RUB­BER COM­POUNDS

Fast Ford - - All About… Tyres -

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.

The com­pounds used will have a huge im­pact on the tyre’s char­ac­ter­is­tics, per­for­mance abil­ity, and wear rate

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