Quote:
Originally Posted by Bill
Bobby, what have I messed up? Go to it ...!
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You're doing great!
Since I am having a slow day at the shop, I will go ahead and bore everyone some more with this.
Most metric tire sizes are divided into 3 parts (or the one most commonly used, on our vehicles and OE tire sizes)
1. You have your section width, which is the measured width between the two bulging parts of the sidewall, when the tire is loaded at it's nominal carrying capacity. This is the first number in your tire size (Ex 225)
2. Now you have your aspect ratio (aka section height). This is a ratio (or percentage) of the section width, and is the measured distance of the sidewall from the ground to the rim. It is the second number in your tire size. (Ex 75, which means the tire is 75% as tall as it is wide)
3. The last number is the rim diameter. It is the width of the hole in the middle
of the tire. (Ex 15)
Now to the part about widths. Each tire size made is supposed to conform to an industry standard (but can vary a little). It all has to do with the section width, and just exactly where the sidewall bulges out. Remember that the section width is supposed to be the measured width at nominal loaded capacity. Some tires have this bulge higher or lower up on the sidewall, which can account for different tires from different manufacturers being a little taller or smaller than the competition. It's not supposed to be much difference though.
When you read a manufacturer specification chart for a tire, it will have a recommended rim width range for each size tire. You can see this on the chart below. (ex 5.5 inches to 7 inches for a 215mm section tire)
You also may notice another part on the chart called the "Measured Rim Width". This is the true rim width that the tire was measured upon to get the numbers that are on the chart. (the 215mm tire was measured on a 6" wide wheel).
If you place a tire on a narrower or wider wheel, it does affect the section width measurement (and the overall diameter slightly). We've been told that the section width is affected between 1/10 and 2/10's of an inch for every 1/2 inch in rim width. If you mount it on a narrower rim than "measured rim width", it narrows the section width and stands the tire up just a hair taller....and the opposite is true with wider rim widths.
Are we bored to sleep now?
