In the context of tyres, comfort is a broader concept than noise. It basically covers acoustic comfort (i.e. noise) as well as the overall driving comfort (tyre vibration, resonance effects, shock absorption, etc.).
Where does tyre noise come from?
Tyre noise is understood as the level of sound that is unpleasant or even causes discomfort to the driver, passengers and bystanders. Sound consists of vibrations travelling through the air in the form of sound waves.
The source of the vibrations may be a solid body (a tyre in this case) which stimulates the air to vibrate, creating pressure waves all around it. In principle, the more the source (tyre) vibrates, the greater the potential noise is generated.
Noise is generated by a rolling tyre
When a vehicle is in motion, there is a constant interaction between the tyres and the road.
The main sources of noise can be listed as follows:
- When the tread blocks come into contact with the surface of the road, the vibrations of the tyre belts cause wailing and booming sounds from the tread.
- The most prominent interior noise we hear is tyre cavity resonance which is caused by the air that fills the space between the tyre wall and the wheel itself. This air vibrates when the tyre is moving in contact with the road and can be heard within the car.
Tyre comfort and noise are tested by various methods (photo: Vredestein).
Every tyre has a natural frequency – the frequency at which it vibrates most strongly. Problems arise when this frequency is close to the frequency of the stimulation (which results from, for example, the roughness of the road).
Then, instead of absorbing noise and vibrations, the tyre amplifies them. A greater problem occurs when this frequency is also close to one of the natural frequencies of the vehicle. This may be the reason why some car tyres are regarded as quiet by some drivers, and by others as noisy. In other words, the tyre, the vehicle, and the road surface (roughness, frequency of lateral bumps, etc.) are all significant factors.
In many cases the high level of noise of a tyre is also the result of errors, imperfections or conscious decisions made by the manufacturer (for example, in order to improve other important performance parameters such as grip).
What is the acceptable level of tyre noise?
Tyre manufacturers generally don’t have any problem meeting the current noise requirements set for tyres. Differences recorded between tyres in tests amount to 6–8 dB(A) on average. Many tyres are 4–6 dB(A) below the limit, but a large proportion are within 2dB(A) below the limit.
The limits are determined in UNECE Regulation 117 (uniform provisions concerning the approval of tyres with regard to rolling sound emissions and to adhesion on wet surfaces).
They are as follows:
For reinforced (XL) tyres the limits are raised by 1 dB(A).
However, with the introduction of the tyre labelling regulations, the noise limits will be made stricter, and the classification of tyres based on width will be slightly reorganized. The new limits are as follows:
For reinforced (XL) and M+S tyres the limits will be raised by 1 dB(A).
What is tyre comfort?
One of the functions of a tyre is to absorb shocks and suppress vibrations. In some cases they may perform poorly, or even become a source of discomfort to some drivers. However, it should be remembered that comfort is a subjective concept.
Different people may consider different things to represent discomfort, and may have different comfort limits. This depends largely on perception levels, but also on cultural factors, individual predispositions and the particular situation.
Another aspect of tyre comfort is the psychological comfort which a tyre gives the driver. This results from a driver’s trust in particular tyres and the predictability of their behaviour.
Sources of discomfort in tyres
Reasons behind greater tyre discomfort generated by tyres:
- a manufacturer’s conscious decision to reduce the comfort in order to improve other parameters;
- the characteristics of a particular model resulting from its design;
- low quality and errors in production.
The main factors are the shape of the tread pattern, the tyre’s construction and its hardness in the shoulder area. A tyre must be constructed so as to provide a certain balance between rigidness and elasticity.
Particular tyre models, which have to meet requirements relating to speed and load indexes, may have reduced ability to absorb vibrations. The larger a tyre is – the greater its diameter and width – the lower is the level of comfort provided.
Tyres with higher sides, which are normally relatively narrow, are regarded as providing the greatest comfort. They may indeed be quieter and absorb road surface unevenness better. However they are more susceptible to tilting and rocking, for example when cornering or overtaking a lorry, and this is often a cause of dissatisfaction with tyres as reported by drivers.
How comfort effects other tyre parameters
Mechanical discomfort resulting from vibrations is directly linked to noise. Low-profile tyres, which are normally wider, generate more noise, and their greater rigidness leads to greater discomfort from their lower ability to absorb shock. If the level of vibrations is caused by non-uniformity of the tyre resulting from the production process, then vibrations and noise will usually be felt simultaneously.
Remember that every tyre represents a compromise between different parameters. In the case of comfort, if a tyre were to be made quieter, it would be at the cost of reduced tyre grip on wet surfaces and resistance to aquaplaning. Tests have shown, however, that there is no definite link between a tyre’s noise level and rolling resistance.
Another way of designing a very quiet tyre is to use small tread blocks. However such a tread is not visually attractive, and we should remember that many users attach great importance to the visual design of their tyres.
How are tyre noise and comfort tested?
External noise testing methods. There are many methods for determining the levels of noise and comfort provided by a tyre. Noise is usually tested using methods similar to those used in official tyre approval tests. These are known as coast-by methods.
Such measurements should be made within a test zone in the form of a square measuring 20 by 20 metres, with microphones placed in the centre of the zone at a distance of 7.5 m from the axis of the vehicle.
The track should be made of an aggregate with a size of not more than 8 mm. Unfortunately, this type of surface does not correspond to all types of roads encountered in Europe. Consequently, tests carried out using this procedure do not provide a complete picture. It may also happen that manufacturers optimize their tyres for these conditions, and so they may be somewhat noisier on other types of surface.
In practice roads may be smoother (mainly in large urban areas), but they are also often rougher, with significantly larger aggregate sizes of around 14–15 mm, as typically found on motorways.
According to the methodology, the driver enters the test zone at a specified speed, and then engages neutral gear and turns off the engine. At least four measurements are made within the range of 70–80 km/h, and four in the range 80–90 km/h.
Testing area for tyre noise using the coast-by method. There are also methods called “drive-by” and “cruise-by”. In a drive-by test, the vehicle enters the measurement zone at a speed of 50 km/h in second or third gear, and then accelerates while passing through that zone. In the cruise-by method, the vehicle passes through the test zone at a constant speed.
Internal noise levels – tests with dummies
This test is performed with a test dummy sitting in the passenger seat, with microphones in its ears connected to a recording device and a computer. The test procedure is as follows: the vehicle is accelerated to a specified constant speed (e.g. 80 km/h), and the measurement equipment is then activated to record the level of sound for a defined length of time. This is repeated several times.
Another good option are tests carried out by experienced test drivers. These tests involve comparing a given tyre model with a set of reference tyres. They are usually carried out in the following sequence:
- reference set
- test set 1 relative to reference set (several rounds possible)
- reference set
- test set 2 relative to reference set (several rounds possible)
- reference set
- ...
- reference set
Comfort testing methods
Depending on which aspect of comfort is to be tested, various tests are carried out:
- To assess shock absorption on a bumpy road, the vehicle is driven on such a surface at a relatively high speed (e.g. 80 km/h). The driver and/or suitable sensors evaluate the amount of vertical acceleration when driving over bumps, the time and effectiveness of shock absorption, and the tendency to produce resonance effects.
- To assess the effect of driving over a single obstacle, a test is made where the vehicle passes over a single bump, step, pothole or manhole cover, suitably prepared on the track, at a speed of around 40–50 km/h. The driver assesses the amount of vertical acceleration and the noise accompanying the impact.
- To assess effects related to the shape of the tread, two tests are carried out. The first aims to determine the effect of the tread pattern during travel. It involves driving at a constant speed of 80–100 km/h. The second test determines the level of wailing and booming of the tread during changes of speed. For this purpose the test vehicle is accelerated to above 100 km/h, and is then slowed down to a halt. These tests are carried out on a smooth road.
- To assess road noise (i.e. to determine overall noise) the driver travels over a section of rough road at a speed of around 80–100 km/h and then describes their impressions.
- To assess comfort on turns, tests are carried out on tracks consisting of a series of turns with different radii.