The noise made by tyres and their general comfort is one of the key performance criteria for many drivers. Why is it so important? In this article, we explain where excessive loudness of tyres comes from and what tyres to buy to travel comfortably.
Traffic noise is unfortunately a frequently neglected problem. The noise made by tyres while driving can ruin the pleasure of travel.
Tyre noise
Noise should be understood as the sound level causing an unpleasant auditory experience or even disturbing the driver, passengers and the environment. Sound is the vibrations propagated in the air in the form of sound waves. The source of vibrations can be a solid object (in this case a tyre), which stimulates the air to vibrate, creating pressure waves around it. Basically, the more the source trembles, the more noise it generates.
In the speed range from 50 to 120 km / h, tyres are the main source of noise emitted by a moving car. Noise can be troublesome not only for the driver, but also for the environment. It turns out it is annoying mostly for people who live by the roads. The accumulated noise made by tyres of dozens of passing cars, causes these people to be constantly in a noisy environment. Traffic noise has become a serious problem in the field of environmental protection and this is why its level is described on tyre labels.
Noise can also be limited by the use of modern surfaces that reduce its level by as much as 7-10 dB (A). Unfortunately, they are on average 2.5 times more expensive than regular ones. Most often, the noise is reduced by limiting the speed on highways and motorways which run close to places inhabited by people.
The car user may also have an impact on reducing traffic noise. It can be done in two ways:
proper care of the exhaust system,
using silent tyres (a car driving on quieter tyres emits up to three times less noise than the same car on ordinary tyres).
In this article, we will focus on the latter.
What are the sources of tyre noise?
Each tyre has its own resonant frequency, that is, the frequency at which it trembles the most. The problem occurs when this frequency is close to the frequency at which forces are applied (resulting, for example, from road roughness). Then, the tyre instead of absorbing vibrations and noise strengthens them. An even bigger problem is when the frequency is additionally close to one of the car's resonant frequencies. This may be the reason why one tyre model is considered by some drivers to be quiet, and by others to be loud. To summarise, it is important what we drive (car), on what surface (road roughness, frequency of transverse faults in the roadway, etc.) and with what tyres. In many cases, the tyre noises are also due to errors, shortcomings or deliberate decisions of the manufacturers (in order to raise other, more important feature - for example, adhesion). At this point, it is worth drawing attention to how important for the drivers’ comfort is the cooperation between tyre and car manufacturers and specialists building roads.
Noise emitted by tyres is particularly annoying in big cities.
During the car's movement there is a constant interaction between the wheel and the ground. As the tyre rotates, it is constantly deformed, and its tread blocks hit the ground. The main phenomena generating noise are as follows:
impacts of the tread blocks when they are touching the road surface and moving away from it, due to arising vibrations of the steel belts, which cause the effect of wailing and rumbling,
compression of the air in the tread grooves, which is connected with two effects: air resonance in the network of tread grooves (including turbulence in places where channels meet), vibrations of air released from the back of the tyre, turbulence of air flow between the wheel and wheel arch,
vibrations of tread blocks during contact with the road and their expansion when leaving the contact surface,
pressure in the tyre too low.
How tyre comfort should be understood?
The concept of tyre comfort is directly related to their loudness. One of the functions of tyres is absorbing shocks and damping vibrations. Shortly speaking, the tyre is to be an element of the vehicle that provides travel comfort. In some cases, it may fulfil this role poorly or even become a source of discomfort - rumbling tyres can even take away the pleasure of driving a car.
We should remember, however, that comfort is a subjective concept. Each person may have a different sense of discomfort, and the limits of comfort may vary for different people. Excessive noise of summer tyres may not be disturbing for some people. It depends mainly on the level of perception, but also on cultural factors, the specific situation and personal predispositions. Another aspect of tyre comfort is the psychological comfort they provide to the driver. This is a result of the driver's trust in specific tyres, and the predictability of their performance.
Why some tyres are not comfortable?
Where do the so-called ‘noisy tyres’ come from? The reasons for the discomfort generated by the tyres include:
a conscious resignation by the tyre manufacturer from its comfort in favour of other parameters,
characteristics of a given model resulting from design assumptions,
low quality and production errors.
This is mainly the result of the tread pattern, tyre structure and its massiveness in the shoulder area. In the tyre's construction, there must be a certain balance between stiffness and flexibility. Some tyre models that must meet the requirements of achieving the speed index and load index can lose the damping properties.
Tyre noise index
The bigger the tyre, with larger diameter and width, the less comfortable. Reinforced carcass tyres also offer less comfort (that is why it is not worth buying tyres with a load index higher than the one dedicated for the car).
Tyres with higher profiles, usually relatively narrow, are considered the most comfortable. In fact, they can be quieter and better at cushioning road roughness. However, they are more susceptible to tilts and rocking, for example when passing a truck, or in a bend, which drivers often quote as reasons for dissatisfaction.
The features reducing tyre noise include:
directional tread without closed spaces,
tread surface of various shapes arranged asymmetrically and irregularly,
transverse grooves shaped so that their entrances and exits do not coincide with the tread edge,
high softness of the rubber mixture
There is also often a question about the noise of winter tyres compared to summer tyres. Silent summer tyres are in this respect statistically only slightly better than modern, almost equally silent winter tyres.
Modern, quietest winter tyres provide a high level of comfort, even with a larger number of sipes on the tread surface.
The type of surface does affect the noise level.
The quietest tyres
What is the acceptable noise level for a tyre?
Meeting the current noise levels required to obtain approval is not a problem for manufacturers. The differences recorded during tests between individual tyres amount to an average of 6-8 dB (A). Many models (especially the quietest summer tyres) are 4-6 dB (A) below the limit, but a big part also approaches the limit at 2dB (A). These limits are set out in Regulation No 117 of the United Nations Economic Commission for Europe (UNECE) - Uniform provisions concerning the approval of tyres with regard to rolling sound emissions and/or to adhesion on wet surfaces. The so-called decibel table is presented as follows:
| Nominal width of tyre section | Permitted sound level dB(A) |
|---|
| <145 | 72 |
| 145 - 165 | 73 |
| 165 - 185 | 74 |
| 185 - 215 | 75 |
| >215 | 76 |
For reinforced tyres (XL) the limits are higher by 1dB (A).
However, together with the enforcement of the tyre label provisions, the noise limits were made more stringent and the division into classes by width was reorganized. The new tyre classification is as follows:
| Nominal width of the tyre section | Permitted sound level dB(A) | Difference dB(A) relative to current limits |
|---|
| <185 | 70 | from -2 to -4 |
| 185 - 215 | 71 | -4 |
| 215 - 245 | 71 | -5 |
| 245 - 275 | 72 | -4 |
| >275 | 74 | -2 |
For reinforced (XL) and M+S tyres, these limits will be 1dB (A) higher.
Currently, the best noise category on tyre labels is awarded to models achieving 67 dB (one wave). 68 dB is also a great achievement (still one wave).
How does tyre comfort affect performance?
Mechanical discomfort due to vibrations is directly related to noise. Low profile tyres, which are usually wider, generate more noise, and, due to their low profile and greater stiffness of the sides and the tyre in general, they are less comfortable, because all vibrations are more perceptible. If the tyre is not homogenous due to its production process, and it causes pulsation as it rolls, the passengers will feel both vibration and noise.
Comparing tyres before making the purchase is very important, but it is necessary to remember that each model is a compromise between different aspects of performance. In terms of comfort, if the tyre was to be very quiet, this would be at the expense of wet surface adherence and resistance to aquaplaning. Research has shown, however, that there is no clear correlation between tyre noise and rolling resistance. Another way to design a very quiet tyre is to use small tread blocks. However, a tyre with such a sculpture is not visually attractive, and it should be remembered that many people attach great importance to the visual design of their tyres.
The influence of comfort and noise on the final rating of the tyre is debatable. And although it is important for users, it cannot compete with such parameters as tyre grip or cornering behaviour. However, an extremely low comfort level completely disqualifies a tyre in the users' eyes.
How are tyre noise and comfort tested?
External tyre noise test
There are many methods to determine tyre noise level and comfort. For noise assessment, methods similar to those used during tyre approval tests are usually used. This method is called coast-by. Such measurement should take place in a square-shaped test zone of 20m by 20m, with microphones located halfway at the distance of 7.5 m from the vehicle's axis of movement. The track should be made of aggregate with maximum size of 8mm.
Tyre noise test using the coast-by method.
Unfortunately, this type of surface does not correspond to all types of roads that occur in Europe. Therefore, tests carried out in accordance with this procedure do not reflect the whole picture. It also happens that manufacturers optimize their tyres for these conditions, and on other surfaces they may be a bit louder. In practice, many roads are smoother (mainly within urban agglomerations), and many are more rough with much larger aggregates of 14-15 mm, which is typical of highways.
According to the methodology, the driver enters the test zone at a certain speed, then puts in the neutral gear and turns off the engine. At least four measurements are made in the speed range between 70 and 80 km/h and four in the speed range between 80 and 90 km/h.
There are also ‘driver-by’ and ‘cruise-by’ methods. The ‘driver-by’ test is characterized by the fact that the driver enters the measurement zone at 50 km/h on the 2nd or 3rd gear and then accelerates through the test area. The ‘cruise-by’ method is to travel through the test area at a constant speed.
Tyre loudness test in the vehicle cabin
Noise test with a dummy
The test is carried out using a dummy sitting in the passenger seat, which is equipped with microphones placed in the ears connected to the recorder and computer. The test is carried out according to the following procedure: the car accelerates to a specified constant speed (e.g. 80 km / h), the measuring equipment is started and it records the sound pressure level for a certain time. The journey is repeated several times.
Subjective noise tests
The comfort and noise tests carried out by experienced test drivers are invaluable. Such tests are carried out by comparison with the reference set. They are usually carried out according to the following procedure:
Rides on reference tyres are therefore much more frequent than, for example, in the case of braking tests. This is to eliminate the impact of the order of tyres on the final results.
Depending on the comfort aspects to be assessed by the driver, various tests are used:
To assess shock damping on a bumpy road, attempts are made to travel on such a road at a relatively high speed (e.g. 80 km/h). The driver and/or appropriate sensors assess the amount of vertical acceleration when driving over bumps, time and efficiency of damping, and the tendency to resonate.
In order to evaluate the effect of passing through a single obstacle, a test is made by driving over a specially prepared single bump, threshold, hole or manhole cover, at a speed between 40-50km/h. The driver evaluates the amount of vertical acceleration and the noise accompanying the impact.
Two tests are carried out to determine the effects related to the shape of the tread. The first is to determine the effect of the pattern while driving. It involves steady driving at a speed of 80-100km/h. The second one is to determine the level of wailing and rumbling of the tread during speed changes. For this purpose, the test vehicle accelerates to a speed above 100 km/h and then decelerates until it stops. These tests are carried out on a smooth road.
In order to assess road noise (i.e. define general noise), the driver travels a section of a rough road with a speed between 80-100 km/h and describes the impressions.
Loud tyres - squeaking
The characteristic humming of summer (or winter) tyres is just one of the types of tyre noise. Another common type of disturbance is squeaking. Where does it come from? Tyre wet grip class depends on slip and friction, which generates noise. With regard to car tyres, we can distinguish two noise levels caused by rubber friction on the road surface:
hissing sound,
squeaking sound.
The hissing sound is created during normal driving when an average grip is needed. It will appear when travelling along a straight line, during gentle braking, acceleration or moving along a large radius curve at moderate speed.
The squeaking sound arises when the required grip level approaches the limit that the tyre/road surface can offer at a given moment. A squeal can be produced during emergency braking, fast acceleration or in a narrow bend at high speed.
Where does the squeaking during sharp braking come from? The ‘stick-slip’ phenomenon is responsible - the tread blocks that are rubbing intensely against the surface cause vibrations which, due to the higher frequency, are heard as a loud squeak. Stronger friction occurs when the tyre is at the limit of its grip, for example during sudden acceleration, sharp braking or cornering at large speed.
The process behind tyre squeaking
1 - the tread block touches the road surface.
2 - the block tilts more and more to keep in touch with the point on the road it touched: energy accumulates during this stage.
3 - the tread block reaches the limit of adhesion and begins to slip, gradual energy release begins and vibrations are generated as the block moves away from the contact point.
4 and 5 - the tread block bends, then moves.
6 - the block loses all contact with the road surface and rapidly straightens up, releasing all previously stored energy. The block rubbing on the surface generates sound. The following illustration presents the behaviour of a tread block.
What determines the squeaking of a given tyre?
the adherence coefficient of the specific surface,
the speed and manner of manoeuvring,
the tyre characteristics - the mixture used, the tread pattern, the arrangement of the blocks and grooves, the number of sipes.
the tread depth and age of the tyre and the associated changes in the stiffness of the tread blocks.
Tyres with worse adherence will be more squeaky. The sound can be amplified by the surface type. Such factors as poor wheel alignment or damaged suspension elements also have the impact on the noise.
Tyre squeaking is not dangerous. It is often quite the opposite - the squeaking acts preventively, informing us that we are approaching the limit of tyre grip.