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14/05/2019
Silence is gold

 The sound produced by the tires

 

Probably not the main problem for tire manufacturers, but certainly one of the many on which technicians are hard at work.

Paolo Ferrini

"Once all vehicles will be electric, the only sound we will hear is the sound made by tires", at least this was the common opinion in several research centres and among some of the world's leading car manufacturers. Whether true or false, some of the engineers must have felt touched where it hurts and decided that the time had come to focus on this matter.

Frankly speaking, no matter how high the label rating parameters are, rolling noise may not necessarily be at the top of a tire manufacturers' priority list, but it is an indisputable fact that this will influence driving comfort and the image a product conveys to the end user. And as such, it can make the difference between a premium and a budget tire.

Every designer’s goal is to reduce the rolling noise of a tire as much as possible, while still guaranteeing the necessary traction and grip. To better explain a development path on seemingly diametrical opposed characteristics, let us take, for example, off-road tires: undoubtedly traction represents a strength, thanks to their aggressive tread designs, but when used on normal roads they can prove extremely noisy. On the other hand, tires with less grooves and tread blocks are quieter on tar roads, but become rather unsafe on wet or unpaved roads. Not to mention that the type of terrain does influence not only the intensity, but also the spectrum of sound. Variations in the latter can be found simply by passing from one tar road to another, if the roughness of the two roads is different.

However, the full/void ratio of a tread design is not the only factor determining a tire's rolling noise. Equally important are the number, shape and positioning of the tread blocks: if they were all made the same way with the same characteristics, they would end up generating a single tone and of an equal level of intensity. All four tires would thus have a single tone, which would be particularly unpleasant for those on board.

In general, we can say that high-performance summer tires have fewer and larger tread blocks designed to guarantee better steering response and greater control. On the other hand, winter tires, require a larger number of smaller tread blocks, and deeper grooves, to increase traction on snowy and icy roads.

The general structure of the tire, particularly the casing, greatly contribute to a tire’s noise level, as they influence the stiffness of the tread and the vibrations of the wheel-tire-vehicle package. Hence the need to develop dedicated technologies - such as, for example, ContiSilent, Goodyear SoundComfort, Michelin Acoustic, Bridgestone, Pirelli NoiseCancelling System and Toyo’s Silent Technology – which could be summed up as a special sound-absorbing cushion inside the tread.

 

ContiSilent, silence is a luxury

ContiSilent is a tire noise-reducing technology developed by Continental, and designed to reduce interior noise on all road surfaces. ContiSilent technology provides for an inner tire absorber, a polyurethane foam, attached to the inner surface of the tread area. Regardless of the temperature, the structure of the foam stays intact.

ContiSilent helps reduce interior vehicle noise up to 9 dB(A) compared to a standard tire. Available for Continental summer and winter tires alike, ContiSilent is compatible with all commonly available rims. Driving performance is not affected and there is no negative influence on mileage and load/speed capability. Fitting on four positions is recommended.

 

SoundComfort, 2019 product of the year

Goodyear SoundComfort technology is a good example of the American company’s commitment towards developing new solutions to meet the needs of consumers and the market. Voted product of the year 2019 by consumers on the basis of a survey conducted in Belgium and the Netherlands on a sample of 5,000 people by the international market research firm Nielsen, SoundComfort technology involves the application of an open-cell polyurethane foam ring on the inner surface of the tire to reduce vibration noise. The result is a noise reduction of up to 4 decibels. This makes travelling much quieter without compromising driving performance, as the foam used is light enough not to affect fuel consumption or the tires in any way.

 

Michelin Acoustic, a ring for everyone

During the 2018 International Motor Show in Frankfurt, MICHELIN unveiled its Acoustic Technology which provides for a custom designed polyurethane foam ring inside the tire. This solution effectively reduces vibrations caused by the contact of the tire with the road by approximately 20% and dampens interior noise when driving on changing road conditions.

 

Bridgestone’s tailor-made solutions

Bridgestone uses predictive software to develop “silent” tires right from the word go, following different paths in order to counteract the different types of tire-related noises. The B-Silent technology, which involves inserting a sound-absorbing sponge inside tires, aims, for example, to dampen the cavity noise, produced because a tire is internally empty. The VPS technology, applied to the tread pattern, calls for tread blocks of different sizes to dampen the noise produced by pumping the air trapped in the footprint. Finally, Groove Fence uses small inserts inside the grooves to limit the aerodynamic noise that develops while a tire is rolling or when fighting air resistance, diverting it sideways.

 

PCNS, feels like travelling with three tires

Twice as many approvals for Pirelli’s PNCS technology over the previous year, rising from 78 at the end of 2017 to over 150. The system designed and launched on the market in 2013 met the increasing demand by car manufacturers, especially in the premium and prestige segments, looking to offer superior comfort to both drivers and passengers.

Pirelli’s NoiseCancelling System, this is the meaning of the acronym, is a technology that allows Pirelli to reduce noise by up to 25%: basically it is as if a car is travelling with only three wheels. Here too, this is made possible by the use of a special soundproofing “sponge” placed in the cavity of the tire, which dampens the vibrations of the air that would otherwise be transmitted inside the vehicle, generating a background noise at the expense of comfort. This "sponge" is composed of an open cell foam that maximizes the interactive surface, intensifying the effect of vibration absorption thus reducing noise. Tires equipped with this technology can be recognised by the fact that they bear a crossed-out loudspeaker pictogram next to the initials PNCS.

 

Silent Technology, a different path

Toyo Tires, on the other hand, followed a completely different path, acting on the longitudinal and vertical air flows generated inside the tire. According to the Japanese engineers, noise is reduced when it passes through a pore. This happens due to the friction that is generated by the walls of the pore crossed by the air and the vortex generated by the sound that passes through the pore. Thus, Toyo engineers came up with a porous film that contrasts the air flow creating a structure through which the sound is forced to pass. In order to intercept the flows in both directions, vertical and longitudinal, an arched perforated film was inserted inside the tire. To do this, a few cylindrical foam inserts were placed in the tire to keep the film in the desired shape. In addition, the hollow structure of the cylindrical foam has a noise reducing effect which, working in synergy with the perforated film, further increases the noise reduction effect.

 

NEWS FLASH: WILL “MAKING NOISE” BECOME MANDATORY?

All new electric and hybrid vehicles sold from July 2019 may be required, by law, to make some noise when travelling below 20 km/h. Currently under consideration by the United Nations Economic Commission for Europe, a new regulation may require from 2021, the installation of a so-called Acoustic Vehicle Alerting System on all electric vehicles, even if already registered. This is to protect the safety of cyclists and pedestrians who, especially in urban areas, may not notice the presence of such a vehicle in their immediate vicinity.

This new regulation considers an emission that covers all frequencies and which, because of its constancy, could be compared to the sound of rain or wind in the leaves or even waves. Each car manufacturer can customize it without exceeding the 75 dB measured at a distance of 2 meters.

Details are still to come and the system still needs some adjustments. For example, a couple of possible amendments might allow the use of these systems not only below 20 km / h, but also at higher speeds, and also to increase safety.

 

THE IMPORTANCE OF PREVENTIVE MAINTENANCE

Periodic tire rotation is a vital preventative maintenance operation. Particularly important is the first rotation in order to reduce the tire wear rate later on, leading to a reduction in the noise produced as well.

Considering the different existing diameters, on average a tire turns about 600 times every kilometre. Each time the front edges of the tread blocks come into direct contact with the road, followed by the rest of the tread block. This is repeated millions of times in the life cycle of a tire, leading to irregular wear which in turn leads to a change in the initial shape of the tread pattern.

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