Pirelli is one of the world’s top six tire manufacturers, with sales of over $3.2 billion. Its product range comprises tires for cars (standard, high performance and motorsport), for trucks, buses, agri-cultural vehicles, earthmovers, and motorcycles. The company has made a considerable technological contribution to the development of quality tires. It’s first car tire patent dates back to 1901, other patents include those for radial ply tire technology (1948), low profile car tires (1970), and today’s new, technologically advanced ultra-low profile car tires, the P6000 and P3000. The Group has an unequivocal and growing commitment to technological process and product innovation. 3% of revenues are plowed back into research in six research centers around the world.
Ing. Federico Mancosu, Head of the Research center in Milan, Italy is actively promoting the use of new technology to reduce tire noise. The center uses several LMS CADA-X testing and analysis systems for tire noise characterization, as well as LMS SYSNOISE for predicting radiated noise. Pirelli is also a leading partner in the ongoing European research project, TINO (TIre Noise Optimization).
One of the objectives of TINO is to detail the surfaces of a rotating tire which actually generate the radiated noise. The approach is completely experimental and is based upon the ASQ (Airborne Sound Quantification) technique. The quantification of the contribution of the different tire surfaces to the sound pressure measured under defined conditions was carried out at the Pirelli Chassis Dynamometer. This facility consists of a 2m rotating drum mounted underground, of which the top is at floor level. The tire is mounted at a free-spinning shaft and is pushed against the electrically powered rotating drum. The speed of the drum is electronically controlled and can be kept stationary for hours. In this set-up, the boundary conditions are very well controlled and very repeatable: the dynamics of the wheel suspension and vehicle are completely excluded.
A standard production P6000 tire was investigated at a constant speed of 50km/h with a preload of 3500N. The measurements around the tire consisted of 3 different acquisitions: operational pressures measured in 336 locations at about 3cm around the tire; and determination of the near and far field acoustic FRFs between the operational pressures and 84 sub-surfaces on the tire. A total 84 x 336 = 28224 FRFs were measured. The analyses have been carried out up to 1000Hz: the far field synthesis of the noise corresponds rather well with the direct measurement and in the near field reasonable results are also encountered. The ASQ method has been validated for tire applications. (A joint paper by Ing. Mancosu and Wim Hendricx (LMS Engineering Services) will be published at the SAE Conference this year).
Another area of Pirelli activity is the development of better models for the prediction of tire noise. This is a very complicated task: tires are extremely non-linear, subject to large static, dynamic and centrifugal loads; they suffer from impact, stick and slip forces; the pumping of air in the tire grooves... When looking at the tire deformations, structural excitations like road rough-ness give rise to a broad variation of vibrations and deformations - some at higher frequencies that can not be neglected.
“At first, the Boundary Element Method using an acoustic-structural coupled model was used to predict the sound field up to the 2nd eigenfrequency of the tire (around 1000Hz)”, said Pirelli Physicist Ennio Cervi. “However this required a very fine mesh density and consequently very long calculation times. Moreover admittance boundary conditions on the surface of the road are not allowed by using the Boundary Element Method. The traditional frequency domain structural-acoustic coupled modeling was put to one side and a time domain calculation was started.”
Pirelli were one of the first SYSNOISE users to adapt the new I-FEM method (see LMS News Vol. 13, 1). I-FEM offers an FE alternative to boundary elements; where a mix of conventional FE elements are used to fill the near field around a radiating structure, com-plemented by one layer of infinite elements stretching to infinity. While the number of acoustic DOFs is higher than with boundary elements, calculation time in general is significantly shorter, because the matrix of the system of equations is sparse. “When early versions of Pre/SYSNOISE became available during the TINO project, the major problem of generating meshes was eliminated” continued Ennio. “We then opted to use I-FEM, since we wanted to analyze in the time domain, and I-FEM gives dramatically faster solutions.” Pirelli use ABAQUS to simulate the dynamics of the rolling tire going over a small 30ms bump, and LMS has developed a special interface to SYSNOISE to transfer the acceleration data to the ‘static’ radiating surface. The noise pattern is then predicted for every 0.5ms of the bump event.
