Time and wear make tyres louder, and quieter – study

The results of a study carried out by the Hong Kong Polytechnic University indicate that two changes through the life of a tyre – tread rubber hardness and tread wear – have a counterbalancing effect on each other. Wing-tat Hung, Yat-ken Lam and Chung-fai Ng, from the university’s Department of Civil and Environmental Engineering, and Randolph Chi-kin Leung from the Department of Mechanical Engineering, presented the findings of their study ‘the impacts of tyre hardness and tread depth on tyre/road noise’ at this week’s ICSV 20 congress in Thailand.

The group measured the two abovementioned aging-related parameters on four Yokohama size 185/65R15 88H C.drive tyres produced in week 31 of 2007. The tyres came from the same production batch and were measured at varying hardness and tread depths. Unlike most previous research into tyre rolling noise, the Hong Kong-based academics did not conduct their study in a laboratory using drum facilities. Instead, they employed an ISO11819-2 certified twin-wheel CPX (close proximity) vehicle that was developed at the university. Tests were conducted at a test track on both hot rolled dense asphalt and porous asphalt road surfaces. 

The four tyres (labelled A, B, C and F for the study) were subjected to CPX tyre/road measurement tests at various stages. Tread hardness and depth were measured both in the laboratory and immediately before rolling noise tests were held at the test track. The study group observed that rubber hardness measured in the laboratory was 2-3 Shore A higher than in field measurements, the difference explainable by the higher outdoor temperature. As expected, tread depth remained constant regardless of whether it was measured in the lab or outdoors.

Measurements on the two road surfaces were carried out at test speeds of 50 and 70 km/h and numerous runs were performed to ensure the data recorded over a 200 metre stretch of road was recorded according to ISO criteria. The results showed the differences in measured rolling noise between the four tyres to be less than 1 dB at both speeds on the hot rolled dense asphalt surface. The differences of the measured tyre/road noise levels on the porous asphalt surface also remained small, albeit the difference between the highest and the lowest level was marginally larger.

As the tread depths of tyres A and C were nearly the same, the difference in tyre/road noise level between these two tyres can be mainly attributed to the difference in rubber hardness. Indeed, the measured noise levels using tyre A is constantly higher than that measured for tyre C (0.6 dB(A) at 50 km/h and 0.8 – 0.9 dB(A) at 70 km/h), indicating that noise levels increase as rubber hardness increases.

The rubber hardness of tyres C and F was nearly identical and therefore the difference in measured noise level between these two tyres can be mainly attributed to the difference in tread depth. In fact, the noise levels measured using the F tyre were always 0.6 to 0.8 dB(A) higher at 50 km/h and 0.5 dB(A) at 70 km/h, which shows that rolling noise decreases as tread depth reduces.

Results

The study found that as a tyre ages, the tread rubber becomes harder, increasing rolling noise. And a reduction in tread depth counteracts this, as it causes rolling noise to reduce. But does this mean a shallower tread depth will cancel out the added rolling noise caused by rubber hardening? Yes and no – it depends upon speed and road surface. 

The results show that rolling noise at 50 km/h on the hot rolled dense asphalt surface rose as tyre rubber hardened and decreased as tread depth became shallower. This same phenomenon was observed at 70km/h for tread depth, but no significant change due to rubber hardness was observed. For the porous asphalt surface, similar results were obtained, except that no significant relationship between noise and tread depth can be drawn for reference at 50 km/h. 

Rolling noise peaked at around 1000Hz on the hot rolled dense asphalt surface, and spectra analysis showed rubber hardness had little effect at this frequency, while tread depth had a significant effect at this frequency range. Tests on the porous asphalt surface showed a peak in rolling noise at around 800Hz, and at this frequency band rubber hardness showed a significant effect while tread depth did not have any significant effect.