Bike Computer Magnet Goes Missing in Action

With only about 42 miles on my new Surly Long Haul Trucker, I looked down and saw that the Cateye Strada computer was showing 0 (that’s zero, nada, zilch) for cadence. That was odd, because it seemed like I was cranking about 80 or 90 RPM.

I pulled over and noticed that the magnet in a nifty plastic housing attached to the crank arm with a Nylon zip tie had slipped down and wasn’t positioned correctly for the sensor to read it. I slid it down about half an inch to where it would register again.

Then it happened again

About a mile up the road, I saw the computer was reading 0 (that’s zero, nada, zilch) again.

At the first opportunity to pull over, I stopped and discovered the reason: the bleeping magnet and its nifty plastic housing had gone MIA (Missing in Action).

I decided that it wasn’t worth looking for it. I searched my tool bag for some Rare Earth Magnets I thought lived there, but discovered that I had taken them out at some point. Rather than muck around, I jumped back on the bike and headed north to Jupiter Island.

I had really wanted to see my cadence, because I’m still learning which gear ratios do what on the new bike.

Time for a fix

I wrote about how I used Goop to attach magnets to the crank arm on my old Trek 1220, so I figured I’d use the same technique this time.

I rooted around in the official Biking Stuff Junk Drawer until I came upon two sets of two Rare Earth magnets from Radio Shack. (My brother has pointed out that I can buy them a lot cheaper through Amazon. Working on the theory that even a blind hog can find a acorn from time to time, I’ll concede that point to him.)

I had just bought a new tube of Marine Goop from George’s Hardware up the street, so I grabbed it on the way out the door, along with a small piece of double-edged foam tape to hold the magnets in place while I determined their best placement.

Those magnets are really strong

The first thing that happened was that the magnets jumped out of my fingers and latched onto my pedal. After digging them out, which was no easy task, I stuck them on the foam tape and tried for the best position. I couldn’t get a reading.

That’s when I noticed that the sensor “sweet spot” was right over the hole where the pedal screwed in. I stacked the four magnets on top of each other and gave the cranks a spin.

Eureka! It worked.

Obviously, I couldn’t apply Goop here or I’d never be able to get the pedal off again, so I’m going to see how they work being held together by pure magnetic attraction. [Speaking of magnetic attraction, I met my wife in February about 43 years ago, so there must be something in those natural forces that hold things together.]

To be on the safe side

I’m going to take my brother’s advice and order someĀ spares to keep in the tool bag.

9 Replies to “Bike Computer Magnet Goes Missing in Action”

  1. You must have slept through that lesson in Chem class. To quote Wikipedia:

    “According to IUPAC, rare earth elements or rare earth metals are a collection of seventeen chemical elements in the periodic table, namely scandium, yttrium, and the fifteen lanthanoids.[1] Scandium and yttrium are considered rare earths since they tend to occur in the same ore deposits as the lanthanoids and share similar chemical properties with them.”

    Most “rare earth” magnets are made of Neodymium. (See )

    Now, aren’t you sorry you asked?

  2. Let me offer you a small suggestion about keeping the magnets in place. Glue the magnets together. Doesn’t take much glue and they will not shift, or get knocked off that way. It did appear in your photograph that you have opted for rather small versions of the rare magnets you might want to go up one size (bigger magnet more “pull” and less likely to disappear) and while I know you think you will never the magnet off, some super glue gel on the first magnet, a thin coat mind you, will be enough to hold it in place but still let you slip a razor blade between magnet and bike to remove if you need it.

    You certainly missed your opportunity to send Adam out to look for some “coast line” while he was in search of the illusive RARE earth magnets…

  3. I ordered some 3/8″ magnets from Amazon. When then come in, I’ll take your advice (wow, twice in one posting. That sets some kind of record.)

    The problem with gluing the magnets in place is that are sticking to the metal of the pedal, which is recessed inside the crank. If I put glue on the first one, I’d never be able to get an Allen wrench in there to take the pedal off.

    I rode 30+ miles Sunday and the magnets didn’t shift. When the new ones come in, I’ll either throw a few in the tool pouch for spares or I may consider sliding the receiver sensor toward the back of the bike about half an inch so I could surface mount the magnets.

  4. Go back and reread what I said about a tiny bit of super gel glue (NOT Marine GOOP) on the first magnet, you will be able to break it loose with the end of screwdriver or something. That particular glue will stick metal to metal, but it won’t bond metal to metal like the GOOP stuff will.

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