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Is it possible to convert the energy you would normally lose when braking your bicycle to rotational energy by attaching an extra wheel which would start to rotate once you use your brakes? E.g., when facing a red light after riding down a hill.

If you could restore that energy, you could regain part of your velocity without as much effort after stopping at the light.

I am quite sure I'm not the first one who had this idea (as I heard about some projects trying to achieve the same thing but then with cars) so I wonder what the technical difficulties are and why I have not seen bicycles like this on the road?

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    $\begingroup$ See blogs.scientificamerican.com/observations/2011/06/24/… $\endgroup$ – innisfree Apr 19 '15 at 20:24
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    $\begingroup$ There's a pithy criticism: "Adding a flywheel to something which has as its main design goal that of being as lightweight as possible sounds weird." $\endgroup$ – innisfree Apr 19 '15 at 20:28
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In Europe hybrid electric bikes are becoming quite popular - you pedal and a small battery + motor gives you an extra push. Such bikes include regenerative braking: when you brake gently the motor operates "in reverse" and charges the battery back up. Given that all the hardware is already there it is not a big additional engineering challenge. It is especially useful in hilly cities - you regain gravitational energy. Your flywheel is a lot of complexity : and the speed matching is particularly challenging since you need to be able to accelerate the flywheel regardless of its speed and the speed of the bike. This requires a continuously variable gearing mechanism.

Electrical storage is just much simpler.

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  • $\begingroup$ The speed matching doesn't necessarily need a transmission. A clutch might suffice depending on the other parameters. All-in-all electrical energy storage is still probably easier. $\endgroup$ – Chris Mueller Apr 20 '15 at 15:33
  • $\begingroup$ @ChrisMueller - a clutch is mechanically easy but energetically terribly wasteful. And it can't work in both directions - if you are trying to add energy to the flywheel, the driving force must be faster than the flywheel; when you want to extract energy from the flywheel, the load has to be moving slower. You need at least some gearing to allow for both those cases. $\endgroup$ – Floris Apr 20 '15 at 15:36
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I'm not a bicycle mechanic or expert, but I would say the main drawbacks of a flywheel system are

  1. Complication of maintenance - the more moving parts, the more often your bike will break down, and the harder it is to repair.

  2. Price - more moving parts means more expense. You need a whole extra gear system to run a flywheel!

  3. Weight/danger - flywheels are designed to store rotational kinetic energy ($KE_{rot} = \frac 1 2 I \omega^2$, where $I \propto m$), and to store a lot of rotational kinetic energy, you either need high rotational speeds, high mass, or both. High rotational speeds mean a lot of wear and tear, or more danger to the user (legs, straps, shoes, etc getting caught in a high RPM system). High mass is annoying for bicyclists trying to lug an extra 15 pounds up a hill. Either way, it's an issue.

  4. Counterproductive - A lot of bicyclists use their bikes to exercise, so it's a bit counterproductive to have an extra system designed to give them less exercise.

Now, that's not to say bicycle flywheels aren't neat - they sure are, and in many ways they're a great idea. But you asked about the drawbacks.

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Another difficulty would be the gyroscopic effect - given the speed of the flywheel, the forces would be enormous, coming down that Alpine pass, you'd probably flip over at the first hairpin bend (switchback in the USA).

Could be solved with dual contra-rotating flywheels, but that just adds more complexity on an already too complex solution. The hybrid electric bike is probably a simpler solution, as well as allowing other ways of charging.

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    $\begingroup$ A byproduct of this scheme could be that it would be easier to keep the bicycle upright when standing on an intersection. If gyroscopic effect adversely affects maneuverability, then the flywheel could spin opposite to road wheels. $\endgroup$ – Nick Alexeev Apr 21 '15 at 3:07
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There are a few fundamental difficulties.

The energy stored in a flywheel is a combination of its moment of inertia and its speed. To get a higher moment of inertia you either need more mass or a larger diameter. The problem being that bicycles tend to be very sensitive to both weight and space. This leaves you with speed, firstly this will require quite a high gear ratio to get from the relatively low speeds of a bicycle wheel to the high speeds required for an effective flywheel as well as a mechanism to engage and disengage it smoothly and transfer between extracting and imputing energy, this adds weight and complexity.

Also a flywheel with a useful amount of energy will have a string gyroscopic effect which is a particular issue on a bicycle unless you have an even more complex system to mitigate it.

Equally high speed flywheels pose materials challenges for bearings and the flywheel itself which is trying to explosively pull itself apart and obviously even a light, low speed flywheel is going to need to be fully enclosed for safety.

The short answer is that with current technology an electrical system with batteries is always going to be more attractive, the technology is mature and well developed and for anything like reasonable cost is going to provide much better energy density and is much easier to package on a bicycle as well as allowing for a much greater degree of control. For example a digitally controlled electrical system can have many different modes of operation without any additional mechanical parts.

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