Where does power come from?

[Fday]

Dude, put down the crack pipe. about 95% of your power is made on the downstroke. Try actually reading Coyle et al.

A good portion of which comes from what is done on the upstroke because it is the passive contribution of the weight of the leg added to the muscle power that adds up to the total force. While 95% of the pedal forces contributing to driving the bike may come there that doesn't mean that 95% of the muscle work is done there, unless, of course, the upstroke is completely passive.

 

[vadiver]

Just FWIW, only this past week I experimented with sprinting, cadence, and technique. I confirmed that I do better in a sprint by using a light gear, very high cadence, and concentrate on spinning circles while sprinting.

Hard gear, pushing down - 850 W
Small chainring (!), spinning circles - 933 W

<snip>
.

This is what I have been doing this whole start of the season. I have been really working on my cadence. I increased it a bit last year and really improved. My goal for this year is to spin most of the time at 100rpm.

This was really cemented home form another thread when a person wanted to work on their power but cannot go faster due to trafic and such. Someone posted, put it in your smallest gear and spin like mad. That made zero sense to me until I tried it. And it worked. Since then I am really working at that cadence.

I figure get the body used to the spin, then start upping the gearing. From what I have found so far on the trainer this is really working.

 

[n crowley]

Other than that I don't EXPECT them to do anything.

Which means, if as you claim there is a 40% power increase, downward pedal power must be increased by 40%. So the question is, excluding the unweighting effect, how can PC's increase downward pedal pressure and what is the necessity for all this torture suffered by the new PC user.

 

[Fday]

Which means, if as you claim there is a 40% power increase, downward pedal power must be increased by 40%. So the question is, excluding the unweighting effect, how can PC's increase downward pedal pressure and what is the necessity for all this torture suffered by the new PC user.

No, downward pedal power doesn't need to be increased 40%, nor does it need to be increased at all, to see a 40% power increase. simply redirecting the direction of the forces to be more tangential can give a large power increase because the net force resulting in torque is the actual force times the cosine of the angle of the force.

Power is simply the amount of work done in a certain amount of time. Work is simply a force exerted through a distance. One can have a lot of force exerted through a small distance or a small force exerted through a large distance and end up with exactly the same power or more.

Lots of things contribute to cycling power, you are only considering one of them. Earlier in this thread I listed the 4 major things by which I believe PowerCranks increase power in the cyclist. You apparently were not paying attention.

 

[vadiver]

No, downward pedal power doesn't need to be increased 40%, nor does it need to be increased at all, to see a 40% power increase. simply redirecting the direction of the forces to be more tangential can give a large power increase because the net force resulting in torque is the actual force times the cosine of the angle of the force.

Power is simply the amount of work done in a certain amount of time. Work is simply a force exerted through a distance. One can have a lot of force exerted through a small distance or a small force exerted through a large distance and end up with exactly the same power or more.

Lots of things contribute to cycling power, you are only considering one of them. Earlier in this thread I listed the 4 major things by which I believe PowerCranks increase power in the cyclist. You apparently were not paying attention.

\
And have no evidence to support the claim.

With the large power short distance vs small power large distance point. This is why the faster cadence generates a lot more power. The effective force on the pedal is less but when you do each revolution the distance is greater. For example. Leaving the defaults the same in A/C with the exception of cadence. Changing from 80 RPM to 100 RPM yeilds the same power, yet the effective pedal force changes from 402N to 502N.

What I have been finding this year is now that I am to the point of being able to sustain the higher cadence, going up a gear is easier than what I was trying last year. Which was staying in the same gear and trying to up the cadence.

 

[Felt_Rider]

This is what I have been doing this whole start of the season. I have been really working on my cadence. I increased it a bit last year and really improved. My goal for this year is to spin most of the time at 100rpm.

This was really cemented home form another thread when a person wanted to work on their power but cannot go faster due to trafic and such. Someone posted, put it in your smallest gear and spin like mad. That made zero sense to me until I tried it. And it worked. Since then I am really working at that cadence.

I figure get the body used to the spin, then start upping the gearing. From what I have found so far on the trainer this is really working.

Quick Hijack

I can't even imagine making that goal yet.
Coming from a competitive bodybuilding background my legs are a little larger, but I am not sure if that is the reason that I simply can't hold a cadence over 80. 80 feels like I am spinning my legs off, but I am very comfortable in the low 70's with bigger gears over the span of 60+/- miles. I am a 2 year old as far as cycling goes and guys in my group are constantly telling me to spin faster (80+) to save my legs, but I am still struggling with this because I stay right with them through out the ride while doing my turn in the front. If I drop to a gearing that will allow me to spin 80+ I get dropped from the group every time. If I stay with larger gears I have no problems.

That being said do you still think it is worth me pursuing this goal of a higher cadence as it is suggested quite often here and from some of my group members? I am interested in your view point since you set this goal for yourself.

disclaimer: I have no intentions of being a competitive cyclist and I am not currently using a PM. I do have a Garmin 305 with a cadence sensor.

 

[vadiver]

Quick Hijack

I can't even imagine making that goal yet.
Coming from a competitive bodybuilding background my legs are a little larger, but I am not sure if that is the reason that I simply can't hold a cadence over 80. 80 feels like I am spinning my legs off, but I am very comfortable in the low 70's with bigger gears over the span of 60+/- miles. I am a 2 year old as far as cycling goes and guys in my group are constantly telling me to spin faster (80+) to save my legs, but I am still struggling with this because I stay right with them through out the ride while doing my turn in the front. If I drop to a gearing that will allow me to spin 80+ I get dropped from the group every time. If I stay with larger gears I have no problems.

That being said do you still think it is worth me pursuing this goal of a higher cadence as it is suggested quite often here and from some of my group members? I am interested in your view point since you set this goal for yourself.

disclaimer: I have no intentions of being a competitive cyclist and I am not currently using a PM. I do have a Garmin 305 with a cadence sensor.

We are very similar. When I started back cycling 3 years ago I was very comfrotable in the 65-75 range. Big gear slow turning, could keep up and climb fairly well. Last year I worked on the road to bring it up to 80-90. I started to gain speed (I do not have a PM on my bike) and was less tired at the end of rides. I think a lot of this also had to do with getting back in shape. I could not spin at 105+ without my legs feeling like they were going to fall off and fly away.

I am now able to ride at 100 RPM on most of my routes. On my trainer last night I was able to sustain 120rpm for about 5min (then the block changed) so I think it can happen. It was very hard for me to do this on the road for some reason. Working on the trainer has made it "seem at least" easier. I am now able to go up a gear or two on the road and not drop the cadence.

The jury is still out on me since I am just now able to get to the road.

 

[Felt_Rider]

thanks
I'll start focusing more on the spin bike during the week.

 

[beerco]

A good portion of which comes from what is done on the upstroke because it is the passive contribution of the weight of the leg

That would be nice except that for non power cranks, the weight of the down leg cancels the weight of the up leg. Unfortunately for powercrank users, that's not the case.

There are losses due to the reciprocating of the legs but no type of crank or pedaling technique can eliminate that.

 

[Fday]

That would be nice except that for non power cranks, the weight of the down leg cancels the weight of the up leg. Unfortunately for powercrank users, that's not the case.

Why do you say that is a bad thing for PC'ers? Forcing the user to pull up forces the user to use more muscles. If the goal is to increase power, it seems to be, that using more muscles would be a good thing. And, it would seem that allowing the up leg to cancel out part of the down leg by robbing from force that could be going to the wheel would be a bad thing.

There are losses due to the reciprocating of the legs but no type of crank or pedaling technique can eliminate that.

Yes, I agree, they cannot be eliminated. But, they can be minimized. By my calculation, the magnitude of the losses associated with that receprocating action increase with the cube of the cadence! If one is pedaling at too high a cadence (a cadence beyond their most efficient cadence) they are wasting energy just making the legs go up and down that could be better utilized to drive the bicycle if they would simply lower their cadence some.

 

[n crowley]

There are losses due to the reciprocating of the legs but no type of crank or pedaling technique can eliminate that.

Could you expand on this statement as to where exactly the power losses are taking place.

 

[beerco]

If the goal is to increase power, it seems to be, that using more muscles would be a good thing. And, it would seem that allowing the up leg to cancel out part of the down leg by robbing from force that could be going to the wheel would be a bad thing.

Bzzt, don't work that way. There a tidal losses due to shuttling blood to more muscle. The best use of the limited supply of oxygenated blood is to use fewer but bigger muscles. That's why the HPV crowd always re-discovers and abandons arm+leg power: not only is it less efficient to use the smaller arm muscles, it results in an over all power decrease due to less oxygenated blood flowing to the legs.

 

[beerco]

Could you expand on this statement as to where exactly the power losses are taking place.

If you take a crank and just spin it, it will spin for a long time. If you now attach a couple of rods to each end of the crank with a mass on the ends (kind of like pistons at the end of the con rod in an IC engine or, your thighs at the end of your tibia) and spin it it will slow much much faster because the mass at the end of the rod is changing directions. i.e. it's going up, slows to a stop and then changes direction to go down. Those constant accels and decels use energy.

 

[frenchyge]

Those constant accels and decels use energy.

You know, I've seen that mentioned several times and it just doesn't sit right with me. I no longer have the resources to model it, and probably no longer have the knowledge to calculate it manually ( ), but it just doesn't sit right.

Yes, the thighs and calves are accelerating and decelerating and changing directions and stuff, but my intuition tells me that if you take a 4-bar linkage from hip to knee to pedal to bottom bracket and fix the ends (ie, hip and bottom bracket), that the linkages will all act as a single mass and conserve momentum and energy as they spin, rather than experiencing a net loss/gain of energy at different points in the crank circle, as has been repeatedly stated here (neglecting joint friction, of course).

Someday, I will either get bored enough to crack open the books again, or it will eat at me to the point that I have to prove it to myself, but for now I'll just go back to quietly doubting that the quoted statement is true. Apologies in advance to Frank and Beerco if my gut turns out to be wrong on this.

 

[daveryanwyoming]

You know, I've seen that mentioned several times and it just doesn't sit right with me. ...

I agree, there are frictional losses in any linkage design but there aren't losses due to the reciprocation itself. Part of the problem with the previous analogy is that it's a passive system. The reciprocation we're talking about is how energy is supplied to this system.

A quick Google turned up a very similar discussion on physicsforum concerning recipocating vs. rotary engine designs. These guys including the initial naysayers quickly came to agreement that reciprocation does not in and of itself introduce losses. Have a read here: http://www.physicsforums.com/archive/index.php/t-134636.html

Here's the key argument:

What you are missing is that that energy is exactly what the flywheel is looking for! That's the energy that turns the crankshaft. The kinetic energy of the flywheel increases while the kinetic energy of the piston decreases. There must be ineficiencies involved here compared to, say, a purely rotary engine. Nope. In essence, the energy is put in at combustion and taken out at the crankshaft by the act of opposing the motion of the piston.

I'm sure someone will argue that the human body is a different sort of engine and that this doesn't apply, but I think it extends quite well to whether reciprocation itself introduces losses.

 

[beerco]

I agree, there are frictional losses in any linkage design but there aren't losses due to the reciprocation itself.

Actually, I'll buy that as I was half thinking the same thing while I was writing my prior post.

(It's awfully embarrasing to have actually fallen for some of Frank's snake oil - I think I picked that up in a prior post of his).

 

[dhk2]

If you take a crank and just spin it, it will spin for a long time. If you now attach a couple of rods to each end of the crank with a mass on the ends (kind of like pistons at the end of the con rod in an IC engine or, your thighs at the end of your tibia) and spin it it will slow much much faster because the mass at the end of the rod is changing directions. i.e. it's going up, slows to a stop and then changes direction to go down. Those constant accels and decels use energy.

Your piston engine analogy seems like a good one. Agree it takes energy from the crankshaft/flywheel momentum to accelerate the piston/rod assembly to top speed at midpoint of the stroke (in 1/4 of a crank rev). But then, wouldn't the decel to a stop at TDC return virtually all that kinetic energy of the piston/rod back to the crank? EG, momentum is going into piston/rod mass from the crank for 90*, then it's coming back to the crank for the next 1/4 rev as the piston/rod is slowed.

I don't have the answers here, but it's an interesting topic. I've noticed it does require significant power just to turn over the pedals at a decent cadence, even in the lightest load setting on the gym trainer.

 

[Fday]

You know, I've seen that mentioned several times and it just doesn't sit right with me. I no longer have the resources to model it, and probably no longer have the knowledge to calculate it manually ( ), but it just doesn't sit right.

Yes, the thighs and calves are accelerating and decelerating and changing directions and stuff, but my intuition tells me that if you take a 4-bar linkage from hip to knee to pedal to bottom bracket and fix the ends (ie, hip and bottom bracket), that the linkages will all act as a single mass and conserve momentum and energy as they spin, rather than experiencing a net loss/gain of energy at different points in the crank circle, as has been repeatedly stated here (neglecting joint friction, of course).

Someday, I will either get bored enough to crack open the books again, or it will eat at me to the point that I have to prove it to myself, but for now I'll just go back to quietly doubting that the quoted statement is true. Apologies in advance to Frank and Beerco if my gut turns out to be wrong on this.

Your gut is wrong I am afraid. Most get it wrong because they think that because one leg is going up while the other is going down, that conserves energy. Unfortunately, energy is a scalar so direction doesn't matter and both thighs (the thighs are where most of the losses are, the other parts of the leg are moving more circular) are accelerating and decelerating at the same time. You could make an approximation of the leg movement as being sinusoidal to make the calculations easy and this doesn't take much time. Then you can see the energy fluctuations increase with the square of the frequency which means the power losses change with the cube of the frequency.

It is an "easy" calculation if you just look at the thigh and make those assumptions.

 

[Fday]

I agree, there are frictional losses in any linkage design but there aren't losses due to the reciprocation itself. Part of the problem with the previous analogy is that it's a passive system. The reciprocation we're talking about is how energy is supplied to this system.

A quick Google turned up a very similar discussion on physicsforum concerning recipocating vs. rotary engine designs. These guys including the initial naysayers quickly came to agreement that reciprocation does not in and of itself introduce losses. Have a read here: http://www.physicsforums.com/archive/index.php/t-134636.html

Here's the key argument:I'm sure someone will argue that the human body is a different sort of engine and that this doesn't apply, but I think it extends quite well to whether reciprocation itself introduces losses.

Nope, the problem here is the flywheel (pedals) does not change speed to any degree so the total energy of the system cannot remain constant. If there was no resistance to motion and the flywheel was of similar mass to the legs then such a system has the posibility of not losing energy. But it has no relationship to cycling.

 

[dhk2]

I agree, there are frictional losses in any linkage design but there aren't losses due to the reciprocation itself. Part of the problem with the previous analogy is that it's a passive system. The reciprocation we're talking about is how energy is supplied to this system.

A quick Google turned up a very similar discussion on physicsforum concerning recipocating vs. rotary engine designs. These guys including the initial naysayers quickly came to agreement that reciprocation does not in and of itself introduce losses. Have a read here: http://www.physicsforums.com/archive/index.php/t-134636.html

Here's the key argument:I'm sure someone will argue that the human body is a different sort of engine and that this doesn't apply, but I think it extends quite well to whether reciprocation itself introduces losses.

Good link; should have read it before jumping in with my post. It answers the question well: reciprocation by itself doesn't induce losses since all energy lost to accelerate the piston/rod is recovered. Seems to me the same principle applies to cycling: the recip movement of the leg masses by itself doesn't cause a loss of power.

Of course, in an IC engine, the flywheel/crank rotating mass is heavier than the reciprocating mass of the rods/pistons. In cycling, the opposite is true: the legs are much much heavier than the rotating mass of the crankset.

However, we know it does takes energy to move the legs on the pedals without producing any useful work. At fairly high cadence, say 125 rpm, even with the chain disconnected, the power required to keep spinning is significant (for me anyway).

I know it's a weak analogy, but maybe the power loss could be roughly equated to the energy losses in an IC engine due to internal friction and pumping losses. Putting the car in first gear, revving the engine up to a high rpm and holding it there illustrates the point. Only a small net power is being delivered to the drive wheels, yet a fair amount of throttle (and fuel consumption) is required. A lot of O2 and fuel is flowing to keep the crank spinning, yet net power out is small.