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energy storing mass

Old 10-12-05, 11:51 PM
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highspeedcycle
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energy storing mass

"You might want to look in to energy storing mass such as a flywheel. Spin the flywheel up, engage the rear dirve and off you go."

Anyone got any ideas on this, i must mod a wal-mart bike with this idea.

Lets work this out


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Old 10-13-05, 08:34 AM
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Riding a 60 lb. bike doesn't appeal to me. I'm sure there are a couple other reasons that it wouldn't work but the details are a little fuzzy and I don't want to break out the physics books. Feel free to tinker if you want, though. Sounds like a fun experiment.
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Old 10-13-05, 08:41 AM
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Originally Posted by Mr. Clean
Riding a 60 lb. bike doesn't appeal to me. I'm sure there are a couple other reasons that it wouldn't work but the details are a little fuzzy and I don't want to break out the physics books. Feel free to tinker if you want, though. Sounds like a fun experiment.
Doesn't have to be a heavy flywheel. Energy stored is directly proportional to the mass of the flywheel but the square of the speed (poorly worded). So a faster lighter flywheel potentially stores more energy than a slower heavy one.
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Old 10-13-05, 08:46 AM
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My physics may be a little rusty but I've always liked the flywheel idea. And I keep imagining stationary cranking (maybe waiting for my riding partners) to spin up the flywheel so I have more energy when that first hill comes along. And I don't think of it as cheating since it was my energy that went into it. And it's clean.
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Old 10-13-05, 08:51 AM
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Didn't someone attempt the hour record with a device like this? It would be great for that kind of flat terrain, constant speed application, but the first time you hit a hill you'd be toast.
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Old 10-13-05, 09:18 AM
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Coiled spring type device??? To help out on starts?
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Old 10-13-05, 09:19 AM
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It would be interesting to see how the gyroscopic effect would work in a real world road ride...

Also, I am no physicist, but the concept of a higher speed and lighter fly wheel only makes sense if you can get bearings etc with very low drag, and since you would probably need a freewheel type of drive to allow the fly wheel to spin even when you aren't pedaling, it doesn't seem likely to work as well in the real world.

And also, I don't know that I would want something spinning at 5,000 rpm in between my legs.... I've seen a dremel cutting wheel come apart.

I would love to see some mechanical device to even out the pedalling energy needed for a ride. I thought before that a nice air pressure drive would be nice. Work it out like the brakes/generators on hybrid cars, when you apply the brakes, a pump pressurizes a tank (or the frame tubes) to supply energy for when you need it later.

I remember seeing a design for an automotive system that used hydraulics in a similar way, and it would probably be more efficient but heavier than air pressure.

Or perhaps take a lesson from the hybrids, and use a small electric motor/generator unit to store the energy in a capacitor or battery.
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Old 10-14-05, 02:09 AM
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I've thought about this often while commuting and hitting stoplights. I always picture the flywheel being used as a brake, then spinning while the bike is stopped, and being engaged again to help start at a green light. If I knew how to work with metal I'd hack something together. I don't care how heavy it would be, this idea is fun!
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Old 10-14-05, 03:27 AM
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The trick is how to get power out of the flywheel. It inevitably have to be some sort of friction-clutch, resulting in some loss through heat...
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Old 10-14-05, 03:30 AM
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Originally Posted by highspeedcycle
"You might want to look in to energy storing mass such as a flywheel. Spin the flywheel up, engage the rear dirve and off you go."

Anyone got any ideas on this, i must mod a wal-mart bike with this idea.

Lets work this out
I can only assume a 'wal-mart bike' is something that is on the cheaper and nastier side than most own on this forum. In which case yeah, there's no harm in trying it out. Not on anything dearer though!
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Old 10-14-05, 05:51 AM
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A wal mart bike has so much potential energy stored up in its mass that the best thing to do is just get on it and RIDE! Talk about mechanical efficency.

I think a heavy bike rolls faster once you get momentum up. this is not physics based, i have no science behind my postulation.

But, I rode a 40 pound schwinn devil sled in a big messenger ride a couple of years ago. When we got to the 'no brakes' downhill the schwinn took off like greased lightning compared to people on 19 pound road bikes and rocketed me to the front of the peloton like an express train late out of the station. There's energy in that mass, you just need to start it rolling!
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Old 10-14-05, 12:11 PM
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The first sentence in your post has more genius than you know. It was theorized by Albert Einstein, proven by Lise Meitner and demonstrated by the United States Army.
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Old 10-17-05, 12:00 AM
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I was going to say something about Schroedinger's cat but couldn't tell if he was alive or dead, spitfire.
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Old 10-17-05, 09:19 AM
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I like the "I must mod" part.

Good luck. The complication of bringing mechanical power into a flywheel, in a modulable fashion, then bringing out that power back to the bike, is not as trivial as it seems. Regenerative braking systems have been studied at lenght by many, mostly for automotive applications. The best system so far is to use electrical energy storage, i.e. electric bikes and hybrid cars. Electricity has the advantage of fairly easy energy transport and storage, when compared to other media such as mechanical (kinetic) or pressure systems.

Hey, you could also do a mechanical system using potential energy storage rather than kinetic: you could bring a heavy mass up on a pole as you brake, and use the descent of this mass to drive the back. I have this vision of a bike with a 200 lb concrete block 10 ft up on a structure. A sure marketing hit.
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Old 10-17-05, 10:24 AM
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When I was a kid I made aircraft models with elastic band power. I think this would be a fairly light weight energy storage method for recovering energy while slowing to a stop and getting back to cruising speed again. It avoids the speed difference problems that you must overcome with a flywheel.
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Old 10-17-05, 10:56 AM
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Originally Posted by dbg
Doesn't have to be a heavy flywheel. Energy stored is directly proportional to the mass of the flywheel but the square of the speed (poorly worded). So a faster lighter flywheel potentially stores more energy than a slower heavy one.
Yes, but it would lose momentum more quickly as energy was bled off into the drivetrain.

If you don't want to pedal, take a bus.
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Old 10-17-05, 12:09 PM
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Originally Posted by AndrewP
When I was a kid I made aircraft models with elastic band power. I think this would be a fairly light weight energy storage method for recovering energy while slowing to a stop and getting back to cruising speed again. It avoids the speed difference problems that you must overcome with a flywheel.
I loved those things. But a couple of years ago my kids brought home an updated version that I was very impressed with. It used compressed air as the elastic storage medium and had a very simple engine that was quite elegant. The cheap little plane fell apart but I rescued the fusilage (air storage) and engine so I could play with it. Neat concept. Maybe we could do something similar. Somebody mentioned compressing air in the bike tubes. (It also reminds me of my favorite bike horn that uses compressed air. And it is VERY loud)
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Old 10-17-05, 02:43 PM
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Flywheels were done to death for use in automobiles several years ago. While they didn't work out for cars they have found a niche in backup power systems.

The Germans used a hand cranked flywheel starting system on the Messerschmitt fighter in WWII. If you ever catch an old film clip of one being started you'll hear the whine of the flywheel building higher and higher until the clutch is disengaged and the engine sputters to life.
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Old 10-17-05, 08:16 PM
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Originally Posted by va_cyclist
Didn't someone attempt the hour record with a device like this? It would be great for that kind of flat terrain, constant speed application, but the first time you hit a hill you'd be toast.
Check out previous threads on this. There was no flywheel device, just that the latest hour record bike had a relatively heavy rear wheel. It makes sense in that once the momentum is built up, the flywheel effect would even out variations in rider output. The rider probably figured that the extra effort to accelerate at the beginning would more than pay for itself later in the ride. Wouldn't want to use it on hills though.
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Old 10-17-05, 09:11 PM
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We studied this in an engineering class (back in the mid 80s) where our professor proudly showed a picture of him popping a wheelie on his bicycle when he engaged the flywheel that had just previously slowed him down to a stop. Anyway, one study we examined for an energy storage system had a bus with a huge flywheel that took up a couple rows of seats in the back. I forgot the details on how it stored energy upon stopping and how it delivered it to the drivetrain, but I do remember it worked quite well. Except for one problem: the momentum created would not allow the bus to turn. At times it could literally turn no faster than maybe to do a slow lane change type maneuver. Thus, it was concluded this strategy was not feasible for almost any type of motor vehicle. Overall, weight was a factor too (have to have a considerable mass). Use of an electric motor/generator/battery system was definitely deemed a better choice.

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Old 10-18-05, 09:22 AM
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Keep in mind that there's no such thing as "free" energy. The idea of storing energy is good, but to the other engineers out there (not that the non-engineers won't get this, but it's not all that intuitive unless you've been through the rigors of several semesters of university level thermodynamics), if you draw an energy "system boundary" that includes time and elevation, you won't get any energy out that you don't put in from either your muscle power, or an external source such as an electric or other fueled motor. If you start your ride at the top of a hill, you could store some of the potential energy to get to the bottom, but if your ending elevation is higher than hour starting elevation, you will have to personally input all the other energy used for the ride, plus the friction losses of the spinning mass, etc. This also assumes you have no "left over" energy at the end, i.e. the flywheel is not spinning when you stop. The mechanical inefficiencies will really add up, as will the weight of the "stuff" needed to make it work.

I think this is a great engineering/inventor sort of pursuit, and could be lot's of fun to tinker with, but in the end, my gut feeling (as a practicing mechanical engineer) is that flywheels for human powered vehicles are not "practical." I'd love it if someone would prove me wrong, but my inclination is to bet against it.
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Old 10-18-05, 10:24 AM
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I'm years out from college physics,

Is a flywheel's force contained in the weight or rotational mass??

If it was mass, I could see a large flywheel made out of ultralight ceramics on Fullerene bushings. A hybrid solar/pedal pod car built around a massive relatively light flywheel.
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Old 10-18-05, 11:04 AM
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The energy is stored in the rotating mass. It's a function of the mass, the distance from the mass to the center of rotation. Think of mass as weight, not as force. It's been too many years for me to remember how exactly the relationship is expressed, in physics terms, but for practical purposes, weight and mass are the same. The accelleration of gravity is in there.

But the distance from the center of rotation is important. Power is force through a distance Have you ever been on one of those old "tilt-a-whirl" rides at a fair? You can make it spin faster if you move into the center. You aren't adding any energy, and yet it spins faster. Better yet - figure skaters. They spin faster when they pull all of their mass close to the center of their rotation. They speed up, not by inputting more energy, but by changing where it is located.

For a bicycle analogy, light rims and tires are far more important than light hubs, as far as rotational energy is concerned. The energy to accellerate the tires and rims, rotationally, is far greater that to accellerate the hub rotation. I believe the power requirements will be a function of the cube of the distance from the center (please, some engineer or physicist correct me if I'm wrong on this. It might be the square, but it is large nonetheless.) An efficient flywheel would balance the mass placement with the diameter, to account for all the practical matters of size limitation, etc.
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Old 10-18-05, 11:14 AM
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http://www.upei.ca/~physics/p261/pro.../flywheel1.htm

As far as I can remember, flywheels were used in some buses and experimental vehicles, but better options (rechargeable batteries) and mainly safety concerns (an relatively large mass spinning at very high speeds) made them a less desirable option.
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Old 10-18-05, 11:26 AM
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Originally Posted by va_cyclist
Didn't someone attempt the hour record with a device like this? It would be great for that kind of flat terrain, constant speed application, but the first time you hit a hill you'd be toast.
The joys of physics. Constant speed = no acceleration = how the hell does a flywheel help?
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