires have the same pressure the linear speed of each of them, calculated as product between angular velocity $\omega$ and rolling radius, must be equal to the other ones and to the vehicle one. If one tire is deflated, to reach the same linear speed of the vehicle it must increase its $\omega$. The system measure this change due to the reduced rolling radius and a warning appears on the dashboard.

As usual the real principle is not simple as explained. The dimension of rolling radius i modified by many factors, some of which are:

  • Longitudinal and lateral load transfers;
  • A different load distribution on the four wheels, for example because we are living for the holidays and our car is full of bags;
  • Just driving on a turn, because the four wheels have a different angular speed;
  • On high speed, because the tire is deformed on a radial way;
  • Different wear between one tire and one other.

All this factors must be evaluated by ECU.

Every time that we change tires, also this system must be correctly initialized at the new reference pressures.

One more disadvantage is that the warning message is very generic, no value is displayed, nor in which wheel there is the problem. Just a warning light is displayed, like in the following image.

TPMS warning light

Some indirect TPMS are able to estimate the pressure reduction caused by air diffusion. Through frequency analysis of belt oscillations, linked to inflation pressure, are able to notice the problem and a warning alarm is displayed.

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