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Has anyone ever tried just using a cheap ($5 for example) bike computer but instead of using the wheels, placing the magnet on the crank arms and then calibrating it to show cadence instead of road speed? I may try this but the computer I use only allows a 1 to 3 mm gap at most and that may be difficult to get down there.

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    It should be possible to make it work -- I suggest spacers should be made of something like fairly solid packaging foam, at least for testing, as it's easy to work and not likely to jam anything. The issue you may have is counting revs rather than converting to speed, and of course you want per minute rather than per hour. If you could set a wheel circumference of 1/600 km (i.e. 1.67 m, which is plausible) then the digits should represent something readable while riding (1 rpm would be represented by 0.1 km/h) – Chris H Jan 20 '16 at 8:49
  • Chris H, why don't you post that as an answer? – Emil Vikström Jan 20 '16 at 12:27
  • Do note that you may need to extend the wire somehow, since many units are designed to mount the pickup on the front fork. Also note that a wireless unit would likely not work, as most need to have the pickup closer to the head. – Daniel R Hicks Jan 20 '16 at 13:18
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    It wouldn't have to show exact cadence such as 88 (RPMs), I could calibrate the computer to read 8.8 for example and interpret that as 88. This should be fine since the 1st digit is more significant than the 2nd. For example, I would like to see at a glance if I am in the 80s, 90s, or 100s for cadence but the 2nd digit is not so critical (85 vs 88 for example). Regarding lengthening the wire, my regular computer had a lot of slack leftover after I mounted it so extending the wire may not be necessary but yes something to watch for. – David Jan 20 '16 at 13:25
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    I actually just checked and it appears to be possible since the crank arm comes reasonably close (about 20mm) to the frame in the rearward position so as someone suggested, I could use some spacers to fill in the gap to make it maybe 2mm and just recalibrate for cadence, not road speed. One drawback is in the higher gears it wont update quickly cuz the crank will be spinning slowly. However it should be accurate enough and I usually use the middle gear ratios anyway (34/20 and 34/18 are my 2 most frequently used gear ratios). – David Jan 20 '16 at 16:00
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This has been done before in pretty much the manner you're describing (and its basically the same way that a decent number of commercial cadence sensors work, e.g. Garmin GSC10, which attaches a magnet to the pedal arm and uses the same type of sensor at the wheel).

An example of someone doing this is this instructable. Basically, the guy attached a bunch of magnets to the crankset's granny ring equally spaced due to how his meter read out (*), and bolted the sensor to the downtube near there.

The main challenge is getting the readout to be readable on your bike computer and also aligning the magnets + sensor.

That being said, Bontrager among others sell ANT+ cadence+speed sensors for around 30-40 USD, so you might not be saving all that much by using this if you already have an ANT+ phone/bike computer.

(*) Assume the bicycle computer is programmed for a tire circumference c (which will be set within some range of values prescribed by the computer manufacturer). Each time a magnet passes the sensor, the sensor counts 1 circumference of tire worth of distance. by putting n magnets on the granny ring (which is the closest and therefore easiest to trigger the pickup), each revolution of the crankset reads nc distance. By setting the tire size on the computer to satisfy cn*1 rpm= 1 km/h, the readout of the computer in km/h will be the cadence in rpm.

The main problem is that your computer probably won't read past say 100 km/h. So, you can do something like set circumference of tire to satisfy c*n*2 rpm= 1 km/h which would give you half the rpm as a readout, or similar.

You also need the magnets to be spaced far enough apart (or of suitable strength) that the sensor (most likely a reed switch, less likely an inductor or hall effect sensor) can switch on and off to register each of the magnets.

  • I got some cheap Chinese made bike computer for about $5 shipped (keeping in theme with my cheap $100 Walmart bike). What I would like to do is get another one just like it but calibrate that one for cadence which should be possible. If it is not possible (or very difficult), what I can do as a cheat is to connect it to the front wheel too (on the opposite fork of the other working one), and calibrate it for cadence in my favorite gear which is (2,4) on my 21 speed (middle chainring and middle cog). That gear is 34T driving 20T for a 1.7 gear. So it would be correct in that gear only. – David Jan 21 '16 at 3:37
  • Continuation of previous comment... as my legs get stronger, I can recalibrate for a higher gear such as (2,5) which is 34T driving 18T or even (2,6) which is 34T driving 16T. It makes sense to check cadence when I am at my normal cruising speed cuz as I shift thru the lower gears getting started it is not really informative. On the on road when I am riding 15 MPH or so it is. – David Jan 21 '16 at 3:42
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    If you're going to do all that work rather then its just better to memorize a few cadence vs speed numbers for your favorite gear(s) and then interpolate what your cadence must be in your head from the speed readout on your computer. I know a 52x13 with 27 inch nominal diameter is 13 mph @ 40 rpm, so at 80 rpm its 26 mph. Approximately work backwards to figure out your rpm. 60 rpm would be 20 mph, etc. – Batman Jan 21 '16 at 4:34
  • -Batman. Actually that is a good idea and simpler to get working. My favorite cadence is 88 RPMs since that is a "magic" number since the feet per revolution and the MPH at that cadence are the same. For example, a gear that moves the bike 10 feet of road distance will also give you 10 MPH at 88 cadence. So I should just make a chart like you said of my 3 favorite gear ratios and check cadence that way based on road speed. I could also do this with 3 extra cheap computers, each one calibrated for a different one of my favorite gear ratios. – David Jan 21 '16 at 4:42

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