Instrument Packed Pedal Keeps Track of Cyclist’s Power

Exactly how much work is required to pedal a bike? There are plenty of ways to measure the power generated by a cyclist, but a lot of them such as heavily instrumented bottom brackets and crank arms, can be far too expensive for casual use. But for $30 in parts you can build this power-measuring bike pedal. and find out just how hard you’re stoking.

Of course it’s not just the parts but knowing what to do with them, and [rabbitcreek] has put a lot of thought and engineering into this power pedal. The main business of measuring the force applied to the crank falls to a pair of micro load cells connected in parallel. A Wemos, an HX711 load-cell amp, a small LiPo pack and charging module, a Qi wireless charger, a Hall sensor, a ruggedized power switch, and some Neopixels round out the BOM. Everything is carefully stuffed into very little space in a modified mountain bike pedal and potted in epoxy for all-weather use. The Hall sensor keeps tracks of the RPMs while the strain gauges measure the force applied to the pedal, and the numbers from a ride can be downloaded later.

We recall a similar effort using a crank studded with strain gauges. But this one is impressive because everything fits in a tidy package. And the diamond plate is a nice touch.

13 thoughts on “Instrument Packed Pedal Keeps Track of Cyclist’s Power

  1. Not bad! I’d be curious to see how these compare to a ‘real’ power meter in terms of accuracy. It’s not the easiest field; bicycles are pretty brutal environments.

    You do need two of them, though. One isn’t enough, as (particularly when you’re tired) the other leg can be fighting the working leg a bit. This won’t measure the (quite small) other components of the pedal stroke (before anyone says it: NO, the crap about your leg pulling up on the backstroke with clipless pedals is an old wive’s tail that has been widely disproven in formal and informal studies.)

    I’m a little dubious of the longevity – MTB is really tough on components and the shock loading of those load cells is going to damage them pretty quickly, I’d think.

    Proper power meters can be expensive, but a little bit of ebay hunting, and you can find pretty cheap used hub-based meters that aren’t very expensive at all (and will work for hundreds of hours on a battery, survive rain/snow/dirt, and get within 2%.)

    1. As for power, you’re absolutely right, I’d be skeptical of the accuracy of the readings. As for needing two, not so much. A good number of pros are riding one sided power meters, Stages namely. Most studies I’ve seen show that that’s more than sufficient for 99.9% of riders. Very few riders have a individual leg power output difference of more than about 2%, which falls in a good power meters confidence interval anyhow.

    1. 200W max what? FTP? CP? 200W for FTP or Critical Power for a competitive amateur cyclist is downright pedestrian, depending on their weight.

      Category 1-2 riders? They can push several times that for minutes on end.

      1. You’re right, somewhere it was calculated that I could put out about 600W, time to do a recalculation now I have the following:
        6ft2,
        79Kgs (12.4st) without a bag of stuff,
        88Kgs (13.7st) with a bag containing a laptop, charger and a few emergency bike tools,

        On the flats I can go 20Mph (Almost 30Mph peak) continuous for about 20 minutes with a struggle to keep that where I oscillate between 15Mph and 20Mph with the average dropping over the next 30mins down to about 10-12Mph average.

        Up hill I can achieve a little over 20Mph for around 5 Minutes before causing exhaustion.

        Down hill is a nice 25Mph and a good >35Mph peak speed

        The limiting factor is how fast my cardiac-circulatory system can pump freshened blood around. I’ve had irregular heartbeats and other issues due to exhaustion and electrolyte depletion (Eating high potassium and sodium content actually helps my neuro-cardiac system avoid glitching out under high stress).

        Just for scale:
        My brother has a 1Kw hub-motor bicycle. I can keep up on the downhill easily. He loses me un the uphill and the flats is a balance between keeping up and keeping alive (Cardiac limit).

    2. I’m an amateur racer and I can easily hold 200w for a few hours. Chris Froome, for example, can hold 450ish watts for about a half hour and he’s about 50 pounds lighter than me.

  2. Blue lights do not belong on a car or bike unless it’s a police cruiser or volunteer emergency.
    The load cells only measure down force. Without clips I pedal with both forward and back force (kick and run) as well as push down. Shoes have the friction otherwise we couldn’t run. Also lifting the rising foot will give even more force instead of lifting it’s dead weight with the other foot. If there is a way of measuring chain tension, that would be comprehensive.
    If you’re into training these tricks are gold.

    1. “Blue lights do not belong on a car or bike unless it’s a police cruiser or volunteer emergency.”

      Yeah, I’m pretty sure nobody is going to give a damn, given you probably can barely see them in daylight. Calm down.

      “Without clips I pedal with both forward and back force (kick and run) as well as push down.”

      Not to any significant degree, you don’t, no.

      “Also lifting the rising foot will give even more force instead of lifting it’s dead weight with the other foot. If you’re into training these tricks are gold.”

      Nope, they’re not, actually. They’re old, commonly repeated myths that have long since been disproven thanks to improved instrumentation on erg bikes used by reseachers…that show that *overwhelmingly* the pedal stroke is dominated by the largest muscle groups in the legs driving downward.

  3. That is a very interesting project. It has some limitations but provides a hell of a lot more information about the power he is producing than I currently have.

    If the blue lights are upsetting I guess you’ll go burko when you actually read the article and find out they change color and eventually go red when when he puts the boot in so to speak.

    For measuring absolute power just measuring the downward force on the peddle has its limitations but when your in the field an indicative measurement that is adequate for most scenarios.

    It would be interesting to put the bike with this pedal gauge on a dyno and compare the results and correlate the results.

    If there was a gauge on each pedal it would be possible to take into account lifting action as well. Something like sqrt ((pedal A – pedal B )^2) should take into account the effect of downward pressure on the up stroke – or lifting for that matter. ( don’t trust my maths )

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