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.

Fuel consumption – European tire label

gasoline

fuel consumption

In the previous post we talked about tire noise, we have deepened the causes and some solutions to reduce it.

Today we talk about the second mark in the European tire label, in particular how tires affect fuel consumption; this parameter is related with tire rolling resistance.

What is the origin of this resistance? Which parameters affect it? Let’s have a closer look.

Hysteresis

Let’s go immediately to the central point: when the tire touches the ground (enters the contact patch), it is deformed by the reaction force of the ground. When it leaves the contact patch not all the energy received is released. This behavior is named as hysteresis of the material.

Hysteresis and fuel consumption

How can we imagine this behavior? What does this concept, maybe still conceptually empty, means? The secret is the viscoelastic origin of the material. Let’s imagine the tire compound as a dish of spaghetti; these are glued each other in some contact points. When an external force is applied to spaghetti, these tend to stretch, but being glued each other a total stretch is not allowed, so the external energy is stored as elastic one.

If gluing points were perfect, when external force is released all the elastic energy would be released too; but in the reality this in not like that because every spaghetto slides respect others and a part of energy is dissipated as heat. This is the reason why the material is called visco-elastic.

So when this dish of spaghetti touches the ground, part of energy is stored and then released, another part is lost as heat. Due to this effect the pressure distribution on the contact patch is not a symmetrical parabola, but the peak is slightly translated onward respect to the wheel rotation axis. The resultant vertical force generates a moment opposite to the driving torque.

Rolling resistance and fuel consumption

The main parts of the tire where energy dissipation is concentrated are:

– Thread: about 70%;
– Sidewalls: about 15%;
– Beads: about 15%.

In order to give some additional information, compared the global vehicle motion resistance, the rolling one has the following influence:

– 20% on highway;
– 25% on backroad;
– 30% on city road;

Parameters that influence fuel consumption

We can distinguish two categories: intrinsic parameters and the ones that can be controlled by the driver.

First group

In the first group of course we can find the compound “recipe”, that with its ingredients represented by natural rubber, styrene, butadiene, carbon black, silica, sulfur and so on, defines the hysteresis level for the finished product, also function of temperature and frequency of road unevenness.

Another important parameter is the thread thickness and void ratio that is related to tire grooves dimensions: the bigger are the channels, the greater are tread blocks deformations, so the more energy is dissipated as heat.

The tire diameter is another influent parameter because the bigger is it, the smaller is the tire bending on the contact patch at leading edge, having as consequence less deformation.

It is not possible work with these parameters, the only thing that we can do is look carefully the European tire label. In order to reduce fuel consumption we can buy “green” tires; working with some data we can consider that if we use a green tire instead “black” ones, we can reduce up to 30% of rolling resistance with a real fuel consumption up to 6%. These numbers are not the truth, but are useful to understand better the phenomena.

Second group

In this group we can find tire pressure, because if it is constantly maintained in the optimal range allows reducing fuel consumption.

Vertical load increase also the rolling resistance, at the same tire pressure. So if for example we take our car for a holiday with our family, and we put on it 2 bicycles, a canoe and the car is full of baggage the fuel consumption is higher also because the tire rolling resistance is higher.

High speed increases rolling resistance because tire is affected by strong waves and vibrations. In addition to being a low efficiency condition, it is also dangerous for the integrity of the tire.

How to reduce fuel consumption

reduce fuel consumption

Going directly to the target, in order to reduce fuel consumption we should read carefully the European tire label when we buy a new tire set and take care of its state, having a periodical inspection in order to check tire pressure and wear, also for safety reasons.

A higher class on the label may mean a tire with a lower hysteresis, but if fuel consumption is reduced, on the other side also grip is reduced, because it depends on the hysteresis too.

The higher class may mean a compound with less percentage of fillers as carbon black, but also in this case in the other side tire wear gets worse.

As usual the optimum is a compromise between what we need.