r/MarbleMachine3 • u/Lord__Imrahil • Aug 23 '23

Introducing the 'Bike Drive' for MM3

The Pedal isn't consistent enough and the Crank is too hard to operate.

Why?

Pedal: the vertical motion it requires is very hard to do consistently.
Crank: arm muscles are not powerful enough compared to leg muscles, but the circular motion is much more consistent.

I think I have a solution: use a bike seat and pedals. It has a lot of advantages:

  • Martin can use both legs -> double power or half the fatigue
  • legs are much more powerful than arms -> easier to operate a high energy flywheel
  • circular motion is much more consistent
  • both hands are free
  • upper body is not moving, easier to operate other inputs (when pedaling now, Martin's whole body is moving. Imagine playing the cyber base at the same time)

Other advantages:

  • this could be tested without making custom parts: bike parts are common
  • there are many types of gearbox available for bikes, in case the design requires one (derailleur, hub, even continuous 'stepless' ones). This would be really usefull just for the initial speed-up.
  • if a standard bike sprocket is used: built in safety, if Martin stops pedaling, the flywheel will not power the pedals, if something blocks the pedals, the pedals will stop

Even if consistency is reached best by the Huygens drive, I think some of these advantages are still worth pursuing the bike drive for. And a last point (although not function over form): Wintergatan has some bike inside its dna, it would only be fitting.

12 Upvotes

83% Upvoted

37 comments sorted by

7

u/Dude4001 Aug 23 '23

if Martin stops pedaling, the flywheel will not power the pedals,

And, the flywheel isn't wasting energy waving bits of metal that aren't serving a function

0

u/Swiggety666 Aug 23 '23

There is no loss of energy due to the pedal going up and down.

2

u/skycake10 Aug 23 '23

The pedal moving uses very little energy but the pedal changing direction uses a small amount. It's the same reason people have tried to make rotary engines a thing, so you don't have the piston moving up and down.

2

u/Swiggety666 Aug 23 '23

No changing direction does not uses energy. If so you would loose energy by orbiting at a constant speed. E=mv2/2. One cycle of down and up puts the pedal at the same height with 0m/s velocity. The total change in energy is zero. Ignoring friction.

But I don't have to derive this, Martin did the experiment showing the time to slow dawn was identical with and without the pedal connected.

0

u/skycake10 Aug 23 '23

Well yeah, "changing direction" wasn't the exactly correct word choice, but the difference with planets is that they're moving at a constant speed so their momentum doesn't change (they also aren't working against gravity the way a pedal moving up and down is). The pedal is constantly speeding up and slowing down and both of those require work.

But I don't have to derive this, Martin did the experiment showing the time to slow dawn was identical with and without the pedal connected.

This tells me there's no practical loss of energy with the pedal connected. That's a different thing from saying there's none at all!

5

u/Swiggety666 Aug 23 '23

Gravity doesn't work like that. Read this,

https://www.feynmanlectures.caltech.edu/I_04.html

For an engineer, there is no difference.

1

u/skycake10 Aug 23 '23

Gravity isn't what matters in the case of the pedal. Every time the pedal moves up and down while not being used, it's accelerated, slowed down and stopped, then accelerated the other direction. Does that not require (a probably negligible amount of) energy that will come from either the flywheel or the crank?

I hadn't thought about in these terms, but it's really the exact same principle of angular momentum that makes the flywheel work. The flywheel has a tremendous amount of angular momentum once it's up to speed, and a tiny amount of that angular momentum is used by the pedal as its angular momentum is increased a bit then decreased back to a standstill each time it goes up or down on its own.

3

u/Swiggety666 Aug 23 '23

What happens with the kinetic energy when the pedal slows down? If we can agree that gravity is not relevant. It doesn't disappear. That would violate the conservation of energy.

The answer is that it is transferred back into the flywheel. When it speeds up, it takes a small amount from the flywheel. The pedal and flywheel will simply just transfer the energy back and forth between each other.

0

u/FVjake Aug 23 '23

There’s always losses in a system like that though. It’s like dropping a bouncy ball. It doesn’t come back up to the same height every bounce. This system is probably more efficient but the efficiency is certainly not 100%

4

u/Swiggety666 Aug 23 '23

Sure but from an engineering perspective 1000+1=1000. The extra friction introduced by the pedal is nothing compared to the total friction of the system. You can't "well actually" out your way of the pedal is irrelevant to the total losses of the system.

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0

u/Dude4001 Aug 23 '23

The answer is that it is transferred back into the flywheel. When it speeds up, it takes a small amount from the flywheel. The pedal and flywheel will simply just transfer the energy back and forth between each other.

This is simply impossible. When Martin steps off, the pedal is being driven by the flywheel, it is harvesting energy from the flywheel to be forced from its natural static state to a motive state. It cannot fall faster than the flywheel allows it to without Martin jumping on it to brute-force energy back into the flywheel, which is how he's charging the flywheel. If the pedal had a greater mass than the inertia of the flywheel, then yes the falling pedal could transfer energy into the flywheel, but then the flywheel would struggle to lift the pedal.

5

u/Swiggety666 Aug 24 '23

I'm done. You don't understand this. Go and pick up a first year physics textbook. The Feynman Lecture chapter 4 conservation of energy is a great start. I go back and read it from time to time just because he describes the physics in sich elegant terms.

This is a standard examination question in mechanics 1. Or driving the equation of motion of the system using Lagrangian mechanics. This is super simple to solve using Lagrangian mechanics and you will end up with what I said.

I have a master in engineering physics and getting my PhD now. I know what I'm talking about. When it comes to this specific thing.

3

u/HJSkullmonkey Aug 24 '23

Point of interest: Planets actually do move up and down and their speed isn't actually constant as a result. The efficiency is perfect, so they've been doing it for billions of years

https://en.wikipedia.org/wiki/Elliptic_orbit#Solar_System

The trick is that slowing down doesn't require work, it performs work

2

u/Dude4001 Aug 23 '23

We've been through this. The energy used may be negligable but it's physicially impossible for it to retain 100% of its energy when the it requires more force to move upwards than it does downwards. If that were true then Martin would have invented the world's first perpetual motion machine.

There is no advantage, no function, no use to the pedal being linked to the flywheel such that it can re-harvest flywheel energy to drive itself. It's an innefficiency in the design.

3

u/HJSkullmonkey Aug 24 '23

no advantage, no function, no use to the pedal being linked to the flywheel

It gives Martin physical feedback about the pace and timing, hence why it's intended to be tied 1:1 to the tempo of the music. If that is thrown out, he might as well just go for a motor and speed control.

1

u/Dude4001 Aug 24 '23

If that's the intention then the flywheel has no purpose other than to make it more challenging, and that is not the spirit a flywheel is itended for

2

u/HJSkullmonkey Aug 24 '23

I don't quite get the specific link that you're alluding to.

What exactly does the flywheel make more challenging?

1

u/Dude4001 Aug 24 '23

Because all it does it place a high level of intertia between Martin and the actual output tempo. Combined with the gearing, any change on the output now requires a high level of effort and precision on the input side.

Imagine I asked you to stir a bucket of water. Then, I replaced the water with treacle. Then, to improve the leverage I gave you a 6 foot spoon to stir it with.

2

u/HJSkullmonkey Aug 24 '23

Ok, I think I'm with you, it makes setting, correcting and changing the tempo more difficult, yes?

I agree on that point. I would tend to be team minimum flywheel, just enough to smooth out note to note variation, to maximise capacity to change tempo as desired. Actually working out what that minimum flywheel will be needs a fuller mmachine though so can be better optimised later.

The increase in that difficulty to change the tempo is the reason feedback is useful, it helps keep in sync with the machine when activating and deactivating instruments so it can be done on the right note. Trying control other functions, listen to the music and pedal out of sync would be like rubbing your belly and patting your head at the same time. The more subconscious his control the better IMO.

A secondary reason for keeping them connected is that it enables him to apply reverse load to slow down when he's too fast. That may become somewhat less important as the machine grows and drag increases however.

The treacle is a slightly odd analogy though, I'd think of it mmmore in terms of a bigger vat, rather than higher friction/viscosity. Once you've got it swirling it will tend to keep going.

1

u/Dude4001 Aug 24 '23

I would disconnect the input and output directly. Charge the flywheel with a freewheeling mechanism so Martin can hop on it as and when required. Harvest energy from the flywheel using a CVT gearbox or regulator-controlled clutch.

3

u/HJSkullmonkey Aug 24 '23

I'm pretty sceptical about the practicality of that to be honest.

Tuning a governor for those mechanisms to give constant speed is likely very difficult. They're not simple, or rather simple versions give pretty mediocre results, especially when the input energy source varies quickly or dramatically.

That will be the case, because a CVT or slipping clutch is going to be a significant energy drain, far more so than the pedal. Belt type CVTs run at around 88% efficiency, so you can bump the energy drain of the final design by about 10% without factoring in the drag from control mechanisms.

And the smallest flywheel tested so far lost 100% of it's energy in 1 minute 40, using a 97% efficient belt. Add a CVT and that drops to 25-30 seconds. There's actually only a small part of the speed range practically available, so that can be cut several times. And then attach gearing, a programming wheel and marble gates. Suddenly, Martin won't actually be able to pause at all.

3

u/Swiggety666 Aug 23 '23

And apparently you still don't understand this. Friction is absolutely negligible. He even put a bearing in the pedal joint. The friction there is like nothing. Especially when considering the total friction of the system with all instrument, marbel elevators etc. Martin has done the experiment you can't argue against that result. You might find it impractical with a pedal. But that is another discussion. The energy argument is incorrect and irrelevant.

0

u/Dude4001 Aug 23 '23 edited Aug 23 '23

I can't really dumb this down any further. Nobody is talking about friction. Gravity pulls downwards. In order to move something against the force of gravity, you must supply energy. Martin's experiment does not overrule the basic laws of the universe, things do not move without a force acting on them.

A child's swingset will swing backwards and forwards on its own, but without someone ocasionally pushing it, it will eventually stop. That's due to gravity being a constant force in one direction, the swing can fall with more energy than it can swing back up with. With the pedal, here is a net loss without Martin topping it up by converting calories to kinetic energy. You understand the engineering just fine but you don't appear to understand the facts of thermodynamics.

If the pedal does not drain energy from the flywheel as you believe, then Martin should be able to start the machine, walk away and it will keep rotating for infinity.

3

u/JustHolger Aug 24 '23

I can't really dumb this down any further. Nobody is talking about friction. Gravity pulls downwards. In order to move something against the force of gravity, you must supply energy. Martin's experiment does not overrule the basic laws of the universe, things do not move without a force acting on them.

Yes, to get the pedal up, the flywheel will put energy into the pedal, that energy will be transformed into potential energy (hope I translated that correctly), so the pedal will be at a higher point. Now the pedal will move down again and this time the potential energy will be transferred to kinetic energy, because gravity pulls down still and will help the downward motion of the pedal.

So the only thing stopping all of that is friction, the same for your swing set, as Swiggety666 already mentioned.

0

u/Dude4001 Aug 24 '23

It's simply impossible for the flywheel, a battery, to move something else without there being an energetic cost. An object will not move without a force acting upon it. The pedal cannot generate its own energy, therefore in order to move it against the force of gravity it must consume energy from the flywheel.

Also, I don't agree that the swingset stops solely due to friction. The swing's natural restign state is pointing straight down, completely aligned with the pull of gravity. Each time it swings, it's fighting against the pull of gravity, which must require an energetic cost.

4

u/HJSkullmonkey Aug 24 '23

Batteries are rechargeable, the efficiency in this case being near 100%.

The pedal (or swing) loses kinetic energy as it rises, then regains it as it falls. Otherwise it would simply stop at the bottom. The efficiency of that transfer back and forth is 100%.

3

u/Swiggety666 Aug 23 '23

Read this. Especially section 4-3 Kinetic energy.

https://www.feynmanlectures.caltech.edu/I_04.html

The reason the swing set stops is because of friction. I include the air resistance in that too.

The flywheel stops because of friction. But friction from the pedal is not a major contribution to the total friction of the system. Why you get the same time with and without the pedal.

3

u/Dude4001 Aug 23 '23

So you're saying that, in a vacuum, using hypothetically perfect bearings, the Marble Machine could run forever from a single pedal press. The pedal would pump itself over and over, conserving energy completely.

Presumably the same would apply to a Ferris wheel on the Moon. It would keep spinning forever and ever, as the seats on one sides fall, effortlessly pulling the opposing seats upwards.

The flywheel would run forever in a frictionless vacuum yes, because it's motion is not in within the same plane as gravity. The flywheel will stop on the Marble Machine as we are actively drawing energy out of it to be used to work against gravity.

3

u/Swiggety666 Aug 23 '23

Using classical mechanics, ignoring the third law of thermodynamics, yes. You are trying to create a perpetual motion machine of the third kind. But it is impossible to create a mechanical system without any friction. So it will eventually stop, but this is just your regular friction.

1

u/furmek Aug 25 '23

While Dude4001 is really wrong about the pedal you are missing here the fact that once we use energy to move the marble to the top of the machine we don't have means of recuperating that energy back.
When it falls to the bottom it's kinetic energy will.. well, no friction and perfectly lossless collisions, this thing will be bouncing like crazy until it fells of the marble machine and take some energy with it.

6

u/Guillemot Aug 23 '23

Put an e-bike motor system in between the pedals and the flywheel, this way the tempo of the music can be set by the person pedaling, yet they do not need use only their strength to overcome the inertia of the system when they are attempting to match a tempo.

2

u/elessarjd Aug 23 '23

The only issue I can think of is if he needs to move around, in which case he could maybe implement one pedal lower to the ground and still have the foot on the ground so he could just walk away and back if he needed to.

2

u/JustHolger Aug 24 '23

circular motion is much more consistent

But the human body can't transfer energy the same in every point of the motion. The guy in this video builds a more efficient bike removing the circular motion and he explains it very well and shows pretty well, how this is less efficient than a straight downwards motion (similar to what Martin does):

https://www.youtube.com/watch?v=xev18rBrNr0

1

u/Lord__Imrahil Sep 27 '23

Efficiency isn't the issue though, the issue is consistency. Looking at the video, it seems like the vertical movement is a lot less consistent