Very Easy Cadence Meter for Your Bike <$12

Intro: 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.

Comments

[jmshnsy]

Thanks for a really good idea. I notice that your computer has 3 digits to display the speed; so if you used 1 magnet and programmed the wheel circumference to 1667mm(?) the computer would display 1/10 of cadence... 126rpm would read 12.6 km/h this both simplifies the setup and allows all normal pedal speeds to be displayed.

[jzink0883]

I had installed a CatEye Velo 9 speedometer with four magnets. My display read 1/2 actual cadence. Per your post, I went to one magnet. Easy to read on the Velo 9. Thanks!

[Sailor Bob]

Your theory is good ,but in reality its hard to read the tenth's. Any sunlight or reflections make it almost imposable sometimes. When I first set it up I had 2 magnets and used 4166 as the circumference and had to double the reading. I stayed between 30 and 40 (60-80 RPM). Now I am running 70-90 RPM 60 just feels too slow now, but when I get up around 99 I find myself starting to bounce. I know some of the complaints on the some of the Cadence computers is the readout is too small for the Cadence. The Cat Eye Astrale would let you switch the Cadence to the larger display. If I start having a Cadence over 100 RPM's I'll just set it up to double the reading. Don't be fooled by the price of this computer, it's very accurate and durable if you set it up right.

[jzink0883]

Used a CATEYE Velo 9, 08/11/2020
UPDATE 08/15/2020: Direct readout of cadence. Per a comment in this thread by jmshnsy, Sset wheel circumference to 166cm. Magnet(s) needed on only one spoke of the chainwheel. 8.6kph on the display equals 86 rpm. I found it easy to read on the Velo 9. Thank you jmshnsy!

Because of the high percentage of negative online reviews for the Schwinn computer, I decided to use a CATEYE Velo 9. The Velo 9 was $33 at Amazon. The Velo 7 ($30, slightly smaller display) also would have worked, but it was on backorder.

I used neodymium magnets. At $10 for a 10-pack (.315” Dia. x .118” Thk) at Home Depot, they’re more expensive than ceramic, but also MUCH stronger. My smallest chain ring is dished and has four spokes. To get the magnets closer to the sensor, I stacked two magnets on each spoke. In over 50 miles of riding, the magnets haven’t moved at all. I may secure them with silicone glue.

The maximum tire circumference that can be entered on the Velo 7 or the Velo 9 is 299cm (2990mm). The tire circumference numbers used to set up the Schwinn computer are too big to enter on the Velo computers. Per the original post, the number for four magnets is 4166mm. Half of 4166mm is 2083mm. For my four-magnet setup, I entered 208cm as the wheel circumference on my Velo 9. I have to double the displayed kmh number to get the actual cadence. Not a big deal for me.

UPDATE 08/15/2020: Direct readout of cadence. Per a comment here by jmshnsy, set wheel circumference to 166cm. Magnet(s) only needed on one spoke of the chainwheel. 8.6kph on the display equals 86 rpm. I found it easy to read on the Velo 9. Thank you jmshnsy!

Using a GPS speedometer app on my phone and the Sheldon Brown gear calculator (https://www.sheldonbrown.com/gear-calc.html), I checked theoretical mph vs. actual mph at multiple cadences and at multiple speeds. Actual mph was always within 0.5 mph of theoretical mph.

I’m an old, fat, out-of-shape guy (71, 5’9”, 230#) who has gained weight during the COVID-19 pandemic shutdown. Getting serious about riding the bike (My typical daily ride is 17miles at an average of 11 mph.) has gotten me back down to my pre-pandemic weight (218#).

Per the cadence meter, I pedal faster than I thought I did; a good thing. My “recreational” cadence is about 60. My “exercise” cadence is 76-86. My “too fast, shift to a higher gear” cadence is anything above 90.

I wanted a cadence meter that was cheaper than my bike. I wanted to avoid dealing with wireless. While my “CATEYE” solution is more expensive than the original poster’s “Schwinn” solution, I’m very happy with the results.

IMPORTANT NOTE: In the picture of the magnet packages, read the warnings on the packages of neodymium magnets. Not toys! Keep them away from kids!

My thanks to the original poster for coming up with the idea and doing the math!

[csiebe]

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.

[DacianM2]

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??

[Phil B]

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-...

[EileenL10]

I think you can consider this item , double wireless cadence speed cycle computer made in China, less than 35USD.

http://jointsport.com/product-detail/2350-cadence-...

[boblangdon]

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

No you can't.
Link????

[KevinK21]

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

[Benapain]

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.

[DanL15]

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.

[balczezzz]

Hi,

I'm doing a bluetooth speed&cadence sensor based on a custom 9+ DOF IMU board. Everything including the code and 3D printing files is open-source and you can check it out here:

https://hackaday.io/project/5286-ble-intertial-mea...

[kgtateddy]

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

[BullN]

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.

[Phil B]

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.

[Benapain]

exactly...

[Yard Sale Dale]

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.

[gdl1228]

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

[BullN]

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.