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;
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:
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.
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%.
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.