Cyclocomputer mph to cadence conversion
 

I'm cheap and "I don't need no stinkin" speedometer and I read here somewhere that its possible to mount the wheel sensors of a cyclocomputer on a crank arm and calibrate the computer so it reads cadence.

Anyone done that and if so, which computer and how? Thanks.

Interesting thought here. Logically speaking, you measure in miles or km per HOUR but revolutions per MINUTE, so I think if you found and entered the value that would be equal to 1/60 of a mile or km (to trick the computer into thinking you are getting 60 revolutions per mile). I don't know if I derived this correctly, but take whatever value your computer instructions say to enter for a 700x23 tire size and multiply that by 12.95 for American and 7.94 for Metric. Let me know if that works.

If you don't have your computer instructions or just want to know which calculations I looked up to determine those values, check this site out.
http://sheldonbrown.com/cyclecomp_c.html

Well, the math is so simple you don't need no stinkin' Sheldon Brown web site to figure it out;)

If you're pedaling 100 RPM, then you want the computer to think you're going 100 MPH, so it displays "100" (I'll do it in English units and leave the metric as an exercise for the reader). So you want:

100 RPM = 100 MPH

A mile is 5280 feet, and there are 60 minutes in an hour, so 100 MPH = 100 * 5280 / 60 = 8800 Feet per Minute, so:

100 RPM = 8800 FPM

Therefore:

1 RPM = 88 FPM or
1 revolution = 88 feet

You want your computer to think that the circumference of your wheel (the number usually entered for the wheel size) is 88 feet.

This makes the possibly-unsafe assumption that (a) your computer will display three-digit speeds (i.e. greater than 99) and (b) your computer will accept bizarrely large wheel sizes (e.g. Godzilla's pennyfarthing). You may need to settle for a wheel size of 8.8 feet, and have the cadence display be 1/10th the real value (so a display of "10.3" means a cadence of 103 RPM).

Ed "Slide Rule" McNierney

One way to fool the computer into giving correct readings with an abnormally large wheel is to fit two wheel magnets 180° apart. That way the computer sees two pulses per revolution and you set the circumference at 1/2 the true value. It's a technique used by Penny/Farthing riders who don't mind the anachronism of a modern cyclometer on an "antique" bike

:D :D :D :D :D

Yes, that's true, but it's not practical when you want to mount the magnet on the crankarm, as San Rensho wants to do.

If San Rensho needs to buy a cycle computer why not buy a Cateye Astrale and set it to show cadence/time, cadence/avg. speed, cadence/distance, cadence/whatever, or even block out part of the display with black electric tape so it only shows cadence? I'm guessing that would be easier than trying to set the wheel circumference to 88 feet.

This whole discussion is really very interesting from an academic perspective but if San Rensho is really that cheap, a plain speed/distance/time cyclometer is much cheaper than any that also show cadence. Give up and spend the $25, or even less. :)

Enter the wheel size as "167" if the value is required in centimeters, or"1667" if it's required in millimeters, and then set the cyclocomputer to display speed in KPH. The 'puter will display a value equal to cadence divided by 10, i.e., 100 rpm will show up as 10.0. I've been using an old Cateye for this function for a long time now.

I was trying to give an actual value because most computers don't ask you to enter the circumfrence in feet. All I did was take your explanation above and compared it to a known value to determine the difference. But hey, if San Rensho wants to dial "88" into his computer and wonder why it shows a cadence of 2RPM, that's fine.

I was about to move on when I realized that there are some interesting readings possible on computers if yourig it to read cadence instead of speed.

Things like average cadence, maximum cadence, and total number of revolutions by the cranks.

Then, depending on the objective, if the computer can be set to stop recording time when the bicycle is stopped... You can now ensure that you don't get credit for coasting. :D

EDIT: I just thought of a potential weakness in the plan to use a "normal" computer to record cadence... The wire won't be long enough, and the work and cost of extending it could offset the savings of buying a cheaper computer.

...or, you could skip all the techno-babble above and from time to time count your pedal strokes for 10 seconds, multiply by six, and lo-and-behold, you have your cadence.

Bob

That great, I never thought of the possibilities. Especially max cadence. It would be great to see how high I was able to wind it up after a sprint. Like having a telltale on a race car tach.

Ok. Did some homework at Nashbar. Cheapest computer $15, cheapest computer with cadence $30. So even though my cheapness is nagging me, I'll splurge on this one and get the one with cadence.

Thanks all.

!!! That's a great solution! It will also give an average and a max, plus total time pedaled, compared to the regular computers showing total time rolling, which is higher. Oh, and also a bizarre "distance"! :D

You could mount the computer on the top tube just in front of the seat. Of course then everyone would wonder why you keep staring at your crotch.

One caveat I just thought of...
If you, like me, keep your pedals horizontal when you don't pedal, and hold your right foot back, the magnet and sensor will be right next to each other, and potentially either trip constantly (which could perhaps drain the battery much faster) or rapidly go on and off, going back and forth across the sensor, to give a false reading (very high average and max cadence).