Bicycle Flywheel Stores A Bit Of Energy, Not Much

Kinetic energy recovery systems have often been proposed as a useful way to improve the efficiency of on-road vehicles, and even used to great effect in motorsports for added performance. [Tom Stanton] decided to build one of his own, outfitting a simple bicycle with a flywheel system for harvesting energy. (Video, embedded below.)

The system consists of a 300 mm steel flywheel mounted in the center of the bike’s frame. It’s connected to the rear wheel via a chain and a clutch which [Tom] assembled himself using bicycle disc brake components. The clutch is controlled by a handlebar lever, allowing the rider to slow the bike by charging the flywheel, or to charge the flywheel to maximum speed by pedalling hard with the clutch engaged.

The actual utility of the flywheel is minimal; [Tom] notes that even at its peak speed of 2200 RPM, the flywheel stores a small fraction of the energy content of a AA battery. Practical demonstration shows the flywheel is only able to deliver a small push to [Tom] when riding the bike, too.

Despite the lack of performance, it’s nonetheless an interesting project and one that demonstrates the basic principles of flywheel energy storage. The underwhelming results perhaps serve as a solid indication of why it’s not something we use particularly often, on bicycles at least. We’ve seen [Tom]’s bike experiments before, too. Video after the break.

49 thoughts on “Bicycle Flywheel Stores A Bit Of Energy, Not Much

  1. Cool experiment.. The large flywheels on the rear of trucks and manual transmission vehicles, store energy in the same fashion. Spinning a flywheel in an effort to power a device, has been a frustrating thing. Things will explode at high RPM, and low RPM does not seem to do the job. Nice video..

    1. Although they do store energy, the flywheel on motors are more about stability as the energy is used and replenished multiple times every revolution. The inertia of the flywheel mass dampens the pulsating combustion strokes and helps drive the crankshaft through compression strokes.

      1. What flywheel? Only manual trans vehicles use those, which probably weigh a quarter of the reciprocating mass of an engine, flexplates are even lighter.

        Unless you are talking something like a big single cylinder low RPM setup.

    2. I’ve thought about this over the years, great to see someone actually do it. The stored energy is proportional to the radius and mass of the flywheel and the square of the angular velocity so a faster flywheel might help. How about a motor/generator connection instead of a purely mechanical system? I live on a hill and would love to have one of these.

  2. Well if you ride a bike in the city you know you have to stop at red lights (or are supposed too), and when you do, if the bike is properly “fitted” (?) you have to get off the seat and put your foot down. When the light changes you have to jump back onto the seat, start peddling, and its a bit awkward. With this flywheel you could store your energy from breaking, put both feet down when you stop for better balance, engage the flywheel when the light turns green and have just enough of a push for an elegant take off. Somebody might actually want this thing.

      1. No need for a flywheel for that, batteries are already energy storage devices. The problem is there isn’t that much extra energy to be had – most of the energy used by a vehicle is expended in fighting drag and friction while moving and is unrecoverable. Kinetic energy recovery systems really just improve the efficiency of braking – transferring the energy from momentum to storage so it can be used to reduce the energy needed to accelerate again immediately afterwards. But the whole exercise still needs net energy input.

        1. In 1992 I went biking in the south of Belgium. I had just covered a flat area when I ended up on a stretch where there were hills. Going up the first hill was difficult, but after that riding down I traded potential energy for kinetic and then that kinetic energy carried me up the next hill. Great fun!

          That is not precisely how it went. When going down the hill, you quickly hit 30-35 km/h (20mph) and then don’t go much faster. This carries you up say 5m of potential energy on the next hill.

          If the hill has a height-difference of 100m, then you should be going 160km/h (100mph) at the bottom to recover that 100m of height on the next hill.

    1. The flywheel could be spun fast enough to create a gyroscope effect, and you could even just have both feet on the pedals and not fall over!
      Cornering with a spinning flywheel would be a bit of a challenge though…

  3. On the one hand, cute, fun, interesting. But…. 2200 rpm spinning flywheel at a dead stop. I don’t suppose he was interested in TURNING that thing, was he? I’d guess that may be enough gyroscopic force to hold the bike upright. Turning it would require some force.

    I once saw a Popular Science article that had this idea of putting big flywheels in the back of busses. The picture had the flywheel mounted basically flat against the back of the bus, upright. “Yeah, okay, that’s great if the bus only goes in one direction forever.”

    1. Useless info:

      Pratt & Whitney PW6124 (bypass 5:1) does something around 6000 U/min (N1: fan) and 18.000 U/min (N2: core). Larger ones (bypass more than 10:1) do a little less for the fan and more for the core.

      With a fan diameter of 2 m (80 foots) this is a nice flywheel.

    2. So, your comment led me to wiki, which I now bring back to you:

      This effect (resisting turns) can be counteracted by using two coaxial contra-rotating flywheels.

      The gyrobus was actually manufactured and put into service in a handful of places in the 1950s.

    3. It’ll do a bit. Not a lot.

      5 kg uniform disk 15 cm in radius. Moment of inertia of the disk is ~0.06 kgm^2.

      Moment of inertia of the actual bicycle wheels themselves is usually 2-3 times that, but the flywheel’s likely spinning 10-20 times higher. So the total gyroscopic angular momentum goes up by a factor of maybe 2-4.

      Now *turning*? Don’t think so. That angular momentum’s still small compared to the angular momentum of the bike in the turn itself. Think about it – if you try to round, say, a 10-foot radius corner, *your* moment of inertia is probably around *five thousand times* as big as the disk. Yeah, the disk is going faster than you, but not *that* fast – you’d likely be rounding a corner at a rotation rate of ~5-10 rpm. Your angular momentum change is still like, 10 times bigger than the disk’s.

    4. Bicycles and motorcycles already have to deal with the forces associated with spinning mass, because their wheels have considerable moments of inertia. When we learn to ride a bike, without knowing it, we learn that you have to push forward on the right handlebar (that is, turn the wheel to the left) in order for the bike to lean right and turn right. If you don’t believe this, just try it. You will be amazed at what your muscles already knew. The effect is much greater on a motorcycle, and motorcycle instructors say, “push right, turn right”. An added flywheel is no different from using a heavier rear wheel.

  4. Years ago I saw an application actually using flywheel energy storage. I was touring ABC network in New York and they were using several for their emergency power system. The main system used diesel generators to power the building equipment but these take time to start so flywheels were used to generate power while the generators started up. I have no idea if this system is still used. Anyone know what f this still is the case?

    1. That would make sense but the only use I’ve seen of them was at JET, the fusion research facility. Wikipedia says “Because power draw from the main grid is limited to 575 MW, two large flywheel generators were constructed to provide this necessary power.[43] Each 775-ton flywheel can spin up to 225 rpm and store 3.75 GJ,[44] roughly the same amount of kinetic energy as a train weighing 5,000 tons traveling at 140 kilometres per hour (87 mph). Each flywheel uses 8.8 MW to spin up and can generate 400 MW (briefly).”

    2. The same happens in the emergency equipement for the Gaasperdammertunnel, a highway tunnel in Amsterdam. The flywheels are always kept spinning and also serve as the “starter” for the diesel engines. They are coupled to an electromotor/generator and are always kept up to speed.

  5. Using mechanical connections to a flywheel just never works. The clutch, while necessary to match the changing speeds, dissipates most of the power, and on starting, you can never get up to the speed that the wheels were at when you disengaged the clutch when stopping. A brushless DC motor on the flywheel, and a brushless hub motor on the bike would be much more efficient at transferring energy, and use of switching boost and buck converters allow the transformation from speed in one to torque in the other to be varied as the bike decelerates and then accelerates. That is, you can electrically make the continuously variable transmission that [Tom] points out is necessary.

  6. One facility that I’m in a lot has this kind of setup – huge diesel generators for long-term backup, and a bank of flywheels for “ride through” during the minute or so that it takes for the big diesels get spun up.

    They inherited the system when they bought the building. It used to belong to a semiconductor company and whatever those guys were doing apparently could not survive even the smallest power interruption.

    For the current owners it’s total overkill, but it did come in handy last year when we had regional power outages from wildfires and they just kept on working.

    The bank of flywheels is in a row of otherwise nondescript cabinets in the back of their electrical machine room. There is no external clue about what’s going on inside, no sound except for the soft 60hz electrical buzz and omnipresent fan noise you always find in these kind of spaces, it’s weird to think that there’s a steel and carbon fiber cylinder spinning at tens of thousands of RPM and hundreds of G’s inside the boxes.

  7. I’ll throw this out there, hybrid human-electric drive. Make it a trike because you’re pedaling all the time at a good aerobic pace even at a stoplight. Continuous excersize in a given time. Generator and motor with a lithium or supercap storage. Motor regenerates for braking. The efficiency of things is getting better and better, there still losses but would it work overall? Most of the time on a bike you are not doing much work. So even with some loss you’ll come out even and don’t have to strain at times. Plug-in option extends things.

  8. I knew a guy back in the 90’s who concreted the rear wheel on his bike using the spokes as reo.
    Starting and stopping and turning was hard work, but once you were going you could freewheel forever.
    He said the weirdest part was hitting slippery patches and having the wheel accelerate without pedalling.
    Unsurpisingly the forces involved quickly wrecked the bike frame.

  9. I think it was back in the 70’s there was an article in PopSci or Popular Mechanics magazine about an experimental car with a flywheel in a vacuum chamber. It was combined with a small ICE to give greater mileage.

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