Crank cadence magnets

[Cinci Jim]

A couple of years back I bought for the wife a bike that had a cool little magnet that "stuck" to the end of the pedal that is recessed into the backside of the crank arm. No zip ties, 2 sided tape or anything - very clean presentation. At the time I thought it was cool and should reflect it to our other bikes. Not a priority, but I have thought about it off & on for the past 2 years.

I asked the bike shop and it seemed like maybe a similar magnet was available, packaged for the bike industry but quite pricy.

Last night I finally measured the little magnet and looked online. Seems like they are all of 60 cents or so, but...

I think it needs to be axially magnetized? The one I found has a surface field of 2952 gauss? Is that enough?

Here is the link to what I was thinking:

http://www.kjmagnetics.com/proddetail.asp?prod=D82E

Anyone ever done something like this?

[11.4]

There are a couple options here. One is a flat disc like you suggested (or a rectangular strip or any of the dozens of other shapes you can find). I use these all the time, stuck with a little bit of Barge cement to the back of the crank arm. You don't need more than an 1/8" thick disc and I've not had a problem with 1/16" discs. Nobody ever knows they are there.

Mine come from www.rare-earth-magnets.com. They have a superb selection. You don't need to worry about coatings on them. I have the bare rare earth neodymium magnets and see no corrosion even after years on a bike.

The other option is a cylindrical magnet that actually goes in the hole in the end of the pedal. For a while pedals had a nice round concentric hole in the axle, and used flats on the axle to fit a 15 mm pedal wrench. That hole could fit a small cylindrical magnet. Nowadays everyone has an allen hole in the back, sometimes 6 mm but all mine seem to be 8 mm. It's hard to fit a magnet in there and make it stick, plus you need that hole with its allen-key flats to tighten or remove the pedal. If you have the non-allen style of pedal, you can get an axial field magnet that fits there. It does seem to lose a good bit of field strength from being buried inside the axle, but it does work as long as your sensor is close. I'd prefer just to do the flat magnet glued to the back of the crankarm.

If you have a crankarm like a Truvativ Omnium, which has a recessed shape on the back, it won't hold a flat disc well, so buy a 1/8x1/8x3/4" magnet instead. Also, if you have a disc or trispoke or whatever, get some 1/8x1/4x1" magnets and stick one inside the valve cutout.

[martinrjensen]

any magnet that will fit will work, axially, radially, upside down, inside out, it doesn't matter. I use epoxy and glue my magnet on to the crank just next to the axle pedal shaft you mention. I have knocked it off a couple times and had to put another one on but I don't really consider that an issue. It's a very clean way to add a magnet as you say.