Force at Crankshaft Pedal

[ThirdString]

I want to preface this with the fact I got a C- in dynamics!

I'm trying to analyze a flywheel with a crank attached. A simplification of the design would be a bicycle pedal, crank arm, and a flywheel. All I need to do is calculate the force at the pedal, but just can't do it.
I know the flywheel/crank moment of inertia via 3D modeling, I can measure angular velocity. I know all the geometry, so R is known. What I'm missing is an understanding of momentum & forces apparently. I know that torque = I*w, but I am assuming the angular acceleration (w) is constant. (I can evaluate bearing friction after I figure out the basics.) I have come to understand that torque is only necessary to change (angular) speed. That was a revelation (makes sense though), but it doesn't help me find the force I'm looking for.

My question is: What am I missing? Is there no actual force on the pedal? (Experience tells me your foot is pushed by the pedal, but of course that slows down the bike/flywheel.) Or is there something in momentum that I need to understand better? Or do I have to assume an angular acceleration if I want to find a force? Although my immediate goal is to find the force, I really want to understand the concept I am currently missing.

 

[GregLocock]

Somehwere in ther eyou hit on the right answer. If there is no friction, and no change in angular speed of the flywheel, the torque required is zero, as is the force on the pedal.

Cheers

Greg Locock


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[ThirdString]

So in real-life, there must be an angular acceleration, which is the force that is felt on the foot. If your pedal hits an immovable object while spinning, you have a huge force since the acceleration is huge. If it's spinning free, there is momentum, but no force. On a bike, there would be forces on the flywheel either from the other foot, or from the road turning the tire.
I guess momentum in real-life and engineering have a big disconnect in my brain.

 

[GregLocock]

I think you got a C- on a good day. The main resistance you feel on a bike at constant speed is rolling resistance, plus aero drag, plus any effect due to hills.



Cheers

Greg Locock


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[ThirdString]

Haha, it may have been a D, I've been out awhile. I actually started the MIT online course in dynamics to help understand this problem better. But I think the key step is breaking the mental association of momentum and force is key. Thanks for the feedback, helping me stop the snipe hunt. I'll revisit the problem again after a good night's rest.

I'm working on stopping force (no freewheel/freehub) rather than pedaling force, thus the 2 factors I mentioned. And it's actually stationary (thus the flywheel), so not even the momentum of the bike is in play, just the flywheel.

 

[BUGGAR]

I too have problems with things that go around, so I usually break these problems into their linear equivalents.

 

[hydtools]

Use the rotational form of impulse = change in momentum.

Ted

 

[GregLocock]

The MIT online course in statics and dynamics is excellent. It is approximately 100 times better than anything I've seen before, although it only does the non calculus side of things

https://www.edx.org/course/advanced-introductory-classical-mitx-8-mechcx-0

Cheers

Greg Locock


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