I did a search on this topic, but was not able to find exactly what I'm looking for and don't really want to reinvent the wheel (pun intended) if I don't have to...
I want to adapt a cheap bike computer as a cadence display for a spin bike. What I lack is the computational skill to accomplish that feat. Here's my plan: Since a regular speed sensor on a bike computer simply measures the time it takes one rotation of the wheel and calculates either MPH or KPH from that, I should be able to, by inputting a certain value for circumference of the wheel be able to achieve an RPM read out, right. I realize that most cycle computers don't go up to 100+ MPH, so what I thought was to reduce the readout calculation by a factor of ten. That way 10 MPH readout would mean 100 RPM cadence. And, since most cycle computers read out to the 1/10 MPH, I should get a usable figure. Anyway, I was wondering if anyone had tried that and if so if they would share the calculation or method of calibration ...

Wheel rotation speed on a trainer has no relevant physical meaning. What matters is cadence and level of exertion. A power meter is best for determining level of exertion, but lacking that, a HRM can be a useful indicator if understood and interpreted properly. If you want to estimate how far you might have traveled on a bike, look at how far you typically go on a bike ride in that amount of time at that level of exertion.

Move the sensor and magnet to the crank.

The first instinct is to have 1 MPH = 1 RPM. However, typically you will be spinning up at around 100 RPM, and most bike comps can't really deal with a speed of 100 MPH (or faster), so since it probably reads in tenths, I'd shoot for the RPM reading to be in tenths (IE 10.0 MPH = 100 RPM)

The math is ridiculously simple.

You want 10 RPM to be equal to 1.0 MPH on the comp. Since 1 mile = 5280 feet, and an hour = 60 minutes, in 60 minutes your pedal (which it thinks is a wheel) turning 10 times a minute will turn 600 times. So you want to set up a circumference where 600 times = 5280 feet. That means a circumference of 5280/600 = 8.8 feet. Since I think all comps want to be set up in millimeters, convert that. 8.8 feet = 105.6 inches = 2682.24 millimeters.

Set the comp to a wheel circumference of 2682 mm.

EDIT: sorry about duplicating your comments on 100 MPH, tenths readout, etc. I just typed what I normally do for this question.

Thanks for the replies. So if using the KPH readout I'd want to use a wheel circumference of 1667 mm....right ?

I wasn't trying to determine distance, just cadence. I already use a HRM and just wanted to be able to be consistent with my cadence.

I didn't consider putting the sensor on the crank, which, as I now think about it, really is the easiest way to get where I wanted to go...now if I could just figure an easy (cheap) way to monitor power input....

Yes, 1667 is correct if you're in kph mode.

If mounting on the crank is inconvenient, if you stay in the same gear all the time, you could put it on the rear tire (or front I guess if you have a trainer that turns the front wheel) - and include the sprocket-chainring ratio in the calculations (you can do this at the end - if you run, say, 38 teeth in the front and 18 in the back, instead of 1667 use 1667/(38/18) = 789mm circumference.

When I get the computer in I'll get it mounted and report back.

Finally got my bike computers in the mail today. Used the above formula including the calculation for the front wheel (i.e. 52/16) and it worked like a charm. Thanks for the help....

Waldick, you already have the cheapest power meter available, a heart rate monitor.

Very old thread I know but another point, set computer to mph NOT kph BECAUSE most computers' speed display shuts off < 2.5mph or 4.0kph (pushing bike walking or coasting at stop sign/light speed) so at mph setting your cadence will be displayed > 25rpm vs. > 40rpm for kph. So program computer to mph and 268 if sets to cms or 2682 if sets to mms so for ex.- 85 rpm cadence will display 8.5mph. Also you can mount 2 computers on your bike (1 regular + 1 cadence) and compare your computers' stopwatch ride times and know exactly the ratio of pedaling to coasting. You'll also know avg. cadence and max cadence and the odometer and trip distance will tell you your total pedal rotations (multiply 600 rpms x miles).