B arry's
Tire
Tech This is a series of articles on the technical aspects of tires, their care and usage. My primary purpose in these articles is to help people understand tires and thereby reduce the risks we all face every day. ..........and since tires is just about the only thing I know about.......... Please drop me a note if you have a topic you want to see: Barry@BarrysTireTech.com |
Tire Sizing One of the fundamentals about tires is sizing. There is a lot of confusion concerning tire sizing and this is going to explain it in general terms and basically how it works. As a general rule, tire sizing describes the physical dimensions of a tire - but that is only part of the story. It also includes the way in which the load carrying capacity is calculated. |
The current system for passenger car tires is like this: P205/60R16
I think it is obvious that given the above, the overall tire diameter is now defined. What is not so obvious is that this form also defines the load carrying characteristics - but it does - and that relationship takes the form of a table (commonly called the load table) where the maximum load carrying capacity is listed by inflation pressure. |
So that you get the idea, here are some other old tire sizes:
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But there are other ways to do tire sizing: 30X10.50R15LT
This form is called "Flotation" - and it has its own set of load tables. |
I hope it is obvious that to define the physical dimensions of a tire, I only need
3 dimensions:
I hope from the previous examples you can see that I can use also ISO (metric) units or English units - although rim diameters have historically been in inches, which continues to this day. - AND - the form the size takes also defines the load table to be used. - PLUS - There are usually separators: "-", "/", "X", and letters, but usually only the letters have meaning. Tire sizing has been an evolutionary process and as time has gone on, some lessons have ben learned. |
First is that you may think that "6.95", "30", or "205", are actual dimensions - and they are NOT! They are a code and the actual dimensions will be close, but not exactly that. In order to deal with that - and to simplify things, the Tire and Rim association came up with a sizing system called "AlphaNumeric", which had the form of:
where: What was great about this was that the letters always stood for the load carrying capacity, so it was difficult to go wrong by undersizing. The bad news was that you needed a table to figure out the physical dimensions. But this sizing system started about the same time that radials started being used. Since radials used the same sizing sytem that people were used to seeing, albeit with metric units, the alphanumeric system was dicarded. |
If you deal with tires other than the current sizing sytem for passenger car tires, you will come across other types of sizing systems, but they will all have something in common:
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Barry's Tire Tech - Load Tables Barry's Tire Tech - Dimension Tables |