Off The Grid electricity generating spinning bike

The Solutions Explorer lets you create alerts that match your needs. You can create several alerts and you will receive a notification each time a new Solar Impulse Efficient Solutions is labelled and matches your filters.

View Details

Your Search Alerts will show up here.

Sign in to create alerts for your filters and search terms.

Don't have an account?

Fancy exercise bikes will calculate the power in watts used by the bike. Our comfy but unfancy recumbent bike does not. Furthermore, in calculating calorie expenditures it does not take into account the resistance level set on it. For a long time I've been thinking how to do this, and finally found a method to estimate watts used by the bike.

The technique only works for bikes with magnetic braking (eddy current) systems: it doesn't work for bikes with a mechanical brake that actually rubs a flywheel. You can check that your bike has magnetic braking by reading the manual or by taking it apart or maybe by checking how quiet the motion is (mechanical braking is presumably louder). Magnetic braking systems have the resistance proportional to the speed you are pedalling, which is a better simulation of outdoor biking since air friction increases with speed, too.

To a first approximation, if w is rotational speed (rotations per time), then the resistance force on the pedal is

F = kw,

where k is the rotational coefficient of resistance. Over a time T, a pedal travels a distance of 2πrwT, where r is the length of the crank from the center. The work done is then force times distance traveled:

W = F(2πrwT) = 2πkrw2T.

The power used by the bike is then:

W/T = 2πkrw2.

Given the power in watts, taking into account the conversion factor between watts and kcal, and the human efficiency factor of 0.24, the amount of (kilo)calories expended is about 1/ times the watts times the time in seconds, or 3.6 times the watts times the time in hours.

In other words, what we need to know is k, the rotational coefficient of resistance for the chosen resistance level on the bike. If the rotational speed w is measured in revolutions per second, then k equals the resistance force at one revolution per second or 60 RPM. If we could set up a motor to drive the bike at a steady speed, we could measure k easily. But that would be difficult to set up. But driving the bike by a human has inconsistent force. One could also use pedal force sensors, but these are expensive.

It took me a long time to figure out how to measure k cheaply, and finally I found a fairly simple method involving a luggage scale, a camera, inconsistent speed and some simple calculations. Basically the idea is that you rotate the crank by hand via the scale, thereby directly measuring the resistance force. However, because it's hard to rotate the crank at a constant speed, you need to take multiple measurements of the resistance force, and that's where the camera comes in.

Powrloo Product Page

There is a complication to the above. In addition to the speed-dependent resistance from the magnetic system, there is also plain old mechanical friction in the system. This is generally taken to be independent of speed, however, and hence is easier to measure. So the actual model for the force resisting the user is F = kw + f, where f is the mechanical friction. And the power is:

2πrw(kw + f).

Finally, I will do a rough estimate of Peloton equivalent resistances.

You now need to set up the bike and camera in such a way that you can capture the scale value in all the positions. I had a horrible time dealing with glare on the scale's display and the on-and-off backlight on the scale. I modified the scale not to have a backlight (the easiest way is just to open it up and cut the wire to the backlight, or you can add a backlight switch). Additionally, you need to be able to move around the bike without interrupting the camera's view much.

I was willing to compromise and allow some data to be lost due to my arm being in the way and occasional glare.

My setup was to put the bike on its side, and have a large tripod with a camera looking down from above. The legs of the tripod were spaced widely enough not to get in the way of the crank. The camera (a Sony A7R2) was set to manual exposure and manual focus, so as to maximize visibility of the scale display throughout the rotation. I recorded the video at P60.

Another way would be to have two people, with one hand-holding the camera and following the scale.

Of course, the ideal would be to capture the data from the scale directly via a battery-powered microcontroller. But that would be much more complicated, and I went for the simple and, I hope, good enough solution.

You will want a video viewer where you can see either milliseconds or frame numbers, and can go back and forward by either a single frame or a specified period of time. I went with Adobe Premiere Pro. I transcribed the start time and end time (m:s:frame) for each set of rotations, and the number of rotations, as well as the force data from the scale in kilograms of force every second for the higher (and slower) levels and every half-second for the lower (and faster) levels. Where glare made the scale display disappear, I checked some neighboring frames to see if data was visible there.

I interpolated missing data (glare and arm-cover) and then for each level I had an average rotational speed and an average force. (For the highest level, I ended up consolidating data from two runs.)

We will have

k = (F-f)/w,

where F is the force resisting motion, f is the mechanical friction, and w is revolutions per unit time. To calculate this, take the average kilograms from the scale, multiply by 9.8, subtract f, and divide by the revolutions per second. The resulting units are Newton seconds. The results ranged from 28 for level 1 and 197 for level 8. The variation in k between levels was quite linear (I just used the graphpad website to graph it) with an r-squared value of 0.99, which makes me pretty confident the data isn't random.

I used a fairly simple python script that takes data in a file called data.txt. The data there includes level numbers, "start" entries with m:s:frame times, "end" markers with the number of rotations and the end time, and in between kilograms of force from the scale sampled every second or half-second, with an "x" marking missing data. The script interpolates missing data, and calculates average rotational speed, average force in Newtons, and our value k for each level, in our Newton-seconds, as well as some sample power data and Peloton equivalents.

Are you interested in learning more about energy generating exercise bike? Contact us today to secure an expert consultation!