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Jack
17-08-09, 03:35 PM
1. Overview
This guide details the information codes moulded onto the sides of tyres.

Confused about the difference between 195/45/R15 and 185/65/R13? Unsure if you should buy a H or V rated tyre? Always wondered exactly what that red dot on the tyre means? Read on.


2. Tyre information.
All tyres have various snippets of information moulded into the sidewall. For the purposes of this guide, we'll be working with metric radial tyres as thats what most people will be looking at to purchase.

So what information is shown on the tyre, and what do you really need to know? The tyres you'll be buying for your Novas will have something akin to this moulded into the sidewall:


195/45R15J 88H

...obviuosly without the pretty colours though. Lets look at what that relates to.

Width
Profile
Tyre Form
Wheel Diameter
Bead
Load Index
Speed Rating


3. Width
This is pretty self explanatory. The first number relates to the width of the tyre - that is, from sidewall to sidewall. Bear in mind that tyres, being rubber, are flexible so there's no "one size for one wheel width" rule, there is usually a range of tyre widths that will fit each rim width depending on how stretched or fat you want the tyre to sit.

The tyre width is expressed in milimeters, ergo a 195 tyre is 195mm wide.


4. Profile
Also referred to as Aspect Ratio or Sidewall Height and tells us how tall the sidewall of the tyre is, from the bead to the tread face. This number is a percentage value that relates to the width - for example, a 195/45 tyre has a sidewall height that is 45% of 195mm = 87.75mm.

Remember, as its a percentage, don't assume that larger profile = taller tyre. This means that a 165/50 tyre has a thinner sidewall than a 195/45 tyre!

5. Tyre Form
This represents the construction of the tyre. Most modern car roadwheels are Radial tyres - referred to as R, however this is used to differentiate between that and Cross Ply, Bias Ply and Solid which are still used on some truck applications, and Semi-Pneumatic.

When ordering standard road tyres, almost all garages/shops will (rightly) assume you are wanting Radial tyres.


6. Wheel Diameter
Another pretty simple one this - it merely details the rim diameter (in inches) that the tyre is designed to fit. Looks a little odd to have an imperial measurement on a metric system, but as most rims are still measured in inches (plus its easier to put "15" on than "381") so its been carried over.


7. Bead
The Bead is the small lip round the inside of the sidewall that grips the tyre onto the rim. Some people mistakenly quote the tyre width and bead code together (for example, "a 6J wheel") resulting in the width being incorrectly referred to as the "J number". :roll:

Bead types come in many styles; J is the most common. However, you may also find JJ, K, JK, D, B, P etc - again, as with Tyre Form, most suppliers will assume you require J bead.

The bead is very important - its the only thing that holds the tyre onto the rim, by way of air pressure inside the tyre. If the bead is damaged or exposed, or the tyre is incorrectly inflated, the bead can fail and lead to rim + tyre parting company!


8. Load Index
The Load Index if a tyre indicates its maximum weight capacity, for each tyre. The number is actually reference to a chart of weights, not the weight rating itself - for example, 88 refers to 560Kg max carrying capacity for the tyre.

Wikipedia has a chart of load indexes here (http://en.wikipedia.org/wiki/Tire_code#Load_Index).


8. Speed Rating
Ever seen two tyres, same size, same manufacturer, same product name, but different price? Smart money is on the speed rating making all the difference. The additional letter here details what speed the tyres are manufacturer rated to - anything over this and they are considered to be running beyond their manufactured tolerances.

The most common speed rating codes are as follows:

P - 94mph
T - 118mph
H - 130mph
V - 149mph
Z - Over 149mph
W - 168mph
Y - 186mph

ZR is also used for "over 149mph". On older tyres, you may find the Speed Rating is inserted before the Tyre Form, for example 195/45/HR15.


9. Other Information
Thats the main tyre information covered, but what else is there?

Maximum Load Pressure
MLP is the maximum safe inflation pressure for the tyre. Always read the manufacturer handbook/information with regards to inflation of tyre pressures, as this will more often than not be well below the MLP.

Harmonic Marking
This is the red dot that appears on many tyres. Its merely for the tyre fitter, and points out the "high" point of the tyre. As its not possible to manufacture tyres to be perfectly symmetrical, all tyres come slightly imbalanced - the red dot therefore identifies the heavy point of the tyre. Usually this will be mounted opposite the inflation valve.


10. Links
RaceLine Central - Sidewalls and Tyre Markings (http://www.racelinecentral.com/tires_sidewall.html)
Tyre Codes and Load Indexes (http://en.wikipedia.org/wiki/Tire_code)
Tyres on Wikipedia (http://en.wikipedia.org/wiki/Tire)