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Ok, I need to reset my bike computer for a 650 wheel. The manual says that the computer has two preset circumferences - 2155mm and 2030mm - I can't believe that I've forgotten this much junior high math, but based on a 650mm diameter, what is the circumference?
Also, why are wheels listed as 650c or 700c, i.e., what does the "c" stand for? My understanding is that 650 means a 650mm diameter.
Thanks and please don't make fun of me more than is obviously necessary.
Bunnyman posted a thread on this long ago, but the short answer is to roll out your wheel after inflating it to the psi level you use. I start with the stem down and roll until the stem is down again and mark both starting and finishing points. Measure the distance between the two points in mms and enter that value if you can, or the closest value.
-Robert
"How wonderful it is that nobody need wait a single moment before starting to improve the world." ~Anne Frank
There are two ways to measure the circum. of the wheel.
1. Take a mark on the wheel, the easiest would be the valve. Roll the wheel until the valve is perpendicular to the pavement and mark the point on the pavement. Rull the wheel one revolution until the valve is again perpendicular to the pavement and mark the point on the pavement. Then take a tape measure and the distance between the two marks in mm.
2. Is to take the measurement from the center of the wheel to the outside edge of the tire in mm, this measurement is the radius=r (this value is not 650mm). Take that number (r) and multiply by 2xpi (where pi=3.14159).
Also not sure why its called 650cc or 700cc cause the c does not mean mm.
James
The 650c doesn't equal the tire diameter, and I don't remember what the c stands for. As a rough, but close estimate, I think 650 tires usually have a circumference of around 1950mm. My preferred method is to take some thread and tape it every few inches to the center of the tire at the desired pressure, until you get all the way around it. Then lay out a tape measure and measure the length of the thread. It has been the most accurate for me compared to rolling the tire like the previous poster suggested, or measuring the tire radius and multiplying by 2 pi. I've also found success by simply emailing the company that sells the tire and asking them. The thread method proved to be real close compared to the response from the company.
Jack
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"What the mind can conceive and believe, the mind and body can achieve; and those who stay will be champions."
If I recall correctly, the 650 is what the diameter would have been with some huge tires on it, and therefore doesn't count. Wheel diameter can vary brand to brand and even tire to tire within brands (eg, 19mm wide vs 23mm wide may have different circumferences). Best to measure.
My method is to sit on the bike, hold the wheel straight, roll exactly one revolution and measure. Make a wet line or chalk line across the tire first can make this easy: measure the distance between the marks left on the ground.
To get it accurate you really do have to sit on the bike and do the rollout. The weight on the tire diminishes the effective radius quite a bit. Changing tires on the same rims can make a surprisingly large difference too.
Yes, thanks for clarifying. Those were my thoughts, though I wasn't totally clear about them.
By the way, I think the 'C' doesn't stand for anything, and is simply the letter after 'B'. There's an article about the history of this somewhere, but I don't recall where.