The Numbers Game – Know Your Tyres

22 Feb by EMAM Team

The Numbers Game – Know Your Tyres

How to read and understand your tyre lettering to make safe and effective enhancements

The sidewall of our tyres features a whole host of numbers and letters. As an example, on my Suzuki GSX-R750, these start with 180/55ZR17 58. There’s a lot of information contained in these few digits. While most of us simply replace worn out tyres with exactly the same ones and have them fitted by a professional, taking the time to learn a little about why a particular tyre works on a particular bike can be helpful. The numbers act as a code, which, when understood, helps us choose the right tyres for our bikes.

Width or without you

The first number, 180, refers to the width of the tyre. This is the distance from one shoulder across the crown to the other shoulder. This is the complete patch of rubber that is available to come into contact with the road surface. Fitting tyres that are overly wide can cause problems, including rubbing final drives and chains as well as huggers and swingarms.

It’s also important to remember that rubber expands when heated. Just because it looks right on a static bike in a workshop, doesn’t mean it won’t wear through its neighbouring components and possibly even fail! At the very least, deviating drastically from the maker’s recommendation will dramatically alter the handling as well as the gear ratios. If you ride a Monster but like the look of the Diavel, buy a Diavel and don’t go trying to fit a tyre that’s going to ruin your bike! (we will link this to the blog about choosing the right tyre)

Ratio Rational

The next number is 55. This is called the aspect ratio and gives the tyre’s sidewall height as a ratio of the tyre’s width. Overly tall sidewalls will deform the crown of the tyre and make it unstable, so we strongly recommend avoiding choosing a tyre with a sidewall that is too high. Similarly, a sidewall that’s too low will destroy the tyre’s ability to grip, in effect over-stretching its body.

Speed Read

Next up are the letters ZR. The Z denotes a speed rating, of which there are several. The most common ones are H, W and Z. H is good for speeds of up to 200kph. H is the rating most likely to be found on a ‘tall tourer’. The next most common one is W. These are good all the way to 270 kph. The Z rating on GSX-R7750 is the highest speed rating and features on all ‘HyperSports’ bikes. The R denotes that the tyre is of a radial construction and is by far the most common construction type on modern tyres.

Finally, there is a single double digit number, in this case 17. This is the circumference of the wheel rim.  Bizarrely, it’s still measured in inches despite just about everything else in our two wheeled universe being decidedly metric.

Understanding these numbers and letters allows us to deviate from the original equipment recommendations safely. On my GSX-R750 the manufacturer’s original fitting was a 190 section but the 180 that I’ve chosen allows the bike to steer a little faster as it ‘falls in’ quicker. The aspect ratio remains at 55, so the character of the bike’s handling remains unchanged when ridden in a straight line and is unaffected by rapid acceleration. The speedo reads a little optimistically, but I am able to get my knee down a little faster and for longer at track days! Remember though – you must always be mindful when changing your tyre sizes from OEM manufacturer specs because of roadworthy issues. If in any doubt, or without the proper knowledge, don’t make the change.

Learn all you need to know about choosing the right tyres for you by downloading our eBook:

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