Almost every motorist has experienced the ``chamber music'' that is generated by his tires when running at constant speed on the highway. Lulled by the monotonous sound, he can be startled when the volume increases from a soft refrain to a rock band concert due to a change in the road surface. He may then assume he has a mechanical or tire problem.
Any roughness of the road surface will beat a tattoo on the pneumatic tire's drum-case, because the pneumatic tire is much like a musical instrument. When inflated, it forms a taut hollow skin that is as responsive as a drum.
Different grades of roughness produce different sounds that can be distinguished when a vehicle moves from one type of road surface to another. Small variations in surface texture can produce sounds quite different in pitch.
Tire noise can be heard outside the car, but also is noticeable inside, where it stirs body panels to vibration.
Efforts to reduce noise from passenger tires have been aimed mostly at the comfort of the car occupants. Tires can be made to run quietly. The cross-section size, its structure and the materials used in its construction play a specific role in the tone of the ``music,'' while the tread represents the most important factor contributing to overall tire noise.
Types of tire noise
The tone emitted by a poorly designed tread pattern can vary from a howl to a roar, or a low whistle to a loud hum. A well-designed tread pattern limits itself to a quieter, swishing song.
Noise generation in a tire is always the result of a design compromise. From a technical point of view, there are three different types of tire noise.
The first is vibration in the tire body that results from stimulation of the casing and tread by road irregularities.
The second type is mechanical pumping of air. The tread is curved both circumferentially and radially so, when this curvature comes into contact with the road, it becomes flat.
This flattening causes the tread elements to move, squeezing the tread grooves when contacting the road and spreading them when the tread leaves it. Tire noise grows stronger with highway speeds and larger loads.
This applies in particular to noise emanating from the air trapped between the tread ribs and the road. Pressure differences are equalized when the grooves open up, but they do so in an unstable pattern that creates noise.
The third type of tire noise is the aerodynamic effect as the tread leaves the road surface. When the tread leaves the highway surface, it springs back to its original shape. It does not do this immediately, but continues to vibrate like a guitar string that has been plucked. These vibrations affect the tire body, resulting in an audible sound.
Tread pattern's impact
The tread pattern may excite noise vibrations that pass through the taut sidewalls of the tire to the vehicle itself, where it is dampened, isolated, amplified or conducted, according to the structure of the car.
When the tread elements are evenly spaced around the tire's circumference, they produce a clearly identifiable note that rises in frequency as the speed increases.
Not surprisingly, an aggressive-looking pattern of chunky tread blocks will create greater patter and rumble on the highway than a tread design composed largely of circumferential ribs and grooves.
Furthermore, any accurately repeated pattern will generate a particular note, or combination of notes, and harmonics that will vary according to the speed of the vehicle.
This is overcome by introducing deliberate irregularities into the pattern, slightly varying the size of the tread blocks or the zig-zag pattern of grooves to dampen the sound rather than amplify it.
Tire noise is an environmental problem mainly with trucks and buses. It can reach such high volume that it becomes a disturbance to the residents along a highway.
As a result, we are seeing more and more construction of ``noise abatement barriers'' or ``Chinese walls'' mandated by departments of transportation.
Over the years, highway surfaces have become much smoother. Highway design improvements have reduced sharp curves and steep grades so that higher speeds can be maintained.
At the same time, there has been a remarkable reduction in motor and body noise by vehicle manufacturers. But these improvements have made tire noise much more noticeable.
To reduce noise, tread design engineers have adopted variable pitch treatments and revised the shapes of tread elements.
Tires that came on the market as recently as five or six years ago, and were considered at the time to incorporate the ultimate in quiet-running features, have almost without exception been re-designed. This has been undertaken at considerable cost.
It has been established that the radial tire makes a worthwhile reduction in road roar. The exact differences will vary from car to car.
In general, the radial tire is more comfortable and quieter than the bias-ply at high speeds-that is to say it creates and transmits less noise of the road roar variety-because of the low dynamic rigidity of the radial ply casing.
In this brief treatise on the causes of high tire noise and the possible solutions, I hope it is clear that the tread pattern plays a key role.
I have often wondered why retreaders continue to use tread designs totally unsuited to the desired purpose. Many problems caused by the use of bias-ply designs or so-called ``neutral'' designs could have been eliminated if proper designs had been used.
Retread mold manufacturers-historically slow to change-need to be quicker to adopt many of the improved design characteristics originating in the new-tire manufacturing industry, as must their retreader customers.