It is a perception that tyres are black and round - however,
that is only because they look better in black than they do
in grey - the colour of natural rubber.
The black comes from an additive - Carbon Black; a filler
that extends the rubber and is part of the complex chemical
mixture that helps keep your tyres in contact with the road.
Tyres can also be coloured: There is no reason why you shouldn't
have yellow, blue, or green tyres - except they will probably
not look so smart after a couple of hundred miles.
Every modern tyre has a wound steel wire beading, encased
in rubber, a rubber sidewall that is built from several layers
of rubber compound, and a tread layer that is built from a
textile and wire belt, possibly multiple layers of "cushion"
rubber and a hard wearing tread rubber. The whole tyre is
lined with butyl rubber lining. That, in a nutshell, is what
a tyre consists of.
Of course the reality is much more complex, with specially
developed wires and textiles, adhesive coatings on the wires
to make sure the rubber sticks to them, different types of
bead wire technology, different types of rubber compound to
vary the comfort and handling characteristics and different
belt structures, tread compounds and patterns and linings.
That is without even considering the different types of Silicas
and Carbon Blacks and other additives thrown into the mix
to make your tyre perform ever better.
How a tyre is constructed makes a big difference to how it
performs. The aim of the tyre is to keep your car in contact
with the road. Everyone understands that. However, in a perfect
world all roads would be of the same specification. They would
all have the same microsurface. They would all have the same
characteristics and a steady, stable temperature, and it would
never rain. Then the tyre manufacturer could concentrate on
developing the perfect compound that created the ideal microsurface
in the tyre to match the road surface perfectly in a slick
tyre. That would give incredible grip ( in fact probably too
much).
However, the real world has as many different road surfaces
as there are roads, and the weather changes from day to day
and the tyre has to be a compromise, further complicated by
the wide range of vehicles in the marketplace. So, we have
tyres with different rubber compounds - you can actually feel
that for yourself by handling a high performance sports tyre,
and a conventional tyre designed for high mileage and everyday
use. There is a clear tactile difference between the two tyres
- this is especially noticeable when the tyre is hot. The
high performance tyre will be stickier and softer. That is
part of how it maintains its contact with the road surface.
It is also one of the reasons why high performance sports
tyres don't usually last as long as more run-of-the-mill tyres.
Tyre techies talk about hysteresis a lot. If you want to
understand your tyre you need to know about hysteresis. Hysteresis
is what happens when you distort a block of rubber. Every
twist of a block of rubber results in heat generation. That
heat generation is energy being lost through the rubber block.
The heat generated can build up in a tyre as it distorts at
the tread block and in the tyre casing as it rolls and its
"standing wave" is generated. That heat breaks down
the rubber compounds in the tyre. That action is called hysteresis.
A tyre with a high hysteresis level might be very grippy,
for example, but may not have a long life. Heat destroys tyres.
We started off with tyres being round and black and then
told you that they didn't have to be black. Well, in fact,
neither are they round. It isn't rocket science to understand
that when the tyre is on a vehicle and carrying the vehicle
load; that it distorts in contact with the road and so has
a flat contact patch - the tyre is not round.
However, as the tyre rolls under load, there is a gentle
bulge in the shape of the tyre both ahead of the contact patch
and behind the contact patch - this is the tyre's standing
wave. Looked at in slow motion it is possible to see this
distortion quite clearly.
We have already outlined how distortion creates heat, and
heat breaks down the tyre. Now we have a rolling source of
distortion that is ever present when the tyre is moving. It
is important to minimise this distortion, whilst maximising
the contact patch on the road, and this is done through maintaining
a correct tyre pressure at all times. The lower the tyre pressure
the bigger the distortion, the greater the heat and the more
likelihood of the tyre breaking down.
In an under-inflated tyre at high speed the tyre becomes
almost triangular in distortion as the standing wave moves
around the tyre. So, when you have a puncture on the motorway
and you run for even just a few hundred yards as the pressure
drops - the heat build up will destroy the integrity of your
tyre long before the rim cuts through the sidewall. That is
why if you get a slow puncture you need to get it fixed sooner
rather than later. Running at lower than specified temperatures
will destroy your tyre sidewall and make it pre-disposed to
blow outs.
Speaking of slow punctures: If you wondered why modern tyres
get slow punctures more often than sudden pressure loss, it
is because modern tyres have a soft butyl liner inside the
casing that stretches around any penetration and gives a seal
that helps prevent air loss: Which is also why it is not always
a good idea to remove the penetrating object from your tyre
tread if you want to drive on to the nearest refuge or service
area. As soon as you pull the penetration out the tyre deflates.
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