Very Easy Cadence Meter for Your Bike <$12

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Introduction: Very Easy Cadence Meter for Your Bike <$12

Cadence is the speed at which the pedals turn, measured in Revolutions Per Minute.
Inexperienced riders tend to pedal too slow which will put stress on their knees.

I always wanted to know how fast I was pedaling but I didn't
want to spend a lot of money to do it. Most experienced riders
have a Cadence of 70 to 90 RPM's. When I started using the meter
I found 60-80 to be my norm, but since then I moved up into the
70-90 range without realizing it.

Here is my solution for less then $12 and super easy to do.

Step 1: $10 Cycle Computer

I bought this at Walmart, but many other
stores carry them. They are very accurate
and simple to use. They also turn on
automatically when the magnet passes
the sensor.

It will also record your maximum and
average Cadence plus the number of
times the crank turned. It will also
keep track of the time you are actually
pedaling. When compared with the time
on the speedometer it will tell you
how much coasting you did.

The only drawback is the meter only
reads to 99.9, but you can set it to read
1/2 and remember to double the reading.


Step 2: Computer and Harness


Here's what you get.

1. Computer
2. Computer harness with bracket
and sensor.
3. Wheel magnet and zip ties which I
didn't use because I like the black
ties.

Step 3: One Dollar Magnets

I picked these up at a Dollar store.
You can use about anything, just
check that they are fairly strong.

Phil B found 1/2 inch ceramic magnets
at Radio Shack for $1.99

Step 4: Remove Magnets


I lucked out finding a pack of
5 which you use all of them on
a road bike.

On a Mountain bike you can use
4 due to the difference in the
crank.

I removed the magnets by cutting
around the outside of the magnet
with a knife. Be careful, you
don't want to pry on the magnet
and break it.

Step 5: Place Magnets

Place the magnets on your smallest
chain ring.

I used 5 so the spacing was even
around the crank.

You can use 4 on a Mountain bike.

If the magnets are fairly strong they
will stay put, but they did move when
they were hit with the hose. Be careful not
to get water in the BB when you wash your bike.

If you have aluminum chain rings and bolts you
will have to glue them on. I used rubber
cement on mine anyways.

Step 6: Mount Sensor

Mount the Sensor on the front down
tube as shown. Let a space about
3 mm from the magnet with the end
of the sensor about 1/2 way over
the center of the magnet.

Don't mount the computer bracket to
the handlebar yet, that way you see the
readout easier.

Set the computer by instruction manual
using 3333 as the wheel size and
the speed reading to KM not Miles.

You can check the pickup by rotating the
crank backwards with the kickstand up
and the bike leaning against something.

Once you are happy with the pickup you
can mount the computer to the handlebar.

Step 7: Sensor Position

Here is a second view of the sensor.

Step 8: Computer Set Up

This is the set up for this computer.

Hold left and right buttons down at the same time to reset,
the whole display will flash then go to the wheel size.

It will show 2124 the default setting with the 4 flashing.
Change the 4 to a 3 by pressing the right button.
Once it is at 3 press the left button to move the
cursor over to the next number and set it to 3.
Repeat until the number is 3333

It will then go to the KM/Miles mode.
Since it comes up as KM first just press the
left button to set it as that. The right button
will switch it back and forth between the two.

You set the time by holding the left button down for
about 4 sec when the display is showing the time.
You can select 12 or 24 hr mode then you
will set the time, hours and minutes.

If You you mess up just reset by holding both buttons for
a few seconds and start over.

How it works.

Say you want it to read 60KM at 60 RPM's
60km is 1km per minute or 1000 meters.
1000 meters is 1000/60 or 16.666 meters per sec.
Since there are 5 magnets for each rotation 16.666/5
is 3.333 meters per 1/5 second or 3333 MM.

Each time a magnet passes the sensor it is recording
as if it is moving 3.333 meters.

Mountain bikes or comfort bikes with 4 magnets.

16.666/4 would be a setting of 4166

The reason you need this many magnets is that the max
wheel size you can input on this computer is 5999.

I hardly ever have a Cadence above 90 so the 99 limit is
fine for me. You can cut the number in half and
remember to double the readout if you like. That will
set the top limit to 199.8 RPM's

To check your work go to this site and print out
a gear ratio chart using your tire size, front
chain ring, rear cassette size and the cadence
you want. You can cut it out and tape it
on your bike if you like.
http://www.sheldonbrown.com/gears/

Hope you enjoy this project as much as I do.

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67 Discussions

0
user
csiebe

2 months ago

Thanks for posting this. I had this idea myself but you saved me the math. I have a 4 spoke crankset and I found that my cadence results were more consistent when I used 2 magnets instead of 4.

Hello! I have some doubts, using the equation with 3333 will read 60km/h to 60rpm, but at 90km/h will mean 90RPM??

Because if i use the same formula for 90km/h to 90rpm i have to input 5000 in the wheel size. So, the formula is accurate at the targeted RPM (60 with your example, 90 with mine), but at other ranges??

0
user
Phil B

1 year ago

I remembered your cadence meter Instructsble and adapted it for use as a monitor on a NordicTrack ski machine. I found a wheel diameter setting of 0470 on the Schwinn cyclometer works very well and gives readings very close to what I get for the same effort on an actual NordicTrack monitor. One advantage of using the Schwinn cyclometer is that I can see the duration of my exercise session in minutes simultaneously with the ground speed in miles per hour. On my NordicTrack monitor I must punch a button to switch between the two.

The NordicTrack monitor I have is twenty or more years old. Electrolytic capacitors on the circuit board and contacts on the touch buttons are aging considerably, although they still work. The bicycle cyclometer gives me new parts, even though they now cost in excess of $20 for what was $12 in 2008.

I do not know if you are still active on Instructables or not. You can see an Instructsble on my adaptation at https://www.instructables.com/id/Easy-Monitor-for-...

>>You can get a cadence computer on ebay for around $5-$6

No you can't.
Link????

This is a pretty ingenious idea! That being said:

You could get by with just one magnet, the speedometer comes with just one magnet because that's all that is needed. This would be better if you are stopping and starting a LOT.

You can get a cadence computer on ebay for around $5-$6

You can just mount 1 magnet, input 166cm or 1666mm as the wheel size, then when you read the speed just multiply by 10 so if you see 9,4 km/h you have a 94rpm cadence.

1 reply

I'm guessing more magnets would lead to more accurate measurement?... well this $20 piece of s*** PC I got from local store only goes up to 2999 (thousandth digit capped to 2), so I'll have to go with your way. Thanks for the tip bro.

i dont get why you use 5 magnets instead of just one..... like on the wheel can someone explain me??

5 replies

6 years later I hope this still helps:

It is because the computer only allows you to set the circumference of the tire between 1 - 9999mm, yet we need it to think the tire has a circumference of 16666.67mm. So if we divide 16666.67 by 2 we get 8333.335mm, but then it would have to spin twice as fast, so we cheat the system by adding a second magnet to make it think its spinning twice as fast.

On a road bike using 5 magnets is easier to space as the spokes do the spacing for us, but then you need to divide 16666.67mm by 5 which equals 3333.33mm and use that as your setting.

You can use as many magnets as you want provided you comply with the limitations of the computer you are using.

If you choose to use one magnet, you will have to set the diameter to 1667mm. this will have the effect of moving the decimal point so that 8.8km actually means 88rpm. Useful if you want to achieve a cadence above 99.9rpm.

See my comment below. I used one magnet stuck in the crank arm recess at the end of the left pedal. I programmed the cyclometer for a wheel size of 1667. It worked great. I ignored the decimal point for a direct reading of my cadence. I think the author used multiple magnets so the two display numbers would both be large, not the smaller decimal place number. He said the sun may glint off of the screen and make the smaller number hard to read. I did not have enough wire to reach the handlebar and mounted the display unit on my top tube near the headset. My body shields the display unit from the sun. It worked for me that way. I was concerned that magnets stuck to the caps of the chainring bolts could come off if I hit a big bump on the road. The recess at the end of the crank arm seemed like good insurance.

For each revolution of the device measured (wheel or crank arm), the magnet triggers the computer that 1 revolution has been made, and it calculates speed from how many times per minute the magnet triggers it and multiplies by a formula.

each gear operates differently it can change the way it calculates the speed

This is the best instructable EVER!!!

I am currently making my 2nd one and I ran into the problem of only being able to find a modern speedo that sets the tire circumference by the diameter (ie: 5" to 31").

I solved the problem the following way:

1. The magic number is 16666.67mm as 1 revolution per second will give a speed reading of 60km/h or 1km/minute which is equal to 1rpm.

2. As the speedo will only allow us to set the tire diameter in inches we need to divide the magic number by integers until we find a circumference that will most closely resemble the allowed settings. Eg: 31" = 787.4mm which gives us a diameter of 2473.7mm.

3. After trial and error I found that 16666.67mm / 8 = 2083.33mm. The diameter of a 26" tyre is 2074.7mm. This is the closest you will manage to get using the range of 5" to 31" tyres so I agree that this will not be the most accurate sensor, however the error will be extremely small between 0-99.9rpm that you wont need to worry.

4. Now we need to make the computer believe that a tire with a 26" diameter is spinning at 8 revs per second which will equal 1rpm of a tire with circumference of 2074.7mm. To do this we need 8 magnets. ONLY 8, NO MORE, NO LESS.

5. Place these magnets with equal spacing around the small ring.

6. The rest is the same as the guide above.

7. Set the computer to tire 26" and you're good to go!

NB: This is in no way as accurate as the older speedoes that allow you to set circumference in millimeters, but the margin of error using these exact steps is 0.414%. If you can get your hands on the older speedoes then I definitely advise that over this method, but this will work almost as well if you can't.

I used it for more than a year before I found a good wireless Sigma with a cadence sensor.
(sorry, no photos - this construction is now history)
And I really appreciate, that the author decided to spread this simple and efficient lifehack.

The only difference in my design was a magnet and sensor mounting point. I used a magnet on the crank and a sensor on the chainstay (the sensor cable had a perfect length for that).

Black is the absence of light, but absorbs light readily - it's why dark things get hot in the sun. White is the presence of all light and reflects light - it's why light things stay cooler in the sun. Some things are treated to resist the damaging effects of UV light, and because black things are more rapidly damaged, they are more frequently treated. In my experience, even when plastics are treated to withstand prolonged exposure to UV, they still break down in a matter of years rather than decades.

How to calculate de cadence in te crank arm with 1 magnet. The arm messures is 160cm