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.
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?
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.
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.
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.
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.
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
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.