Got Rolling Diameter, Dummy?

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We don't know about you, but the staff of the Antisocial Network would greatly prefer that someone who can tell his or her hiney from a hole in the ground be the one to answer our questions. As a middle-school teacher once told one of us, the truly smart person isn't the one who knows the answer, it's the one who knows who to ask. In the age of the internet, though, we find that the truly dumb person is the one who pretends to know, asks someone who does know, and then garbles the answer. You know, people like some-time music critic and PolySci graduate Madison Velding-VanDam of eHow.com, who stepped way outside his comfort zone to bullshit his way through an explanation, of sorts, of "How to Calculate Rolling Diameter."¹

Of course, to answer that question, one needs to know what "rolling diameter" is – but Madison didn't seem to know:

"Rolling diameter is the measurement of a tire or wheel's surface that touches the ground. Due to their circular shape, it is very difficult to accurately measure rolling diameter of a tire or wheel with only a ruler or measuring tape."

Yeah, sure: but what is rolling diameter? Well, it's not the "measurement of a tire or wheel's surface that touches the ground"; it's the total diameter of the rubber tire as opposed to merely the diameter of the wheel. Why's that important? Well people who want to put oversized decorative wheels on their car need this information so that they don't exceed the amount of rubber that will fit in the space. If you know the trick, it's easy to do. Velding-VanDam obviously didn't know the trick. What Madison said to do is more or less correct: Determine the tire's section height s Rolling diameter is approximately wheel diameter + 2s

And yet, even though it took him more than 116 words to spit that out, his answer remains essentially useless. Why useless? because he didn't tell his readers how to determine this mythical section height; and yet it's as easy as pie!

Here, let us tell you: all you need to do is

1. Read the tire sidewall. A typical tire size contains three bits of information, section width, aspect ratio, and wheel diameter. Take, for example, the tires used by the AN company car: 265/65R17. The first number (265) is section width in millimeters, the second (65) is the tire's aspect ratio (as a percentage), and the third number is the wheel diameter in inches. 
2. Calculate section height, which is section width * aspect ratio: 265 * 0.45 = 119.25
3. Convert section height to inches and double it: 119.25 / 25.4 = 4.7 * 2 = 9.4
4. Add the doubled section height to the wheel diameter: 17 + 9.4 = 26.4

The rolling diameter of our tires is 26.4 inches. There: wasn't that easy? and it included both the formula and instructions on how to get the information you need! And you don't have to deal with ambiguous partial instructions like those Velding-VanDam published:

"Find the width and aspect ratio, which is the ratio of the shape's longer dimension to its shorter dimension, for the tire, wheel or circular object for which you wish to know the rolling diameter."

We already know what an aspect ratio is, Madison, but what does it mean in this context? and how do you find this information? Omitting that information renders your post completely useless to most people, which is precisely the sort of content we single out for the Dumbass of the Day award.

¹ The original has been deleted by Leaf Group, but can still be accessed using the Wayback machine at archive.org. Its URL was ehow.com/how_7485075_calculate-rolling-diameter.html

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