Reading your Tyre’s Message

January 12, 2007 at 3:38 am 1 comment

Tyres are a very tradeable commodity in today’s international trading world. Some countries use them to buy other currencies, and soon after a currency has been devalued, you will find tyres from there on your docks.

They are all made to similar technical, performance, and dimensional standards through a series of interlinked Tyre and Rim Association Standards world-wide. The major bodies are Tyre and Rim Association, U.S.A., Japan T & R, Australian T & R., and E.T.R.T.O. (Europe). So a Romanian made tyre fits a Peugot in Brazil- it’s no accident. These Standards cover the basics, but individual countries may require other information to be available to the tyre consumer. So manufacturers try to cover all the bases, by putting a veritable essay on the tyre sidewall some of which is relevant, some (a lot), is not. For example, U.S.A. legislation requires performance information on tread wear, wet road/braking information. The ratings given are very high, the reason being that they are related to a bias ply tyre, which hasn’t been around as a passenger tyre now for over 20 years! Tyre technology improvements have made them meaningless.

So let’s stick to the basics of a tyre size code. Let’s take 195 /75 R15 92H as an example. Now I have to get a little bit technical, because measurements of a tyre are made under strictly controlled conditions, otherwise they too, would be meaningless.

195 – Dimensions in millimetres of the section width of the tyre (with raised lettering excluded), when mounted on a specified rim (the measuring rim), inflated, stabilised for a period because they grow a little bit when inflated for the first time), and then measured at its widest point. The rim has to be specified, because section width increases with increasing rim width. Every young car enthusiast knows that!

75 – The ratio of section width to section height. Section height is measured vertically from bead ledge to the crown of the inflated tyre.

R – The tyre is of radial construction. Nowadays, they nearly all are. If there is no “R”, then they are not Radials.

15 – The nominal rim diameter. “Nominal” because it’s not exactly 15 inches, and how would you measure it on a taper anyway (see interference fit article). In fact to measure a rim accurately requires specialised equipment called a ball tape. Certain manufacturing tolerances are permitted, so each rim diameter and profile has its own ball tape measure, with the tolerances built in.

92 – A code for the load in kilograms that the tyre can carry at a specified maximum pressure, in this case 630 kg. If for example, it was 96, this signifies that the tyre can carry a higher load, at a higher inflation pressure, so it has a stronger casing. The smallest code is 0 (zero), which carries 45 kg. That’s probably a wheel barrow tyre. The largest (so far) is 279, which carries 136 tonnes (136,000 kg), the size of which you have probably guessed as those giant machines that work in the mines.

H – A code for the speed capability of the tyre at specified load, temperature and pressure. In this case, it’s 210 km/h. The range of speed categories go from A1 (5 km/h) to A8 (40 km/h) for slow moving equipment. Then B to Z covers 50 to 300 km/h in 10 km/h steps, with 0 (Zero) omitted because it might get confused with O, and H out of sequence (because it was there first!)H comes after U, then follows V, Z, (same reason), W, and Y. X is omitted because the first radials (Michelin) were designated X, and this could breed confusion.

Z is different. ZR with no other speed category means the tyre has been designed to fit the performance of a particular high performance vehicle. They’re on your Ferrari or Lamborghini. ZR plus a speed category following, carries that speed category as defined e.g. 275/40ZR17 93W has a maximum speed of W (270 km/h)

Each tire has a speed rating based on how well it reaches and sustains a specific speed. A higher speed rating generally means that the tire will provide better handling. Things to think about when looking at speed ratings:

  • The code indicates the speed capability after pressure adjustments.
  • The higher the code letter, the higher the speed capability.
  • Higher speed ratings confer greater ability to cope with severe conditions of speed and heat. They perform more reliably under duress.

Carbonblack TyreXchange displays a illustrated summary of tyre sizings as shown on your tyres.

Entry filed under: Tyre Sizing. Tags: , , .

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