The proposed reduction in tire noise that came into force as legislation in 2012 stimulated a lot of interest in the underlying mechanisms responsible for radiated noise.
Observation of the noise signatures of various tires, from a variety of manufacturers, revealed among other things a modulation or pattern within tire noise indicative of resonances. Were these lab effects or due to the structure of the tire?
Experimental and analytical modal analysis of a wheel is pretty straightforward for a freely suspended wheel and possible for a stationary wheel attached to a vehicle. Neither of these is, of course, the case we were interested in. We needed to know which were the dominant resonances when the wheel was in service on different road surfaces, at different speeds.
The acoustic modes of the tire cavity were much easier to calculate and there was some published research but no indication of the acoustic pressures being generated within the tire cavity for a running tire. This, rather fundamental, gap in knowledge for the cavity resonance was perhaps due to the lack of instrumentation capable of making the measurement. The problems involved in producing a tire cavity microphone (TCM) were diverse rather than insurmountable but the market for such a product was very small.
The primary cavity mode resonance at approximately 200Hz is clearly audible inside many vehicles and even in some considered premium. How tire constructional variations might reduce the amplitude of cavity resonances was judged as likely to be of immediate value and interest to both tire and vehicle makers. The TCM was developed in 2010 for a broad cross section of tires.
Some interesting features to emerge were:
• Tires from different companies had dramatically different primary cavity resonance noise levels by at least 6dB.
• Construction detail could affect the primary cavity level by 6dB.
• A change in primary cavity amplitude of 6dB, in a prestige vehicle, was all that was needed to go from inaudible to annoying.
• The inserting of acoustic foam glued to the liner reduced the primary cavity mode by 6dB.
An example of complex spectral map for radiated noise is shown in Figure 1. The degree of modulation of the internal cavity modes as a tire runs down from 100km/h to 30km/h is shown in Figure 2, and the strength of the internal cavity modes in Figure 3.
The level of the primary cavity resonance depends on the energy feeding it. Tread pattern noise for speeds between 30km/h and 80km/h may coincide with 200Hz resonance, and should this happen, the cavity level can reach 140dB. Once excited, the mode can persist even though the vehicle speed may change slightly. Hitting a discontinuity in the road will also inject energy into the cavity which will respond strongly at the resonant frequencies. The primary cavity mode may then remain excited even if tread and road surface inputs are relatively low.
To fully understand the generation and sustain mechanisms for the cavity modes it is necessary to understand how energy flows through the tire. The source of energy is predominantly the tire’s interaction with the road surface. The interactions as each section of tire is forced through the contact patch is complex.
Figure 4 shows the complete transit of the contact patch for a summer tire. The data was obtained using a small accelerometer embedded in the liner, not particularly easy to attach but possible. The resulting figure clearly shows the very high acceleration going into and coming out of the contact patch.
The peak accelerations should be very similar; any differences greater than 20% almost certainly mean that the accelerometer has not remained firmly attached to the liner. Inspection of the graph shows that before and after the contact patch there is a wave present in the tire. These two waves, pre and post contact patch, are at different frequencies and are a function, to some extent, of the speed. For radiated noise purposes these waves may be important as they are typically at 800Hz and 1300Hz for 80km/h. These two third octave bands often dominate the radiated noise, the 1300Hz third octave being particularly sensitive as the A weighting curve emphasizes this band 0.6dB.
The work done as tire rolls under load must eventually appear as heat. This heat raises the temperature of the tire until a point of equilibrium is reached, i.e. the heat carried away by the airflow around the tire plus the heat flowing into the rim plus the heat transferred to the road equals the heat generated by the tire’s rolling resistance. The point where the tire is at the highest temperature is almost certainly the region where most work is being done and rolling resistance generated.
Figure 5 shows the temperature distribution across the tire, from shoulder to shoulder. The shoulders can be seen to be at some 5° hotter than the bulk of the tread. It would be reasonable to conclude that the transition from tread to sidewall is a region that should be optimized.
There are however some secondary effects due to the temperature of the surrounding air, the temperature of the road, and the nature of the road surface. Are these secondary to the point that they can be ignored or do they influence the performance of the tire? If they are important then they need to be quantified to ensure that any new regulatory targets are well chosen, for example road construction standards enforced rather than tire noise targets lowered would probably make freeways quieter.
The first step, after deciding which targets are possible, practicable and achievable, is to make sure that the testing regime, required by the regulations, can be fairly applied. Initially the targets have to be set where the best tires are today. These targets will need to be progressively lowered to improve our environment. The arbitrary lowering of targets does not achieve improvements if the problems are intractable or, for example, if the improvement in radiated noise level has been achieved by reducing wet grip.
For tires to improve, measurement technology must exist that can accurately record the small changes that guide a successful development. If we do not know and understand exactly what is happening how can we possibly improve?
Computer aided design and modeling can speed up design but only if the calculations and predictions can be validated. What to measure and how has always been the challenge, and for many years the testing system has been a vehicle and expert driver. Humans can be sensitive and discriminating detectors but they are also prone to non-linear responses. The development process requires the sensitivity and discrimination but without the non-linearity.
Laboratory testing will furnish almost every conceivable parameter and perhaps in the future, laboratory testing will capture the complete envelope of possible use. That day is not with us yet and will be a long time in coming.
Instrumentation that can help us understand what is happening has always been and will remain central to our objectives.
Figure 1: Typical radiated noise from a summer tire, entry microphone – note the resonant nature of the response
Figure 2: Typical tire cavity time history – note the very strong resonances
Figure 3: Tire cavity resonances measured inside the tire cavity. N.B. the five harmonics based upon 208Hz
Figure 4: Acceleration measured on the tire liner through the contact patch at 90kmh
Figure 5: Tire temperature running at 100km/h and 80 load on steel road wheel