I am designing a bicycle that can generate electricity for a science project at school.

My design incorporates a flywheel. You see, flywheels are used in cars. In essence, these are regular wheels, just heavier. This added weight adds inertia to the wheel, thus keeping in going even if no energy is being inputted. In terms of cars, the alternator is powered this way.

I am replacing the back wheel of my bike with a flywheel, which I can thus use to potentially generate electricity. The more weight I add, the more inertia the wheel has and the more electricity can be created over a period of time.

Based on my calculations, if I choose an appropriate wheel I can get to very high RPM's using pulleys, sprockets, gearboxes, and other engineering mechanical ideas. The bicycle will be stationary, so I won't be going anywhere. It will be designed in such a way that I can pedal and the wheels will move, but I won't go anywhere. Exactly what I need.

One tradeoff however. I cannot make the wheel so heavy that I cannot pedal efficiently. I need the wheel to be the heaviest it can be without it being so heavy that I cannot push it and get it going.

I am a relatively strong, fit, male sixteen year old. I participate in many sports, such as football and basketball and soccer. I am 5'11" and I run everyday.

What weight should the wheel be?

  • 1
    I'd suggest you do a bit of googling -- there are a lot of bicycle based generator designs. The point of a flywheel in a bicycle based generator would be to smooth out the power delivery of a human. – Batman Jul 22 '17 at 19:37
  • Pulleys, sprockets, gearboxes? Why not just use a normal bicycle drive train. More weight is not more electricity. Electricity will be the work your legs deliver. – paparazzo Jul 22 '17 at 22:23
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    Weight has no effect on the rotational friction of a disk, other than it's impact on bearings. Consider that the rear bearings of a bike wheel are designed to carry about 150 pounds of weight with no significant friction. Make the wheel as heavy as you can reasonably manage. (Note: Place the weight as close as possible to the rim. Weight near the hub does you little good.) – Daniel R Hicks Jul 22 '17 at 23:35
  • @paparazzi with a typical bike setup the rear wheel isn't going to turn much faster than 500rpm. A generator needs more. Many bike generator projects take a belt drive off the back wheel to do that. – Chris H Jul 23 '17 at 10:47
  • It depends whether you have a single speed or can accelerate the wheel more gently working through some gears- a high enough combination of weight and gearing will make the initial pedal strokes very hard (maybe even impossible depending on static friction in the generator/flywheel) – Chris H Jul 23 '17 at 10:50

First, watch this video:

2nd, ditch the flywheel.

An automotive alternator isn't "powered by a flywheel", it's connected to the peripheral belt, and spins at the same RPM as the engine, and is quite content with variable engine speeds. The purpose of the engine's flywheel is because for an ICE, the power generation phase is a very small intermittent portion of the total cycle, and so the flywheel smooths that out a bit.

In cycling, your legs can provide a more constant power input. There is no benefit to the flywheel as that is just additional mass the rider will need to accelerate. When it comes to energy, there is no free lunch and any energy in the flywheel has to be put there by the rider. Unless you are able to efficiently extract energy from the coasting flywheel (you won't), it will present a net loss of conversion efficiency.

Bicycling, and human beings in general, are a terrible source of energy. Most people without significant training will struggle to even momentarily provide 100W, fewer still will be able to sustain it for more than 5 or 10 seconds.

  • +1 but you're being a little pessimistic about power outputs. Most people who regularly ride/run should be able to sustain 100W for long periods, 200W for a couple of minutes at least. A quick check shows that just the work done against gravity on a recent hill-climb equates to 250W for 6m, excluding drag etc. and I'm only a casual cyclist doing ~100km most weeks, occasionally twice that. (I was going slowly enough that drag and rolling resistance only add ~10% to this). – Chris H Jul 24 '17 at 8:40

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