Bicycle Speedometers for Dummies

Sensor for counting wheel revolutions

The site formerly known as eHow.com, and now known as a whole host of niche sites, is renowned for in AN's office for the lax manner in which its "contributors" were allowed to answer questions. Common sense was never the site's strong suit, a deficiency that was compounded by the large number of writers attempting to educate people on topics they did not themselves understand. Failure of logic was a common problem, and that's why professional journalist William (W. D.) Adkins is back on these pages. This time, he's attempting to explain "How to Calculate MPH for Bikes" at SportsRec.com.

If all you want to know is your average speed on a ride, Adkins pretty much has the calculation nailed: speed = distance ÷ time; which Bill pounded out fairly well (although he needed an awful lot of words to say it) . The problem? There's another, more likely, interpretation of the question: "How fast is my bicycle going?" Adkins, sad to say, ignored that interpretation entirely. What he did say suggests that he either didn't know much about bicycles or he just didn't think much. Here's why we say that. Bill's "data collection" section says,

"If you are using an odometer to measure distance rather than traveling a course of known length, record the starting and ending odometer readings as well."

It is to laugh: how could your bicycle have an odometer and not a speedometer? And if you have a speedometer, it's ve-e-e-ery likely to show you average speed and instantaneous speed. The problem would be solved right there.
But we suspect that the question is from a homework assignment about calculating speed on the fly. How would you do that? It's pretty simple (and it's the principle on which bicycle speedometers work):

1. Measure the circumference of the wheel
2. Mark a point on the wheel
3. Measure the time necessary for the wheel to complete a revolution or, better yet, the number of revolutions in a minute.
4. Convert distance and time to miles and hours.

Here's an example calculation for a wheel circumference of 83.25 inches completing 125 revolutions in one minute. In words, you calculate the inches per revolution times number of revolutions per minute times number of minutes in an hour, and divide that by the number of inches in a mile:

MPH = (83.25 * 125 * 60)  / (5280 * 12) = 9.85 MPH

It's that simple. Of course, a bicycle computer is constantly updating the calculations. In truth it isn't reading the instantaneous velocity but a moving average of the past few seconds... but it's close enough, as they say, for government work.
If you have any sense, of course, you'll realize that Adkins could have easily fit both interpretations of the question into his 300-word post, not to mention that our Dumbass of the Day didn't seem to notice that the odometer is generally coupled to a speedometer. Feh.

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