Tyres The overlooked diamonds

Tyres are often an overlooked resource. It helps to start with the basics, and work your way up

Published:  23 December, 2019

Tyres; the little black miracles that keep us and our precious cargo glued to the road. There is an awful lot to write about within the subject of tyres: sidewall markings, pressures, TPMS, tread patterns, puncture repairs, the rules on mismatched tyres on a four-wheel drive vehicle.
This is why my article on tyres will be split into two parts. However, something told me that for this issue we should go back to basics and by back to basics I don’t mean as basic as the minimum tread depth for a car tyre. 75-year-old Ethel who doesn’t drive now can tell you that. No, I mean the bits you think you know and the bits you need to know as a technician, as a MOT tester or as a centre manager selling your “diamonds” as I’ve heard them referred to in the past.
A jeweller knows all about the product he/she works with and sells. They know that the most precious and expensive diamonds on their shelf were probably mined in India, Brazil or South Africa and made from pure carbon. If it’s a synthetic diamond, it was probably made in a factory in Birmingham. In that respect would it hurt to learn a little more on the subject of tyres.
Let’s begin with the three sets of numbers on the sidewalls of your tyres. The first three-digit number is the width of the tyre in millimetres, the second two-digit number is the height of the sidewall and this is expressed as a percentage of the width of the tyre. Next, we have the size of the wheel that the tyre in question should be fitted to. Then we have the load index of a tyre, this will be located after the rim size, this is the maximum weight that the tyre can carry and this is expressed in kilograms. For example, if the load index number is 84, it means the tyre in question can carry a maximum load of 500kg, whereas a load index of 108 could carry a metric ton all by itself. There are obviously many more load numbers to comprehend, these were a few examples.
Beside the load there will be a speed rating, now. I have no idea why most little cars have a ‘T' rated tyre fitted to them (safe up-to 118mph) or even a ‘H' rated tyre (tested up-to 130mph) fitted to them but it seems the norm. However, considering that a lot of these cars cannot even get to 100mph it seems like a waste, especially when it is illegal to drive your car over 70mph on our roads, even in other countries where the majority of tyres are made, most speed limits don’t exceed 80mph.
There are exceptions to this of course. Maybe one of those nutters who puts a motorcycle engine in a Smart car and make it go 160mph for example. That’s all well and good, but you’ll never find a tyre for a Smart car that will handle that kind of speed, plus if you like driving at more than 160mph then you have no regard for rules anyway. Please do not put your ‘T’ rated tyres on your homemade Smart-bus and take it to the Isle of Man or indeed your gran's private airfield for a play. If you do please send me the video though. I do love things like that, but I also like safety too.

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