Quote: Original post by Anonymous Poster

Quote: Original post by darookie
A final note on Power = Force * Speed is that this formula only applies if and only if force and speed are constant, which surely doesn't apply.

Wrong. The power at time t is the force at time t times (dot product) the velocity at time t. Nothing needs to be constant.

Quote: Original post by darookie
Otherwise power is ΔWork / ΔTime, which fits nicely into a physics simulation (since you have a Δt).

ΔW / Δt = ΔW/Δs * Δs/Δt = force*velocity

Yes.(agreed (agreed with this anonimous poster))

Quote: Original post by Anonymous Poster

Quote: Original post by crocomire
from http://www.analyticcycling.com/ForcesSpeed_Page.html:
"Speed is estimated based on rider power and the forces acting on a rider from wind resistance, rolling resistance, and gravity."

What is meant by this "rider power"? How can they use Power as an input variable if Power is computed from Force?

The "rider power" is the power the rider puts into the pedals, ie at what rate he is doing work (as seen from the outside - what goes on inside his muscles is another story). From the power and a bunch of other parameters you can calculate the riders steady state velocity, just like in the page you are refering to. That way you don't have to worry about pedal forces and gears and transmission and so on, but it all comes down to how detailed a model you want to use.

The rider power is typically measured on a cycle ergometer. The power, together with the riders weight and size says quite a lot on what level of performance you can expect from him, so I think it is a good starting point for what you're trying to do.

Another thing worth mentioning, rider's power, as seen from outside, depends to rotation speed of pedals.(e.g. at big speed you can't follow pedals with legs, at small speed you don't pull with proportionally more force. It's result of how muscles works)
Assuming there is variable transmission that always keeps rotationspeed of pedals constant, we can assume constant power of rider and then given that drag is f(v)
, at steadly velocity, we have
f(v)*v=power of rider
(so power of rider is equal to power pumped into environment with all these drags)

edit: by the way... it's one "Mr. Jekyll / Dr. Hyde" anonimous poster, or that's different people?

[Edited by - Dmytry on June 13, 2005 7:14:12 AM]

Crocomire
I hope you take my advise in good way I just want to save you some wandering in a vicious circle of nonsense.
There are existing laws that predict exactly what you one to do. In fact this devices even exist in real like.
You need to read the first and second law of thermodynamics, in particular the first law, it states:

"I a close system the change of energy is equal to the heat minus in the system minus the work realized by the system"
U = Q – W

U is the energy change,
Q is the heat in the system
W is the work

U is how much energy is lost in the process, and it is always positive.
Q is the amount of total energy in the form of heat (calories)
W is the mechanical work (potential and kinetic energy)

To relate Heat to temperature you need to look at second more obscure law, entropy
"Entropy is the ratio of Heat to absolute temperature".
These two laws is all you need to do your simulation trust me these will solve yor problem.

This is how mechanical engineers do this stuff by using the old reliable laws. If you keep reading recommendations of people with litle knowledge of algrabra but that do not know what are they taking about, lots of reasons as why the laws are wrong and some how an impulse and perhaps some simpletic integration by manipulating some algebra on an firt order euler numerical integration, and soon you will end up doing and interative impulsive system with euler-verlet integration that will overthrwon the laws of thermodynamic.

If you read some text book or in the web about first and second law of thermodynamic I guarantee you will solve your problems, I suggest read a text book so you get the fundamental, in the wet some time you may get some incorrect information that you will only be capable to filter if you know the basis.

There are books that come with plety of tehery and solve problems that will help you how to use this equation.

one little thing:

Quote: Original post by Anonymous Poster
This is how mechanical engineers do this stuff by using the old reliable laws.

speak for yourself: im a mechanical engineer, but this is certainly not how i would approach the problem...

Quote: Original post by Eelco
one little thing:

Quote: Original post by Anonymous Poster
This is how mechanical engineers do this stuff by using the old reliable laws.

speak for yourself: im a mechanical engineer, but this is certainly not how i would approach the problem...

Could you tell me how you would approach it?

Another related thing:
Bicycle efficiency and power

I just added my two cents
If you check those thread machines or micelle electrician machines. Those are especially thermodynamic devices.

The use a concept engineers with experience since the industrial revolution.
Essentially is this in a close ideal system the energy lost is zero.
However not system is ideal but in the system is isolated enough the energy lost is minimal

So practical engineers figure out that if the come up of a way to insulate the system well enough then they can used and approximation o the first law of thermodynamics
And say

0 = Q – W

now with this law and the first law that relate the temperature to calories (heat)

All you need is a thermometer and a speedometer, to measure the how much heat or calorie a person burn in a training machine.
This is way you see that every treading machine manual or instructor recommend the user to wear a swept pans and jackets. Is that want to make the system as close as possible by preventing energy from escaping.
And that is how billion of dollars are spended each year craeting beeter issolation systems.

This wonderful principle had been made possible the creation of Boiler, Internal combustion engines, air-conditions, refrigerator, and a whole bunch of other devices.

Quote: Original post by Eelco
one little thing:

Quote: Original post by Anonymous Poster
This is how mechanical engineers do this stuff by using the old reliable laws.

speak for yourself: im a mechanical engineer, but this is certainly not how i would approach the problem...

of course not you want to invent a new law of physics, so you can solve it with and impulse.

Quote: Original post by crocomire

Quote: Original post by Eelco
one little thing:

Quote: Original post by Anonymous Poster
This is how mechanical engineers do this stuff by using the old reliable laws.

speak for yourself: im a mechanical engineer, but this is certainly not how i would approach the problem...

Could you tell me how you would approach it?

ofcource.

you ask a lot of questions, but lets focus on the one you deem the most important one.

Quote:
my main question is: how do I relate the energy a biker spends (per time unit) to a force on the pedals?

as dmytry said, this involves some biology. when i was first told energy was force*distance, i immidiately considered it bullshit, since youre muscles obviously consume power when pushing against a static object. however, i was wrong. muscles are rather complicated, and certainly not an 'ideal' powersource.

how do you define the amount of energy the biker spends? all chemical energy used in his body? if so, you should first determine which fraction of that is put to use in the muscles involved in riding his bike. when you have this, then there are the biological inefficiencies involved in muscles. muscles have a certain optimum working speed/force. if you want to do it right youd need data describing the muscles various relations for instance its load vs speed curve. this is crucial if you want to capture phenomena such as a biker not being able to keep pace in low gear at high velocities.

also, i imagine there are some time dependant characteristics. you can put much more force in a single short push, where all muscles can contract according to their optimum, than over a prolonged period of time, such as pushing against a static object, which is accomplished by different muscle strands taking turns in maintaining the force, and getting tired on top of that.

if you cant find any data, we can do nothing more that guess a little. first i suggest you try and find some of it, and when we know in what format it comes, if at all, lets take this a step furter.

Quote: Original post by Eelco
how do you define the amount of energy the biker spends? all chemical energy used in his body? if so, you should first determine which fraction of that is put to use in the muscles involved in riding his bike. when you have this, then there are the biological inefficiencies involved in muscles. muscles have a certain optimum working speed/force. if you want to do it right youd need data describing the muscles various relations for instance its load vs speed curve. this is crucial if you want to capture phenomena such as a biker not being able to keep pace in low gear at high velocities.

See what I tell you crocomire:
All this is nonsense rhetoric. Because it does not tell you how to quantize the energy you want to measure, instead it sends you to find the data describing the muscle various relation (whatever the heck that means, and how you do that?) it is just speculation that essentially is leaving you in limbo and all confused.

Somebody is already blaspheming against the design of the human body. They say it is inefficient. I happen to think the human body and biology is that more efficient machine that exists. Imagine is manage to extract and store energy from biological resources with very little waste. If you wait long enough I bet somebody will bring the relativity into the problem.

The problem is not so difficult the first law of thermodynamics is the only law that relates mechanical work to heat stored in a system. Just read more about it and you will see that it makes sense.

Quote: Original post by Dmytry
Another thing worth mentioning, rider's power, as seen from outside, depends to rotation speed of pedals.(e.g. at big speed you can't follow pedals with legs, at small speed you don't pull with proportionally more force. It's result of how muscles works)
Assuming there is variable transmission that always keeps rotationspeed of pedals constant, we can assume constant power of rider and then given that drag is f(v)
, at steadly velocity, we have
f(v)*v=power of rider
(so power of rider is equal to power pumped into environment with all these drags)

f(v) would have to include not only drag forces but also gravitational forces from uphill and downhill cycling. As far as transmission is concerned I think that on a modern racing bike you can assume near optimal cadence and efficiency under most racing conditions.

Quote: Original post by Anonymous Poster

Quote: Original post by Dmytry
Another thing worth mentioning, rider's power, as seen from outside, depends to rotation speed of pedals.(e.g. at big speed you can't follow pedals with legs, at small speed you don't pull with proportionally more force. It's result of how muscles works)
Assuming there is variable transmission that always keeps rotationspeed of pedals constant, we can assume constant power of rider and then given that drag is f(v)
, at steadly velocity, we have
f(v)*v=power of rider
(so power of rider is equal to power pumped into environment with all these drags)

f(v) would have to include not only drag forces but also gravitational forces from uphill and downhill cycling. As far as transmission is concerned I think that on a modern racing bike you can assume near optimal cadence and efficiency under most racing conditions.

Obviously, if he goes uphill or downhill.
Yes, if it's about real sport and not just recreational, we can assume that bike is good and is tuned to this specific rider, and rider changes gears optimally enough. (see link, Bicycle efficiency and power)