Lastik-yol gürültüsü
Tire-road noise
Yazar: FATİH BAY
Danışman: DOÇ. DR. AHMET GÜNEY
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Otomotiv Bilim Dalı
Derece: Yüksek Lisans, 1998
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Gürültü = Noise ; Otomobil lastiği = Automobile tire ; Taşıtlar = Vehicles ; Yollar = Roads

Özet:
ÖZET Yüksek hızlarda seyreden motorlu taşıtlarda lastik-yol gürültüsü, toplam taşıt gürültüsü içinde en büyük etkiyi oluşturmaktadır. Ancak bu gürültünün incelenmesi ve kontrol edilmesine yönelik çalışmaların azlığı ve bir ölçüde çelişkiler içermeleri sebebiyle, literatürde bir boşluk bulunmaktadır. Lastikle yol kaplaması arasında oldukça karmaşık bir ilişki vardır ve lastik-yol gürültüsü çeşitli mekanizmalar sonucunda ortaya çıkmaktadır. Ancak, lastik-yol gürültüsü temel olarak lastik yanak titreşimlerinden kaynaklanmaktadır. Profiller arasında bulunan havanın sıkışmasıyla temas yüzeyinde oluşan hava hareketleri (Hava Pompalama - Air Pumping) ve temas yüzeyinin ön ve arka kısımlarında oluşan hava rezonansları (Horn Effect) da gürültü oluşumunda etkili olan mekanizmalardır. Lastik-yol gürültüsü seviyesi taşıtın çalışma şartlarından, lastik özelliklerinden ve yol kaplaması özelliklerinden etkilenmektedir. Lastik özelliklerine bağlı olarak, taban desenini geliştirmek, lastik malzemesinin sertliğini azaltmak ve lastik karkasının katılığını arttırmak, oluşan gürültünün seviyesini azaltacaktır. Fakat tüm bu değişiklikler lastiğin performansını olumsuz yönde etkilemektedir. Lastik-yol gürültü emisyonu yol kaplamasının gözeneklilik miktarı ve akustik yutuculuk özelliklerine bağlı olarak da azaltılabilmektedir. Genel olarak, gözenekli ve pürüzlü yüzeyler düzgün yüzeylere göre daha sessiz olmaktadırlar ve yol tutuş özellikleri de daha iyidir. Yol kaplamasında yapılacak geliştirmelerle, gürültü seviyesinde daha büyük iyileştirmeler sağlanabilir. Taşıtın çalışma şartlarına bağlı olarak ise, taşıt hızının artması ve tekerleklere tahrik veya fren momenti uygulanması, lastik-yol gürültüsünün artmasına neden olmaktadır. Lastik-yol gürültüsü ölçümünde kullanılan metotlar genel olarak Serbest-geçiş metodu, Laboratuvar tambur metodu, Römork metodu ve Römorkla serbest-geçiş metodudur. Tüm bu metotların kendilerine özgü olumlu ve olumsuz özelikleri mevcuttur. Uygulanacak metot belirlenirken, bu özellikler yapılan çalışmanın amacına bağlı olarak değerlendirilmelidir. Tek başına lastik-yol gürültüsünü kontrol eden ve sınırlayan mevcut bir yönetmelik yoktur. Lastik-yol gürültüsü ölçümünü tarifleyen uluslararası standartlar ise Amerika için SAE J57 ve Avrupa için 92/23/EEC ile sınırlıdır. XII

Abstract:
SUMMARY TIRE-ROAD NOISE Road traffic noise originates not only in the power units of vehicles, including engine, exhaust, air intake and transmission, but also in the interaction between tires and the road surface, so-called tire-road noise. Tire-road noise generally dominates over the other sources altogether above about 100 km/h for all vehicles and about 60 km/hr for modern small cars. On wet roads, speeds can be much lower to give the same degree of tire noise. Since the noise from the power unit (power-train noise) is subject to regulations with increasingly tighter limits, tire-road noise will become even more important in the near future on a relative basis. Tire-road noise is emitted both into vehicle cabin, i.e. interior tire-road noise, and to the roadside environment - exterior tire-road noise. The interior part is a very important sales argument; thus it is handled efficiently by the tire and vehicle industry in order to obtain the best balance between the various performance aspects, including the acoustical environment. However, no specific demands have been formulated on exterior tire-road noise - either from customers, public or authorities. Figure 2.3 illustrates the location of the various sources of noise for a typical diesel engine goods vehicle tractor unit and Table 2.1 gives an example of the relative magnitude of the noise from the separate sources for a vehicle of this type operating in top gear at a steady speed of approximately 50 km/h. A similar distribution for a passenger car is given in Table 2.2. When assembled into a complex vehicle, the rank order of these sources and the combined noise level can change due to the screening effects of the vehicle body and reflections from the road surface. Several possible sources of tire noise have been reported by the workers in this field. These include aerodynamic noise from the rotation of the wheel and tire, noise from the vibration of the tire surface, and pressure fluctuations caused by pumping of ah in the tread grooves in the contact patch. The dominant noise generation mechanism of cross bar tires is the sidewall resonance with the tread vibration caused by the collision between the lug blocks and the road. In general, variations in tread pattern have small effects on the overall levels of tire noise. However, tires with regular transverse features are systematically noisier than tires with predominantly circumferential patterns. As the angle between the displacement direction of the tire and the diagonal grooves decrease, tire noise level also decreases. Reducing the hardness of tread rubber and increasing the stiffness of tire carcass also lowers tire-road noise emission (Figures 3.4, 3.5). XIIIThe principal road surface characteristics that effect tire noise generation are surface pattern and the acoustical absorption characteristics of the surface material. Porous or pervious surfaces are known to suppress the air pumping component of rolling noise. Also their acoustical absorption characteristics are quite good when compared with the conventional dense surfaces. As a result, tire-road noise on this kind of porous surfaces are found to be lower. But the main beneficial effect of these surfaces arise on traffic noise for free flowing traffic at high speed and they perform poorly at low speeds, the reason for this is the effect of texture on the generation of rolling noise. Similar to dense surfaces also with porous surfaces the unevenness of the road surface induces vibration of the tire carcass and the tread system, which are radiated as noise. The peak noise in dB(A) of a coasting vehicle measured at 7. S m along the normal to the centreline of the vehicle path is related to the vehicle speed by an equation of the form: LAmax = A + BLogi0V (3.2) where LA,max is the peak noise level, V is the vehicle speed (km/h), and A and B are constants depending on the tire characteristics and surface texture. In accelerating condition, tire noise emission increases as torque is applied to the wheels. There are no present regulations on noise emission from tires of vehicles. The two only available international standards describing the measurement of tire noise are SAE J57 (Figure 4. 1) and 92/23/EEC. The four methods being used for the measurement of tire-road noise level are coast- by method, trailer method, laboratory drum method and trailer coast-by method (Figures 4.2-4.7). In coast-by method, a test vehicle equipped with test tires is rolling with the engine switched-off (coasting-by) past a road-side located microphone. The microphone is 7.5 m from the centre of the test track or road lane on which the vehicle coasts. The maximum noise level (A-weighted) is recorded when the vehicle coasts by, usually with FAST time constant. It is also common to make a recording of the frequency spectrum at the moment of peak A-weighted noise. The method is often used to classify both the road surface and the tire influence on noise. Trailer method requires fewer test tires than the previous method and is not very sensitive to noise from traffic. The test tire is mounted on a trailer which is towed by a vehicle. Close to the test tire, generally within 0.1-0.5 m, one or more microphones are located. The noise level is measured as an average over a certain time interval, generally 4-60 s. Most trailers have an enclosure around the microphone and test tire in order to provide screening from wind and traffic noise. Some trailers may utilize more than one test tire. Laboratory drum method makes use of a drum facility (drum diameters of 1.5-2.5 m are the most common). A test tire is mounted so that it can roll against the drum and one or more microphones is (are) mounted close to the test tire - in principle xivwith similar positions as in the trailer method. Special care must be observed regarding the acoustical environment. It is absolutely necessary to have the drum equipped with a surface which resembles that of a actual road. This can be achieved by fitting a moulded replica of a road surface onto the drum in segments. Trailer coast-by method is a combination of the coast-by and trailer methods. A trailer is towed by a vehicle along a test track and the microphones are located as in the coast by method. However, the noise to be recorded is the noise when the trailer is passing, i.e. the noise must not be recorded at the moment when the tow vehicle is closest to the microphone. The problem of separating out the noise emitted by the trailer from that of the tow vehicle can be relaxed if the drawbar is very long and the tow vehicle is equipped with the most quiet tires available and also with screens over the tires. It may also be necessary to make special measurements of noise from the tow vehicle alone and correct for this noise in some way. It may be claimed that the best way to control vehicle noise at high speeds is to introduce a high speed pass-by test. But a constant-speed test like that is not so easy to make reproducible in practice, since when aiming at a certain speed the driver might want to adjust the throttle opening more or less. In this way, one does not really know how much the engine was loaded at the moment when maximum noise was recorded. It seems better to separate power-train and tire-road noise measurements. This would have the advantage of separating the responsibilities of vehicle and tire manufacturers. If one would test the same tire on different vehicles, one would probably not get fully identical results because of vehicle influence. Vehicles may influence tire noise in these ways: screening by or reflections against body parts, poor wheel alignment, brakes not fully released, air turbulence noise, disturbance from power train noise, rattling of loose parts, noisy suspension springs, influence on tire dynamics by extremely soft or stiff suspension different tire loads, and disturbing bearing noise. The level of the influence of different vehicles is found to be limited to maximum 1 dB(A) at 70-90 km/h. Vehicle influence would, naturally, be highest with the coast-by method, lower with the trailer coast-by method and lowest with the trailer and drum methods. Fortunately, tire load has not been found to influence noise very much. However, the tire width turned out to be the important parameter and the width of course is related to the weight of the vehicles, since wider tires are used for carrying higher loads. Tire-road noise increases with speed by 9-10 dB(A) per doubling of speed. Thus, double speed means approximately double subjective impression of the noise. For reducing the background noise influence, one should go as close to the test tires as practical with the microphone. Of the two main options, a microphone distance of 7.5 or 15 m (used in Europe and USA respectively), the former is preferable from this point of view. xvThere are two really serious problems with background noise, both of them connected with trailers. Firstly, in trailer method, many disturbing noises may seriously limit such measurements. Most measurements are feasible if one excludes the lowest frequencies. The other serious problem is for the trailer coast-by method, in which it is necessary to limit the influence of the tow vehicle noise emission. The most obvious and simple way to correct for this noise is to make separate runs for the articulated tow and trailer vehicle and for the tow vehicle alone, note the maximum noise levels and subtract the tow vehicle maximum noise level from that of the combined tow/trailer vehicle (Figure 4.9). Test surface porosity might cause a sound absorption effect. Therefore, it is essential to make sure that the surface air voids content does not exceed 8 % and/or that the sound absorption coefficient over the interesting frequency range does not exceed 10 % (according to ISO 10844). Slightly higher absorption values may influence the sound propagation, eve n if such absorption occurs only outside the wheel tracks, between the test lane and the microphone. A special problem is the influence of sound propagation when using the trailer method with a near-field measuring point. Here the problem is the reverse: A sufficient absorption effect is not measured. It has been found that a limit of maximum 5 m/s is suitable for the wind speed. At higher wind speeds, the propagation can be affected but, moreover, there will be low-frequency noise in the microphone induced by the air turbulence. For temperature effect, an influence of around 1 dB(A) per 10°C has been recorded (noise decreases with increasing temperature). Tire-road noise emission is often clearly directional. Most commonly, noise emission is stronger in the directions towards the front and rear of the tires. To at least some extend this can be explained by the so-called horn effect. When the available methods are generally reviewed, coast-by method appears as the one which is the most representative of actual traffic. This method gives fair and accurate results. Its harmonisation with related methods is maximum and works equally well for automobile and truck tires. Advantages with the coast-by method are that it is not "traditional", it is not complicated at it requires no extra equipment. A disadvantage is that the method is influenced by test vehicle type and condition. Also, it requires more tires than the others, which means more labour with balancing and exchanging tires. On the plus side, however, the use of a number of tires will automatically give an average over more than one tire sample and directional effects will be somewhat averaged. As an indoor method, drum method has the major advantage of being independent of climate and weather conditions. However, the method is not representative f the actual traffic. Firstly, noise is measured in the near-field, not in the far field where a listener would be. Secondly, the drum is curved, but this effect is relatively unimportant. Thirdly, it is necessary that the drum be equipped with a replica road surface. xviTire ranking with this method is less fair than with the coast-by method. However, the method will give very accurate and repeatable results. Harmonisation with other methods is very poor, but it is very suitable for research purposes. The main disadvantages with the trailer method are its lack of representativity and the background noise problem. Regarding accuracy, it is almost as good as the drum method. Harmonisation with other methods is very poor (except with the drum method) and requires expensive equipment. However, tire testing with this method is very fast and efficient. Representativity of the trailer coast-by method is somewhat poorer than for the coast-by method but absolutely better than for the drum or trailer methods. This method is less influenced by test vehicle characteristics and condition. Tire ranking will be less fair and accurate than for the coast by method. Repeatability and reproducibility will be poorer due to tow vehicle noise. Tire noise can be reduced from present-generation levels but only at the expense of safety. It appears likely, therefore, that the noise from future vehicles will be limited by the noise generated by the tire-road surface interaction. Further improvements in tyre-road noise are more likely to arise from improvements in the design of the road surface. XVII