RUBBER COMPOUNDS
The specific compounds used for different tyres will vary dramatically. Tyre manufacturers invest fortunes in developing different tyre compounds in order to meet specific different criteria, and as such the exact ‘recipe’ for each compound is always a closely guarded secret.
There are hundreds, if not thousands, of different tyre compounds in production, and there are a vast number of different elements that go into each compound to give its specific characteristics.
A typical road tyre will contain a number of different rubber compounds, some natural and some synthetic; each designed to do a specific job. Some will help keep the air within the tyre, others will allow the sidewall to flex, and others will help bind the nylons and steel belts together.
The compound most people refer to is the tread compound, as this is the part of the tyre in contact with the road and therefore has the greatest affect on a tyre’s performance. The tread compound itself is also made of various different
elements, including complicated polymers and occasionally trace amounts of silica. These help a tyre’s resistance to abrasion, tears, or cuts, and also prevent the tyre from loosing its integrity and going very soft and almost goo-like.
However, the tread compound consists of three main ingredients; natural and synthetic rubbers, carbons, and oils. As a general rule of thumb the greater the rubber content the more grip the tyre will offer. But this comes at the expense of longevity, as softer, grippy tyres wear faster than harder compounds containing more carbons.
When you hear people talking about ‘hardness’ and ‘softness’ of tyres they are not referring to how hard the tyre is to the touch, but instead are talking about how rubber molecules in the tyre interact with the road.
The rubber used in tyres goes through a process called vulcanising. This is a process that converts rubbers and polymers into a more durable material by adding elements such as sulphur. The addition of sulphur creates crosslinks between the polymer
chains and rubber molecules, which has the effect of making the material less sticky but more durable.
One of the main differences between harder and softer compounds is due to how the addition of sulphur affects the rubber molecules. In a softer compound (which feature a greater percentage of rubber) the crosslinks caused by the vulcanisation process are fewer in number. This means the rubber molecules and polymer chains are longer between crosslinks than in a harder compound. The longer the rubber molecules are, the more they can interact with the road surface, therefore giving grip. In a harder tyre compound the rubber molecules are more restricted, and are unable to interact with the road surface as much, therefore giving less grip.
However, a softer tyre will wear more quickly than a harder tyre. It is the movement of the rubber molecules and the friction they generate that provides the grip we crave, but at the same time this friction causes the tyre to wear. So you need to choose a compromise between the levels of grip you want and the amount of time you want them for.