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A girl's guide
to tyres
How to sound like you know
what you're talking about!
Background
Tyres have to cope with the
considerable forces generated by accelerating, braking and
cornering, while helping to insulate the car’s occupants from
road surface irregularities. We expect them to provide good
grip, have low road noise and have a good lifespan. In
addition to all of this, we expect the humble tyre to do this in
all types of weather. All in all, a fairly tall order!
In recent years, modern tyres
are almost always of radial construction, the trend being to wider and lower tyres,
with most cars having
power-steering. Without power-steering, the road contact area of
wide, low-profile tyres would make the steering effort
excessive, making turning very difficult and requiring arms of
steel to turn the steering wheel in any direction at low speed.
A few things you should
know about tyres
Choosing tyres for your
vehicle is very much ‘horses for courses’ and your local tyre
specialist will be able to advise you which tyre best suits your
vehicle. The specialist will consider what type of car you
drive, what sort of roads you use your vehicle on and what you want or
expect from a tyre.
So what should you look for between visits to
your local tyres specialist?
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How can I tell if the
tyre has a safe amount of tread on it?
The primary purpose of the
tyre tread is to allow the tyre to provide grip on a surface
that is wet, unless the tyre is designed for racing on a dry
racing track which requires a smooth tyre surface. At
100km/h, on a wet road, an average tyre has to move more than five litres of
water a second from the tyre/road contact patch — that is where
the tyre touches the road surface. To displace this water the
tyre surface has grooves moulded into it. It is very important
that tyres have enough tread to maintain grip in wet conditions.
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The minimum legal tread
depth is 1.5mm across three-quarters of the tyre width
and around the entire circumference. Modern tyres
usually have a wear strip set in the central grooves.
When the tread is worn down to this strip, the tyre is
at minimum tread depth and should be replaced.
Pictures: Above left
is a new tyre; note the wear strips in the tyre grooves
(highlighted in white for clarity). With the worn tyre
(above right) the wear strip is nearly level with the
tread surface. |
Does it matter if the
tyre is a bit soft or slightly deflated?
It is very important that
the manufacturer's recommended tyre pressures are used and
maintained. If the pressure is too low or too high the tyre may
overheat, wear rapidly and it will affect handling.
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You will find these tyre-pressure figures in your car handbook and sometimes
stuck on
the driver's door jamb. Inflation is best checked with a tyre
gauge — usually part of the air hose at your service station.
Over or under inflation
reduces tyre contact with the road and hence grip. Telltale
signs for a persistently over-inflated tyre may be more wear
through the centre of the tyre tread, while an under-inflated
tyre will probably show wear on the tread at the edges of the
tyre.
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Is there anything else I
should be looking for?
An occasional check of the
tyre sidewalls will pick up any damage caused by stones or road
debris. Also look for uneven wear of the tread i.e. wearing on
one side. This can indicate wheel-alignment or suspension
problems.
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What do those numbers on
the side of the tyres mean?
Tyre size:
For cars, usually expressed as width in
millimetres i.e. 195 is 195 millimetres measured across the width.
Tyre construction:
The vast majority of tyres on cars and SUVs are of radial
construction symbolized by R. Absence of a letter means the
tyre is of cross ply construction. It is possible this may also be
the case on some vintage or classic cars.
Aspect ratio:
Have you heard people talk about low profile tyres? This refers
to the ratio of width to height of the tyre (see diagram on the
right).
Speed performance rating:
Expressed as a letter for maximum designed speed, L is 160km/h, S
is 175km/h, H is 210km/h and V is above 210km/h.
Wheel-rim diameter:
Usually expressed in inches e.g. 14 inches.
Example: 195/70 R14 91H means Tyre width is
195mm, Profile is 70, Radial construction, rim size is 14 inch
and speed rating is for a maximum speed of 210km/h.
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Above picture:
Ratio of width to height is expressed as tyre profile.
The more sqat the tyre the lower the profile.
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Other tyre information
and tips
Space-saver tyres
were developed for cars with limited spare-tyre storage space.
These are quite common in Japanese imported vehicles. These are
designed as an emergency-only tyre to get you to the nearest
place that can fit a full-size replacement tyre. You should be
aware that a space-saver tyre has a designated speed limit
compared to normal tyres. Check your car’s handbook if your
vehicle is fitted with one of these tyres as a spare.
Run-flat tyres:
Usually a car-manufacturer-fitted option, for the same reason as
the space-saver tyre. An emergency-only tyre to be replaced at
the earliest opportunity. The tyre has a self-sealing function
when punctured.
Trailer tyres:
Remember that the same checks and standards with regards to
tread depth etc apply to trailer tyres as they do to cars.
If you have any doubts about
the safety or condition of your vehicle’s tyres, check with your
local tyre specialist. This article must not be used in place of
specialist advice. |
