Bicycle Mechanics - Installing cadence magnet on crank with hollow back?

I picked up the cadence sensor kit for my Sigma 1606 wireless today. The crank arms (Bontrager Race Lite) on my bike (Trek 2100) are hollowed out on the back side, and the magnet is too wide to fit in the "trough", and too narrow to span both sides. It wants to sit with one side down and one side up so it is crooked. Anybody have a suggestion?

Add a shim.

Add a shim.

I thought about that, but the relief is round, so unless I make a WIDE shim that will bridge the relief it won't improve matters. The kit came with a shim to space the magnet closer to the sensor, but it actually makes the crank-to-magnet fit worse. I guess I'll have to do it that way, but I was hoping to be able to use the adhesive tape that is on the magnet to help keep it in place.

Sometimes you just have to get creative.

I can understand wanting to do a proper job of it...and you can. Try carving some lightweight materials to create your shim. A little time with a Dremel or file and some foam tape should get you what you need. Your saving grace is that this is not a highly stressed component. As long as the magnet passes within the sensor's range and doesn't shift while riding, you're golden.

Bonus: If done right, it won't show so your worksmanship matters less than the function. ;)

Rtv?

Fill the void in that area with some 'Touch 'n Foam' insulation, level the area, then use a double sided sticky to hold it fast.

If the magnet is in a casing, remove it from the casing and just install the magnet. Worked several times for me with different computers.

Or, just buy a magnet that fits the hole in the crank behind the spindle. They are everywhere.

try a wine cork. you can carve it up to your specs.

Or, just buy a magnet that fits the hole in the crank behind the spindle. They are everywhere.


http://www.radioshack.com/product/index.jsp?productId=2102642&cp=&pg=1&sr=1&y=5&retainProdsInSession=1&origkw=magnets&x=12&kw=magnets&numProdsPerPage=100&parentPage=search

+1. I use the earth magnets, round ones, just stick them to the end of the pedal spindle. i dont even glue them, the magnetisim holds them tightly enough that i've never lost one, even on my cross bike on BAD washboard.

I just cut some strips of double-sided tape in the lengths that I needed and stacked them until they held the sensor fairly flush. It's not rock solid, but it holds it on just fine.

If the magnet has tabs for attaching with a zip tie, just use a zip tie. It doesn't have to rest an a backing and if the zip tie is tight, the magnet is going nowhere.

+1 on attaching a magnet to the pedal spindle (assuming the spindle is steel). I use 1/2" round neodymium magnets. These fit perfectly over the back of the spindle, but are not so small that they will fall into the hex socket on some pedals. The ones I have are around 1/4" thick and stick out from the crank arm a bit, so they are easy to grab with a needlenose pliers if you need to take one off to get at the hex socket.

+1 on attaching a magnet to the pedal spindle (assuming the spindle is steel). ...

If not, super glue or the like works just fine.

Thanks for the replies, folks. I feel like a complete idiot because I was looking at mounting the magnet more toward the BB than out at the spindle of the pedal (with the idea that the sensor was tucked in better from being bumped). When gmason mentioned a magnet on the spindle itself, the fog cleared a little in the old brain so tonight after dinner I went to the shop and mounted it out near the end of the crank arm and all is well (the hollow cavity stops short of there).

Thanks again!

:beer: Mazeltov!