Another question about cheap speedo into cadence question.

If you can put in 16667 (instead of 1667) it should show 90 cadence as 90kph, rather than 9.0kph.

However, two potential problems with doing this:
1) calibration is unlikely to allow you to put in a number as large as 16667 (hence the 5 magnets thing in the instructables)

2) Even if you could enter 16667 it's likely that your cycle computer would not want to display speed greater than 100kph, so whenever your cadence goes above 100, it would not display!

Sounds likely that you have already found the best compromise.

First off, check what the computer is expecting for wheel size.

Is it expecting a Diameter or a Circumference?
Is it using millimetres, centimetres or inches ?

The size of your crank is irrelevant - you just need the number of times per minute that the magnet passes the sensor.

Here comes the science:

For a reading of 60kph, the computer calculates that a wheel has rolled 60,000m in one hour.

60,000 metres/hour = 1,000 metres / minute

If you think of the wheel as representing your chainring, then at a cadence of 60rpm, the chainring has turned 60 times in that minute. A "wheel" turning 60 times in a minute covering 1,000 metres means that the circumference of the wheel is (1000 / 60) = 16.6666 metres

Divide circumference by Pi to get diameter

16.6666 / 3.14159265etc = 5.305 metre diameter.

Now, a standard 700x23 wheel has about 2.1 metres circumference, or about 0.668 metre diameter, so here is where your problem is.

If your computer doesn't let you set a diameter up to 5.305 metres / 5305 mm / 208.86 inches, then you can't use a single magnet. This is why I suspect the instructables article is using 5 magnets - the computer is getting 5 pulses per revolution, so the configured wheel size can come down to 1.061 metre diameter, or 3.333 metre circumference, which is more likely to be in the range of accepted values.

So your formula is (for cadence using KPH, sizes in metres)

Circumference = 16.666 / number of magnets (Instructables has 5 magnets, hence their value for wheel size of 3333)
Diameter = 5.305 / number of magnets

For MPH, it's the answer from the KPH formula multiplied by 1.609344, so
Circumference = 26.8224 / number of magnets
Diameter = 8.538 / number of magnets

Let us know if that helps?

http://www.instructables.com/id/Very-Easy-Cadence-Meter-For-Your-Bike-12...

ebay, Cateye Astrale.
I think I chucked a couple away a few weeks ago, from the old computer and light retirement home in the garage, sorry

I know what your trying to achieve but to me the calculation seems overly complicated. All you want the sensor to count and calculate the average of your cranks rpm, not necessarily the speed at which they're passing the sensor. Therefore, the distance from the sensor to the BB I would have thought is irrelevant. By this same reckoning the gear ratio should have no influence either.

Sorry I can't help further.