My Haywire Bike Computer

The other day, I was out for a ride when I noticed something odd. My bike computer was behaving very oddly. It registered speeds twice as fast as I was going, then suddenly dropped to half the speed. After I’d ridden about four miles, it showed that I had done closer to nine.

Now, when you’re as OCD as I am about keeping stats, that’s a very uncomfortable feeling. I could still enter the mileage, because my route was one I ride all the time, so I know how far it is. I missed such important stats as average speed, maximum speed, and time of ride.

After traveling about half the route, I pulled over to the curb and adjusted my speed sensor on the bike’s fork. I made sure it was within two mm of the magnet, and I turned the magnet a bit to make sure it was positioned perfectly. Then I hopped on the bike and started pedaling.

Nothing. That is, nothing had changed. It still had me racing beyond my ability one moment, and in the next, wasn’t registering speed at all.

Back home, I reasoned that my headlight could be the culprit. I had installed the headlight just before the ride, because the sun is low in the sky and casts long shadows these days. A headlight on low beam would let me better be seen by drivers, especially if the sun is in their faces.

But I remembered that last winter when I had the light on the bike, the computer ran just fine. Hm. That couldn’t be it, then.

Just for the heck of it, when I rode the next day, I took the headlight off.

The computer worked perfectly.

“Magnetic anomaly!” said one of my biking friends.

That was it, all right. And I recalled that one thing had changed since last year. I have a new computer. The old was was wired. The new one is wireless. The wired computer wasn’t affected by the magnetic field thrown off by the headlight, but it sure plays havoc with a wireless unit.

I guess I’ll just have to make it a point to ride before dusk for the rest of winter.


  1. jon fair says

    What kind of light and what kind of computer are you using? Is the computer transmitting via RF or Bluetooth? I wouldnt think a battery powered light would have the kind of inductance needed to create a magnetic field unless you were running a dynamo. The Nerd part of my brain is going nuts with this one lol!

  2. Midland says

    I’m happy your using your light in the afternoon, please keep using it. I’m a big proponent of using “to be seen” lights”

  3. says

    I had to use a wired computer on my commuter bike because my MagicShine light dumps loads of interference, more than my Sigma could deal with. So much for using one computer on all my bikes…

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