Tyres – The Basic Facts [2007/03]

TyresTyres are sized according to diameter and what’s known as aspect ratio.   Diameter is simple; if you’ve got a 17-inch wheel, it requires a 17-inch tyre.   Aspect ratio is the relationship between the width and the height of the sidewalls.  Say the tyre is sized at 160/70, it means that the sidewalls are 70% of the width of the tyre.  The width is 160mm, so the sidewalls are 112mm high. 

Tyres for sports bikes typically have a low aspect ratio, some as low as 50%, ie a 190/50.  This gives less sidewall movement and effectively makes for a tyre that deflects very little, or keeps its shape under high cornering forces. As well as the obvious, like diameter, tyres must also match the width of the wheel rim.  Tyre sizes are matched to rim sizes for optimum profile,  that is the shape the tyre takes when fitted to that rim.   If you’ve heard people talking about preferring the behaviour of a smaller tyre (like a 180/55) compared to a 190/50, it’s because of its profile on, say, a 6-inch rim.  The smaller tyre assumes a rounder cross section, offering better turn-in and a bigger contact patch while leant over. 

There will always be an optimum recommended tyre for any given rim, and fitting a bigger tyre because you reckon you’ll have more rubber on the road can often mean a trade-off in handling, so unless it’s highly recommended fitment that improves the handling and roadholding, don’t do it. The next things to look at are compounds.  A compound is the blend of rubber used for the tread.   A soft compound will generally offer more grip, but wear faster that a hard compound.   Some tyres are now made with dual compound using a harder band in the centre, where acceleration forces can quickly wear a tyre out, and a softer compound for the edges where the tyre spends less of its time and where a softer compond offers more grip when the bike is banked over. 

Then there are speed ratings.  If a tyre is marked 170/60 ZR17, it’s 1 17-inch rim, ZR-rated for speeds up to 150mph.  A 130/70 H17 is H-rated for speeds up to 130mph, V or VB, are for up to 149mph, S for up to 112mph, and there are other – lots of others.  In fact the more you get into the wonderful world of tyres, the more there is to learn.  

That’s why tyre specialists exist, but that’s no reason to ignore what those expensive black hoops are all about. 

Sponsored by Andy Anderson

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