You’re speeding along enjoying a nicely paced training ride, when suddenly you realize your Garmin is no longer picking up your cadence. If you’re like me, several miles may have passed in blissful whistling before you notice that the sensor magnet on the crank arm has fallen completely off. Vowing never to trust a zip tie again, you pedal home having to count each stroke. Math…yuck.
Instead of hitting your local bike shop or checking Garmin’s website for part replacements, find the cheapest neodymium magnet you can buy, or search your house for the small, rounded variety of magnet. Seriously, that’s basically what that piece of rectangular plastic that fits so neatly on your crank is made of.
Take the magnet and stick it to the inside of your pedal spindle–basically where the spindle screws into the inside of the crank arm. That’s it! Although you may need to slide the cadence sensor forward or back slightly, it should have you free from all that math in no time.
As an added plus, your bike will also look just that much cleaner and faster without those added zip ties. 
A FEW TIPS
If you have hex bolt style pedals–the kind that you install or remove with an Allan key from the back of the crank arm–I would recommend that you use a magnet smaller than the diameter of the spindle opening in your crank arm, and one that does not sit flush. The magnet will still stay put, but you’ll be able to easily pry it out when you need to remove or service the pedal.
One note, though. If you have titanium pedal spindles, you’ll be out of luck here–neodymium magnets won’t stick for you.
Amazon has tons of magnets that will fit. Generally, you’ll want to stick to something smaller than 8mm, but you can play around with the fit if you get a multi-pack of magnets.
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