I n the past few years environmental checks have moved on
Page 40
Page 41
If you've noticed an error in this article please click here to report it so we can fix it.
from tailpipe emissions to include drive-by noise. The EL; has legislated to restrict the total noise made by new trucks, and tyres make a significant contribution. After 1 October European legislation on drive-by noise will cut the current 82 decibels (dB(A)) on the largest trucks to 80dB(A) for type approvals: not surprisingly truck manufacturers are demanding that the tyre suppliers should do their bit.
Tyre noise can be separated into interior noise and exterior noise.
Interior noise is a nuisance to the driver; manufacturers have been working on the interaction between vehicle and tyres to separate and understand airborne and structurally transmitted noise. Exterior noise is the problem the legislators are interested in because it is defined as a nuisance perceived at the roadside as the vehicle drives by.
Measuring the total vehicle noise is done under power by using a microphone 7.5m from the centre of a road at a height of 1.2m. The same set-up is used to record tyre noise but the vehicle coasts by at a pre-determined speed with the engine switched off and the clutch disengaged.
Exterior noise is influenced by several factors including the road, the vehicle and the rolling conditions Tyre manufacturers are united in saying that this is the single most important factor in road noise. Anyone driving up the newly resurfaced section of the M40 will know just what a difference the surface makes— Dunlop's Dr. Roger Williams reckons that it can add up to 8dB(A) to the noise generated by a tyre. Noise is measured on a logarithmic scale; every 3dB(A) increase represents a doubling of the noise level.
To quantify the effect of road surface on tyre noise, Michelin ran tests on a range of surfaces. These proved that porous asphalt surfaces have good sound absorption properties and therefore reduce rolling noise. As an added benefit they drain well, which increases grip in the wet and reduces spray.
This type of surface is only slightly more expensive than the traditional surface but the tyre manufacturers believe the DOT should adopt it on the main motorways and trunk roads.
As a rule of thumb, bigger tyres make more noise. Big trucks have big tyres so their tyres make more noise than small trucks. Body/trailer contours play an important role in the transmission of this noise.
Noise increases with speed, decreases as temperature rises and is changed by the torque effect. It is expected that tyres will have to be type approved from 1998 and these factors will have to be specified. The likely limits are around 79-81dB(A) with the engine off, but this will be measured at a higher speed than the total vehicle test and it will be carried out on an ISO-standard road. These controls are bound to have an effect on retreaded and regrooved tyres.
Slick tyres produce the least noise but, while some truck drivers might fancy themselves as Damon Hill, letting them loose with slicks on is not a likely option. Differences between tread patterns (drive/steer large/small block) accounts for 23d13(A) of the total the noise, says Williams, and low-aspect tyres are about 3dB(A) noisier than standard models.
Dunlop is among tyre manufacturers which believe tyre noise can only be measured accurately in a laboratory, but Michelin reckons that consistent, accurate results can be obtained by measuring noise on the same stretch of track, on a single vehicle and at pre-determined speeds by applying statistical temperature correction.
The tyre companies are not only interested in identifying and reducing the factors that cause tyre noise; they are working on the way this noise is transmitted.
The noise sources can be isolated using advanced acoustics and applying the principle of `retropropagation' (a method of identifying a sound source, its intensity and direction). The main noise sources have been pinpointed as the entry and exit areas of the contact patch.
Most of the noise is caused by mechanical vibration set up by the sudden impact between road irregularities and the tread pattern blocks as the tyre rotates. Sudden relaxation at the exit of the contact patch can also set up vibration of the rubber tread block, which generates more noise.
Air pumping is another contributing factor. At the entry and exit of the contact patch area air is suddenly sucked into and expelled from the grooves in the tread pattern. Air trapped in the grooves as the tyre rolls is subjected to multiple reflections, which leads to 'organ-pipe' resonances.
Propagation of this noise falls into two categories: nearfield and farfield.
In ne.arfield propagation the noise from the entry and exit of the contact patch is amplified by the so-called 'horn' effect as the curve of the tyre and road surface combine into a sort of loud speaker. Farfield propagation is related to the way a given road surface reflects and absorbs the noise.
All the easy steps towards reducing rolling noise have already been taken: further improvements of more than a decibel are expensive and almost impossible to achieve without harming other aspects of a tyre's performance.
Tyre designers and manufacturers might be forgiven for asking legislators: "What do YOU want to give up in order to reduce noise—wet grip, dry grip, cornering or braking?"
However anti-lock braking systems might allow further noise reduction, say the tyre manufacturers. As the tyre won't run locked for more than a split second the grooving could be reduced, moving towards shallower tread patterns and softer compounds which both reduce tyre noise. But while these moves might not affect grip, they are likely to reduce tyre life and resistance to abuse such as kerbing.
If the legislators are determined to reduce tyre noise, all the tyre companies agree that the quickest, greatest and most cost-effective advances can be made by opting for porous asphalt surfaces. This would allow operators to keep products which have been designed to combine good grip, safety, comfort and durability with low rolling resistance (reducing fuel consumption and hence exhaust emissions).
Legislators who need to be convinced need only take a drive down the M40 around Junction 5.
. by Colin Sowrnan