杏吧原创

On the road again

Question: How is car tyre road noise generated? How can it be minimised and
are the methods compatible with other desirable environmental effects? Are there
comparative lists for cars or tyres that can be consulted?

Answer: The generation of tyre noise is complex, involving several
mechanisms. One is vibration within the tyre itself鈥攖he tyre wall acts
like the diaphragm of a loudspeaker, exciting the surrounding air and creating
sound waves.

The initial vibration is caused by the forces acting on the contact patch
(the part of the tyre tread touching the road surface) as it hits the road, and
also when the blocks that make up the tyre tread pattern, emerge from the
contact patch.

The latter effect happens as the blocks rapidly return to their undeflected
position, releasing elastic energy which was stored as they deformed under
pressure. This so-called block 鈥渟nap-out鈥� causes the blocks to vibrate and sound
waves to be generated.

Another cause of noise is the release of compressed air which has become
trapped in the tread grooves of the tyre as it enters the contact patch. The air
is expelled mainly towards the rear of the contact patch and the air pressure
modulations which occur can produce audible sound waves. The noise from this
鈥渁ir pumping鈥� process is in the frequency range from 3 to 5 kilohertz and is
quite different from that caused by tyre vibration, which is in the region of 1
kHz.

It has long been accepted in Britain that roads need a degree of roughness to
provide skidding resistance and to help water disperse from under tyres, which
can cause aquaplaning. But conventional road designs that address these factors
tend to make tyres noisier.

Experimental porous surfaces were invented by the Transport Research
Laboratory (TRL) in Berkshire, initially to prevent water build-up on airfields
by allowing water to drain below the surface. As well as reducing spray in wet
weather, these surfaces significantly reduced tyre noise. This happens because
air trapped in tread grooves is able to leak into the road surface rather than
becoming trapped and compressed. Also, the road surface has fewer sharp edges
which tends to reduce noise.

The TRL has also determined which components of surface texture help to
reduce tyre vibration while ensuring a good degree of antiskidding performance
and hence safety. A whole range of new surfaces with these characteristics have
been developed and are now in use on roads.

As well as looking at road surface design, the government believes that tyre
tread patterns can be modified to reduce noise levels while maintaining safety
levels and is thus supporting further research in collaboration with
industry.

Test methods are also being developed to allow different road surfaces and
tyre designs to be compared in terms of their noise performance.

Speed is also a factor in tyre noise generation. Speed limits have recently
been imposed on some German autobahns to help to reduce noise in residential
areas. Speed controls on Britain鈥檚 M25 motorway around London have improved the
flow of traffic and consequently reduced air pollution, but have had a less
noticeable effect on noise levels.

Ongoing research will also investigate the possibility that reduced tyre
noise leads to lower rolling resistance and improved fuel consumption. However,
the benefits may be difficult to measure because other forms of resistance such
as aerodynamic drag are more significant. More information about the TRL鈥檚 work
on quieter road surfaces can be found on the environment pages of the TRL
website: http://www.trl.co.uk/env.htm.

Paul Nelson

Chief Research 杏吧原创, Environment

Transport Research Laboratory

Crowthorne, Berkshire

On the turn

Question: Why, when driving, does the steering wheel of a car straighten
itself if you remove your hands after turning it? It doesn鈥檛 happen on my
friend鈥檚 Lego Technics car.

Answer: The tendency of the steering wheel to return to the straight-on
position is caused by the caster action of the front wheels. This effect is more
clearly seen on a shopping trolley where the vertical swivel axis of each wheel
is in front of the wheel-to-ground contact point. If you start pushing the
trolley when the wheels are not aligned to the direction of the trolley motion,
the wheels are pulled around into alignment by the drag force between ground and
wheel.

The full explanation is that as the trolley moves forward, the drag force
exerted by the ground on the wheel always opposes any relative motion (or slip)
between the wheel and the ground.

Unless the wheels are aligned to the trolley motion, the drag force does not
pass through the swivel axis, and therefore it produces a turning moment about
that axis which always acts to bring the wheel back into alignment.

In a car, the same effect is achieved by inclining the steering axis and
ensuring that the point where the axis intersects the ground is ahead of the
tyre-ground contact point. The same is true of a bicycle, as you can see if you
hold a broom handle alongside the steering axis of the bike so that the handle
touches the ground. You should see that this point is just in front of the
tyre-ground contact point.

You can demonstrate the caster action on a bicycle for yourself by pushing
the bike backwards and forwards by the saddle while the handle bars are left
free. When going forwards, the bike is easy to push in a relatively straight
line.

However, going backwards is almost impossible because the front wheel tries
to turn round through 180 degrees just like a shopping trolley wheel would. You
will also find when reversing a car that the steering wheel loses its tendency
to centre itself.

Bill Laughton

Southampton

This week鈥檚 questions

Squareheads: Why do hammerhead sharks have hammerheads?

Graeme Bushell

University of New South Wales

Follicular challenge: The individual hairs on a tabby cat have bands of
colour. Because the same colour can appear more than once on a single hair,
these colours are not created by fading. So how do the follicles of the tabby
cat produce different colours in the same hair and what causes the changes in
the pattern?

Rob Gill

London