In this part of our series about how ranking lists of tyres are prepared, we consider how noise and comfort are assessed in tyre tests.

In the context of tyres, comfort is a broader concept than noise. It basically covers acoustic comfort (i.e. noise) as well as mechanical comfort (all types of vibrations associated with the tyres, resonance effects, shock absorption, etc.).

What does tyre noise mean?

Noise is understood as a level of sound that causes an unpleasant hearing sensation or even 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 is the potential noise generated.

Where noise is generated as a tyre rolls

When a vehicle is in motion, there is constant interaction between the tyres and the road. The tyre rotates and undergoes continuous deformations, and its tread blocks strike against the road surface. The chief sources of noise can be listed as follows:

  • Striking of the tread blocks when they come into and leave contact with the road surface, due to vibrations of the tyre belts, which cause wailing and booming sounds from the tread.
  • Compression of air in the tread grooves, which leads to two effects: resonance of the air in the network of tread grooves (including turbulence at the points where the channels meet), and vibrations of the air released from the back of the tyre.
  • Vibrations during contact and decompression of the tread blocks when leaving the contact surface.
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Tyre comfort and noise are tested by various methods (photo: Vredestein).

Every tyre has its 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 a reason why a given tyre model is 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 noisiness of a tyre is also a result of errors, imperfections or conscious decisions made by the manufacturer (for example, in order to improve other important performance parameters such as grip). It should be mentioned how important it is for driver comfort that tyre manufacturers work together with makers of vehicles and with road construction specialists.

What is the acceptable level of tyre noise?

Tyre manufacturers do not generally have any problem meeting the current noise requirements set for tyres to gain official approval. 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 approach to within 2dB(A) of the limit. The limits are laid down in UN ECE 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:

Nominal cross-sectional width of tyre Permissible noise level dB(A)
<145 72
145 - 165 73
165 - 185 74 
185 - 215 75
>215 76

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:

Nominal cross-sectional width of tyre Permissible noise level dB(A) Difference from current limits in dB(A)
<185 70 do -2 do -4
185 - 215 71 -4
215 - 245 71 -5
245 - 275 72 -4
>275 74 -2

For reinforced (XL) and M+S tyres the limits will be raised by 1 dB(A).

What does tyre comfort mean?

One of the functions of a tyre is to absorb shocks and suppress vibrations. In short, a tyre is supposed to be one of the parts of a vehicle that ensures comfortable travel. In some cases it may perform that role poorly, or even become a source of discomfort. It should be remembered, though, 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

The reason for discomfort generated by tyres may be:

  • a manufacturer’s conscious decision to reduce comfort parameters in order to improve others;
  • the characteristics of a particular model resulting from its design goals;
  • low quality and errors in production.

The principal factors are the shape of the tread pattern, the tyre’s construction and its solidity in the shoulder area. A tyre must be constructed so as to provide a certain balance between rigidity and elasticity. Particular models of tyre, 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.

Effect of comfort on other tyre parameters

Mechanical discomfort resulting from vibrations is directly linked to noise. Lower-profile tyres, which are normally wider, generate more noise, and their low height and the greater rigidity of the sides and of the whole tyre lead to greater discomfort from their failure to absorb shocks. 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 grip on wet surfaces and resistance to aquaplaning. Tests have shown, however, that there is no definite link between a tyre’s noise levels 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.

The impact of comfort and noise levels on a tyre’s final ranking is a matter for discussion. Although it is an issue of importance to users, it cannot compete with such parameters as grip or behaviour on turns. Nonetheless, an extremely low level of comfort will disqualify a tyre in the eyes of users.

How are tyre noise and comfort tested?

External noise testing methods. There exist 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 a measurement 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 motion 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 speeds 70–80 km/h, and four in the range 80–90 km/h.

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Testing area for tyre noise using the coast-by method.

There also exist 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 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 pressure for a defined length of time. This is repeated several times.

Also of great value are tests of comfort and noise carried out by experienced test drivers. These tests involve comparison 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.

Runs using reference tyres therefore occur with much higher frequency than in the case of braking tests, for example. This is done to eliminate the effect of the sequence of tested tyres on the final results.

Comfort testing methods

Depending on the aspect of comfort being tested, various tests are carried out:

  •  To assess shock absorption on a bumpy road, the vehicle is driven on such a surface at 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 uniform 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 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 his or her impressions.
  • To assess comfort on turns, tests are carried out on tracks consisting of a series of turns with different radii.