wphamilton

5 or 6 years ago I showed off my foot-velcro straps on one of these threads, but nobody was particularly impressed with my idea. I never thought of using them with bare feet but that would work and you're right it would keep the angular momentum to the minimum.

woodcraft

If I do leg lifts, I get tired because the effort to lower my leg does not equal, nor compensate for the effort to raise my leg.

When I ride up a hill and down the other side, it does not combine to equal no effort.

So in my corner of the universe, weight has considerable meaning.

Perhaps I have not spent enough time with textbooks that tell me otherwise.

kpotier16

Again, we are not really talking about the crank, the feet have to be moved in order to spin the crank, the foots linear inertia is tangential to the crankarm and the acceleration is towards the bb, saving weight will help, move your foot in a circle, then move it in a circle with a 5 pound weight attached. I believe my original calculation was spot on, that the rotational inertia is nearly irrelevant (.027 watts for 90 cadence and 200mm radius) , and no acclerating to 130 will not make a siginificant difference, because the acceleration is purely based on the velocity and radius as legs do not have roational inertia (unless your leg is a wheel), gravity is cancelled out. essentially this weight is only marginally better than framw weight. simply put, you need a sizeable radius to have significane MOI (or a very large mass). I get this is difficult for people to understand and I accept my numbers are not perfect because all parts of the pedal stroke do not have equal power. This being said, I welcome someone else to find something more accurate and calculate it if possible.

Bunyanderman

If you could save that energy in your leg, which you can't, it would be exactly equal. That doesn't relate to centripetal inertia, different type of system.

If you ride down a hill, then back up (equal height, horizontal distance doesn't matter), you will end at the same elevation every single time (no friction).

If ever in doubt, use this. F = ma. With f = force, m = mass, and a = acceleration. With zero acceleration, there is no force. Maybe friction confuses you, I can't tell, or maybe just trolling.

woodcraft

I think that I'm more confused by gravity than friction,

but also statements like '...there is no force', 'perfect compensation', & 'weight doesn't matter' give me a little trouble.

Again, you guys must be really fast with such advantages.

Bunyanderman

Then how is this relevant at all?

If there was no "arm" that moves your leg in a circle, there is no momentum to move you leg back up. With a crank, the weight gained gives more momentum, to which you need more momentum to move it back up. It will always equate, a simple physics book will tell you this.

Bunyanderman

In basic terms, you have two types of energy. Potential energy, and kinetic energy. If one decreases, the other will increase.

Potential = mgh
Kinetic = 1/2mv^2

With an elevated weight that starts to accelerate by gravity, it will turn its potential energy into kinetic energy. If a moving objects starts moving vertically against gravity, it will turn its kinetic energy into potential energy.

To help go between speed and elevation, you set the equations equal to each other. Guess what happens, the mass cancels. gh=1/2v^2

Here is another analogy, if a skateboarder drops into a half pipe, he will start and end at the same height until some other force acts on him. A lighter skateboarder won't move higher and higher, same thing with a heavier rider.

HTupolev

But these situations aren't analogous to what we're talking about. You're comparing something which isn't good at storing incident mechanical energy (a human body) with something that is (a flywheel, essentially).

In a world with perfect mechanisms and no air resistance, if you were on a perfect electric motorbike that could convert energy on the downhill into battery juice, you actually COULD go up and then down a hill with no net loss in battery power.

By the way, the "compensation" concept I brought up is used in the real world all the time in heavy lifting applications. Consider an elevator, with a pully at the top of the shaft, connecting a cable between the elevator itself and a counterweight that weighs about the same as the elevator. Besides acceleration, all the elevator motor has to do is combat air and cable friction; trading gravitational potential between a counterweight and the elevator is basically free for constant-speed motion.
This is what's happening with weight on your cranks.
You're instead envisioning a counterweightless elevator, with a person at the bottom of the shaft hanging onto the elevator cable, attempting to use their arm strength to lift and lower the elevator.

kpotier16

The arm does not move your foot in a circle, your foot moves the arm in a circle. I agree with you essentially, the force is essentially irrelevant, that being said, there is centripetal force that the body must counteract, you cannot ignore this to truly examine this

Bunyanderman

So do you think it would save you actual watts (no acceleration)?

kpotier16

It will save you watts, but not many, I calculated the 200 gram saving at 90 cadence at .027 watts more saved versus 200 grams of static weight. So yes, it will save you power, but we are talking about 1/10000 of your power (assuming 250 watts), this is simply insignificant

woodcraft

OK try this:

Spin your arms in a cranking motion for one minute,

then do it again with three lb. dumbbells.

Notice any difference?


I just did this.

Without the weights, it was not too hard.

with the weights, my arms got quite tired, & my breathing was starting to elevate. I would probably have reached failure within a second minute.

Conclusion: weight DOES matter.

YMMV

gregf83 

Sorry, your thoughts were incorrect before and they're still wrong. The power to turn the cranks at a constant cadence is only used to overcome friction. Your .027 watts is .027 watts too high unless you're estimating friction and I don't believe you were. Accelerating the cranks from 90 to 130 RPM is the only thing that will take any power, although you are correct it will be small.

Bunyanderman

If your arm swung in a perfect circle and you didn't run into forces needed to stretch tendons and muscles, then I'd believe you. Until you can replicate a crank and spindle with your arm, your test is completely irrelevant.

Seriously give an attempt at understanding inertia.

Bunyanderman

This guy has it.

woodcraft

I'm envisioning a person at the beginning of a long bicycle ride, attempting to use their strength to get to the end.

Not a perfect mechanism, for sure.

gregf83 

Were the weights you were spinning in free air or attached to a crank? If they were in free air they weren't balanced and it takes work to move them up and down.

Here's a thought experiment for you: Elevators are typically designed with a counterweight system. The counterweight is sized to equal the weight of the elevator plus a portion of the load but for this experiment assume the counterweight exactly balances the elevator. To move the elevator up an down in this scenario the motor only needs to overcome friction because the system is balanced. Add and equal weight to the elevator and the counterweight and the system remains in balance and it doesn't take any more power to move the elevator at a steady speed.

kpotier16

I cannot help you at this point, you should do some actual research and learn some physics, a rotating object does have centripetal acceleration, this must be counteracted, if there is less weight, then there is less force to be counteracted, this explains why feet sometimes slide off platform pedals, because the foots linear momentum is tangential to the crank, therefore the cyclist must accelerate there foot towards the bb to pedal in a circle. If you dont like my calculation, I can respect that but please do a better one that doesnt ignore all relevant forces

gregf83 

I don't need help and this high school physics problem doesn't need research.

Google can help you but here is a reasonable explanation you can start with: Torque and Rotational Equilibrium

woodcraft

There's a hand cycle machine at my gym with cranks and hand knobs.

I used it last week.

Uses resistance instead of weights, but the effect is the same.

But as Bunyanderman doesn't seem to even believe that my arms got tired,

we may be beating a dead horse (or 'perfect mechanism') at this point.

f4rrest

@woodcraft have you used a teeter totter?

Shoes on a crank are like two people on a teeter totter. If they are the same weight, then the weight of the person being lifted is completely offset and balanced by the weight of the person going down, regardless of how much they weigh.

Unless you have independently operated cranks such as powercranks. Those are seriously a biotch to pedal with heavy shoes.

Bunyanderman

Does the bold help you read?

HTupolev

No it isn't. A hand cycle with extremely low friction and an extremely heavy freewheel would take a while to spin up to speed, but once it's going it would take very little effort to keep it spinning.

woodcraft

I did the dumbbell experiment.

How about you do one?

Ten mile ride, with & ten miles without 5 lb. ankle weights.

It shouldn't matter since they are equal & offset each other, right?

wphamilton

Having lots of weight on the pedals is like putting a disk there, or a flywheel if that's easier to visualize. Change in angular momentum, corresponding to accelerating the bike, is where you'd feel it. Also change in the linear momentum of the bike. It's more force to accelerate the same amount. No acceleration, no harm.

Except going uphill and down, you can't maintain the same speed with the same energy on the heavier bike, which is what the other guy missed. Due to that pesky non-linear air resistance.

You should do the experiment - I think you'll be surprised.