Any Magnet

View from the Wahoo Blue SC installed on my Merida bike, with replacement magnets

Biking apps seem to be built to appeal to competitive people: the notion of King of the Mountain (KoM), e.g. the fastest guy for a given segment is pretty prominent in Strava. I can’t say this interest me much. The current KoM of the A1 segment back from Aikidō is some guy called Matthias W., this had me quite confused. Be assured it is not me.

What I find interesting is to see data about how I ride, where I’m slow, which day I was fast, etc. Any good geek will tell you, more data is better, so I looked into adding some sensors to my bike. Surprisingly these things are not very expensive anymore, so I ended up buying a Wahoo Blue SC speed and cadence sensor.

The device is basically a bluetooth transmitter with two sensor plates that measure the frequency at which two magnets pass by: one on the pedal, one on the wheel. The installations instructions sounded pretty simple, except for one thing: one needs to unscrew the pedal to slip on the crank a rubber band with a magnet.

The thing is, even with the right key, I can’t unscrew the pedal – clearly I would need a longer key, the kind the guy in the bike shop has. There is a bike shop close to my flat, but it is typically closed in the morning, and closed when I come back in the evening. He is usually open on Saturdays, but this Saturday was the national holiday.

Yesterday it stuck me while I was swimming: this device probably just measures the frequency of the magnetic field, so the actual strength of the magnet would not matter, as long as it strong enough, so any magnet would do.

A few months back, I had, on a whim, bought some Neodymium magnets from Deal Extreme, I wanted to use them to fix things to the metallic frame of my office desk, which never really worked. The cool thing with these magnets is that they are really strong, and really hold on to any metal they are stuck to, like the screw that holds the pedal, the exact one I can’t unscrew.

So instead of the small magnet with its rubber band, I just stuck a bunch of magnets to the pedal, aligned everything and it works: I can now measure both speed and cadence of my biking. Sometimes, any magnet does the trick…

2 thoughts on “Any Magnet”

  1. Thank you so much for this post. My pedal was not stuck, but my chain stay was too close to the crank arm to allow clearance for the magnet band. That, and for whatever reason I had the rpm arm sticking up instead of down, so the photo really helped as well.

    I didn’t need the large magnet stack you had, the small magnet I took out of the band fits completely inside the crank behind the pedal post, but it still seems to read reliably.

    Thanks again!

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