Spin Bike RPMs for Dummies

Stationary bike computer

You have to love it when people can find more than one way to show just how unqualified they are to write their little freelance articles (and collect their eHow.com stipends). We were checking an older post for dead links not long ago, and what to our wondering eyes should appear but a new and equally stupid post on a similar topic. The old one was something about bicycle tire sizes, and the new one seemed tangentially related... it's eHowian C. Taylor and his SportsRec.com post, "How to Convert RPMs to MPH on a Stationary Bike."

It's distressingly obvious that C. (his full name appears to be Chad) has no idea what he's talking about. Pretty much anyone who's ever spent more than a couple of minutes on a stationary bike will be flabbergasted by a line in his post. It's in his example, in which he performs some calculations and tells his readers that,

"In the example, if the bike's gauge read 500 RPMs..."

We asked one of our staffers who regularly uses stationary (and spinning) bikes how often he hits 500 RPM. He just looked at us incredulously and told us that it's rare for him to get over 130 RPM, and he's a bit of an animal in spin class. He says that the documentation on the bikes his class uses say that the maximum safe speed is about 120 RPMs... In other words, it is probably impossible for a human to reach 500 RPM on a stationary bike!

Taylor went through a long, involved discussion of how to calculate flywheel circumference and convert that to inches per minute and then to miles per hour. Except perhaps for his two-decimal place version of π, he got the math correct. Yay, Chad. Well, except perhaps for the rubbish about "Measure in inches the diameter of the stationary bike's wheel, inclusive of the tread..." Dude, stationary bicycles don't have tires! But that's not Chad's biggest faux pas. No, where he actually screwed up majorly is in his assumption that the flywheel size has anything to do with RPMs of a stationary bike. In fact, the RPMs display on a stationary bike refers to the rider's cadence, not flywheel rotations.

What Taylor should have told the OQ is that there's only one way to convert RPMs to MPH, and that's to look at the documentation for the bike. There's no industry standard, but docs for the spin bike at ANHQ (a Lemond Revmaster Pro) say that, "Speed at 80 Pedal RPM ~ 18MPH."¹ With that in mind, converting between RPM and MPH just takes simple math:

1. Divide the current RPM by 80
2. Multiply the quotient times 18

For example, an RPM of 100 converts to a speed of approximately 22.5 MPH. That calculation, however, requires a little research on your part... more research than our Dumbass of the Day bothered to perform.

¹ Different manufacturers use different speeds; documentation of the Precor Spinner bikes where our staffer works out say that 80 RPM is about 15MPH. You mileage will vary...

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