Another question about cheap speedo into cadence question.

by ta2dwez   July 31, 2014  

I bought a cheap wireless speedo from lidl (£6.99) earlier in the week and im currently trying to use it as a cadence meter. I have a magnet on the left hand crank arm and the receiver on the chain stay with the display on the cross bar so i can see it by looking down. The possible problem is. I have measured the distance from the BB to the magnet (170mm) i have then multiplyed that by 2 and then multiplyed that by Pi. (170 x 2 x 3.1416) = 1068 rounded down. I have put that into the computer as the wheel size and im using KPH but the read out does not seemright? my comfortable cadence according to the computer is 55 but that number seems low?

i have been looking around online and they mention gear ratio's as well.

Am i missing something in the calculations?

Im not overly fussed as i can use the computer reading as a standard (55) but it would be nice to have the correct cadence.

Any help would be great


10 user comments

Oldest firstNewest firstBest rated

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.

posted by Leeroy_Silk [82 posts]
31st July 2014 - 23:24


Exactly, all I want is the RPM of the cranks turning. But for the life of me I can't work it out Worried

posted by ta2dwez [52 posts]
1st August 2014 - 7:55


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 Sad

posted by andyp [1284 posts]
1st August 2014 - 8:11


posted by the infamous grouse [24 posts]
1st August 2014 - 8:53


That link is where I got the idea from. But I have one magnet on the end of the crank arm

posted by ta2dwez [52 posts]
1st August 2014 - 9:06


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?

neildmoss's picture

posted by neildmoss [219 posts]
1st August 2014 - 10:26


I put in 1667 as the tyre circumference which was asked for by the computer. I used a cadence metronome app from the play store and it matched, i have no idea how the math works Thinking , but it works, im happy! Big Grin thanks for all your help!

posted by ta2dwez [52 posts]
1st August 2014 - 11:36

1 Like

Glad to help.

Looks like the computer is using wheel circumference in cm.

Don't forget to show your friends your average "speed" after a ride too.... Smile

neildmoss's picture

posted by neildmoss [219 posts]
1st August 2014 - 11:46


It's showing 90 cadence as 9.0kph

posted by ta2dwez [52 posts]
1st August 2014 - 12:01


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.

posted by Tjuice [138 posts]
1st August 2014 - 13:01