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  #1051  
Old 01-30-13, 23:34
JayKosta JayKosta is offline
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Originally Posted by FrankDay View Post
...
If the crank speed is ever lower than "required" by bike speed the freewheel will override and the rider will feel that. That simply never happens in normal
riding except when the rider intentionally coasts.
...
====================
Frank,
(and I am thinking about a bike with freewheel, not a fixie)
My viewi of what happens when 'crank speek is lower than "required" by bike speed' is that

1) the bike will immediately begin to slow (decelerate) due to mechanical resistance/friction and wind drag.

2) If the reduced crank speed is slighlty greater than the new slower speed of the bike, the rider will still be producing torque.

3) If the reduced crank speed does product torque, then
a) either the bike speed will increase (if the torque give increase crank speed),
or
b) the bike speed will continue to decelerate, but at a reduced rate because the torque and crank speed are less that what is needed to overcome the slowing of the bike due to mechanical resistance & wind drag.

When the rider is again able to produce high torque and crank speed, the bike will accelerate to its previous speed.

Jay Kosta
Endwell NY USA
  #1052  
Old 01-31-13, 05:37
FrankDay FrankDay is offline
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Quote:
Originally Posted by JayKosta View Post
====================
Frank,
(and I am thinking about a bike with freewheel, not a fixie)
My viewi of what happens when 'crank speek is lower than "required" by bike speed' is that

1) the bike will immediately begin to slow (decelerate) due to mechanical resistance/friction and wind drag.

2) If the reduced crank speed is slighlty greater than the new slower speed of the bike, the rider will still be producing torque.

3) If the reduced crank speed does product torque, then
a) either the bike speed will increase (if the torque give increase crank speed),
or
b) the bike speed will continue to decelerate, but at a reduced rate because the torque and crank speed are less that what is needed to overcome the slowing of the bike due to mechanical resistance & wind drag.

When the rider is again able to produce high torque and crank speed, the bike will accelerate to its previous speed.

Jay Kosta
Endwell NY USA
Jay, that is an interesting way to think about things and I understand what you are saying and I don't want to be nitpicky but that is simply the wrong way of looking at things. Unless you can describe what is going on using math it is impossible to predict what will happen. If you cannot predict you do not understand. The bike speed varies, not because the crank speed slows but because the instantaneous power drops to below that needed to maintain current speed. As long as there is some forward force on the pedal it will be moving in unison with the bike speed because any forward force is always trying to increase the pedal speed (F=ma). The bike speed only drops when the forward force on the pedals is less than the retarding force transmitted to the pedals by the wind and rolling resistance, when adding the two forces together gives a negative number. So, when the forward force is greater than the retarding force coming from the wind and rolling resistance the bike will speed up, when it is less it will slow down. While the variations in speed you describe do occur you have the cause and effect backwards. Everything that happens can be described using calculus derived from the simple formula F=ma and how the parts of the bicycle are mechanically connected and acting as levers. Nothing can be described mathematically using your premise that I know of.

While your understanding of what happens physically is correct you would be better served by describing it in terms that are mathematically correct vs a laymans interpretation as it will give you more credibility if you ever in the future are talking about this with someone knowledgeable in this area.
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  #1053  
Old 02-01-13, 00:39
coapman coapman is offline
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Originally Posted by FrankDay View Post
Here is a study out of Norway that sort of kicks that notion into the garbage can, at least if you think improving efficiency is something to aim for. Now if they will just repeat this and see if power correlates as well and this debate should pretty much end.

I don't agree with everything they say in their analsis but their data is their data.

From the abstract (DC is essentially the size of the force at top and bottom dead center): "Results: Mean work rate was 279 W, mean FCC was 93.1 rpm, and mean GE was 21.7%. FE was 0.47 and 0.79 after correction for inertial forces; DC was 27.3% and 25.7%, respectively. DC size correlated better with GE (r = 0.75) than with the FE ratio (r = 0.50). Multiple regressions revealed that DC size was the only significant (P = 0.001) predictor for GE. Interestingly, DC size and FE ratio did not correlate with each other.

Conclusions: DC size is a pedaling technique parameter that is closely related to energy consumption. To generate power evenly around the whole pedal, revolution may be an important energy-saving trait."


Can you give me a layman's interpretation of this research ?
  #1054  
Old 02-01-13, 01:44
JayKosta JayKosta is offline
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Quote:
Originally Posted by FrankDay View Post
...
The bike speed varies, not because the crank speed slows but because the instantaneous power drops to below that needed to maintain current speed. .
...
=========================================

Frank,

I think our only point of contention is the item above.

My thinking is that since F=ma
F= m(change in velocity / change in time)
or
F= m(dv/dt)
So that a change in speed (dv) can result in a change in F, as well as a change in F can result from a change in speed (dv).

My contention is that in road pedaling, the change in crank speed in the low torque sectors is the cause of a significant portion of the change in F.

I think that most well-trained competitive cyclist have adequate strength to maintain constant torque thru a complete pedal rotation when a 'sub-maximal' amount of torque is used (i.e at a level of exertion somewhat less than 'all out').

In this case, in the low torque sectors it is a reduction in crank speed that yields the reduced torque. This reduction in crank speed occurs because the cyclist has difficulty with the physical movement of maintaining the crank speed due to the needed change in muscle usage and timing/coordination of the pedal stroke in the low torque sectors. I doubt that the cyclist lacks the raw strength needed to produce the torque, but that he cannot maintain the crank speed to achieve the torque.

Probably some testing combination of power meter, and muscle & nerve activation is needed to give a more complete understanding of what is really happening.

---- this sounds like the end of a journal article that concludes with
"Our results indicates that further research is needed that would provide valuable information".

Jay Kosta
Endwell NY USA
  #1055  
Old 02-01-13, 04:06
FrankDay FrankDay is offline
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Quote:
Originally Posted by JayKosta View Post
So that a change in speed (dv) can result in a change in F, as well as a change in F can result from a change in speed (dv).
Well, while I guess one might think that one has to ask what is a change in speed (it is an acceleration) and how did it come about. This is not a chicken or egg thing. In this case the Chicken (Force) definitely came first.
Quote:


I think that most well-trained competitive cyclist have adequate strength to maintain constant torque thru a complete pedal rotation when a 'sub-maximal' amount of torque is used (i.e at a level of exertion somewhat less than 'all out').
While that might be true in space where there is no gravity it is not true in a strong gravitational field if one is just looking at one leg. When one combines the two legs together the sum can look pretty constant but the pedal forces are coming from the resultant force of many different muscles working together, some big and strong and others somewhat weaker. As I stated before it is possible to get an almost even torque around the circle when both legs are included as I can do it, achieving spinscan numbers 95 and above. But, such accomplishments are rare. It is pretty rare to see any cyclist with spin scan numbers above 80. Take a poll of Computrainer owners and see what you get. You will be surprised how "low" most of their numbers are.
Quote:

---- this sounds like the end of a journal article that concludes with
"Our results indicates that further research is needed that would provide valuable information".
No further research necessary. In bicycle pedaling the instantaneous pedal forces dictate both bike and crank speed. Crank speed does not dictate pedal force. For crank speed to dictate pedal force the situation would have to be an outside force driving crank speed against a flacid leg. That could only happen when riding a fixed gear bike and slowing down or going downhill. It could never occur on a freewheel bike (unless one leg was driving the other, flacid, leg and then it would only occur on one leg) except in the case when we see negative forces on the upstroke when the cranks act as one, where pedal speed may play a partial role. This is not the part of the stroke where the bike is slowing down.

I guess some who call themselves "scientists" believe that the universe is only 8,000 years old and that further research is necessary before they will believe the 14 billion year hypothesis. More "scientists" are probably in agreement that the age of the universe is a true controversy than will take your side in the above argument.
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Last edited by FrankDay; 02-01-13 at 04:09.
  #1056  
Old 02-01-13, 14:17
JayKosta JayKosta is offline
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Frank,

Perhaps we just 'need to agree to disagree' about this, but one last question.

With cranks similar to 'Power Cranks', what does the rider need to DO to regain producing torque with a crank arm if/when it rotates out of 180 degree alignment with the other crank arm?

My guess is there are 2 most likely answers -
1) apply more force to the trailing crank arm (by doing what).
or
2) increase the speed of the trailing crank arm to catch-up with the leading arm.

Jay Kosta
Endwell NY USA

Last edited by JayKosta; 02-01-13 at 15:24.
  #1057  
Old 02-01-13, 15:30
FrankDay FrankDay is offline
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Quote:
Originally Posted by JayKosta View Post
Frank,

Perhaps we just 'need to agree to disagree' about this, but one last question.

With cranks similar to 'Power Cranks', what does the rider need to DO to regain producing torque with a crank arm if/when it rotates out of 180 degree alignment with the other crank arm?

My guess is there are 2 most likely answers -
1) apply more force to the trailing crank arm (by doing what).
or
2) increase the speed of the trailing crank arm to catch-up with the leading arm.

A quote you might find useful -
"Some people believe they are thinking when they are just rearranging their prejudices".

Jay Kosta
Endwell NY USA
What does a person need to do when on PowerCranks to start applying torque when they fall out of synch (because they stopped applying torque). That is easy, all they need do is start moving the foot as fast as the other pedal. As soon as the foot is moving as fast as the other pedal which is applying torque it will lock up and the two cranks will remain in the same relative position to each other regardless of how much more torque is applied.

Then, the question you might ask is, if the cranks will remain in the same relative position when I am pushing forward on them how do they ever get to be 180º again? The answer to that is "Easy." All one does is keep the pressure off the one of the cranks allowing the cranks to fall further apart until they line up again and then start applying positive pressure and, voila, they are again locked up at 180º. The clutch will lock up whether one applies a lot or a little positive pressure. Sounds hard to get it just right but it is easy. Most people do this by coasting with one leg at the bottom while pedaling with the other let and then starting pedaling with the coasting leg as the power leg comes over the top. (Edit: It doesn't matter how much positive force is on the right and left pedals or how much difference there is between the right and left pedals, as long as both pedals have a positive force on them the pedals will be moving at the same speed and the cranks will stay in synch.)

This is all pretty simple physics/mechanics stuff. This is basically a conservation of momentum thing. If momentum changes in a system energy must either come into or leave a system. Looking at the bicycle I think it is a convention that we call positive forces as those that tend to speed the bike up and negative forces those that cause the bike to lose speed. Wind resistance and rolling resistance are always negative. Forces due to gravity can be either negative or positive (depending upon whether one is going up or down a hill). On a perfect bicycle with a perfect (no friction) free wheel forces from the pedal that drive the bike can only be positive and the conservation of momentum says the pedals will not slow down unless there is a negative force applied to them. The pedals will not slow simply because the positive pressure is reduced, they can only slow if there is another force larger than the positive pedal force slowing them. So, the bike doesn't slow because the pedals slow but the bike slows because the forces slowing the bike (wind, rolling, gravity) are larger than the forces trying to increase bike speed (pedals, gravity). When the bike slows the pedals slow and when the bike speeds up the pedals speed up because they are all tied together via the chain. It is that simple.
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Last edited by FrankDay; 02-01-13 at 17:47.
  #1058  
Old 02-01-13, 17:52
JayKosta JayKosta is offline
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Quote:
Originally Posted by FrankDay View Post
What does a person need to do when on PowerCranks to start applying torque when they fall out of synch (because they stopped applying torque). That is easy, all they need do is start moving the foot as fast as the other pedal. As soon as the foot is moving as fast as the other pedal which is applying torque it will lock up and the two cranks will remain in the same relative position to each other regardless of how much more torque is applied.
...
================================================== ==
Yes, I agree with this

"... all they need do is start moving the foot as fast as the other pedal. As soon as the foot is moving as fast as the other pedal which is applying torque it will lock up ..."

I had a misunderstanding about how the PC operated - I thought that torque could only be applied to both crank arms simultaneously when the arms were at 180 degrees. From your explanation, simultaneous torque can be applied at any crank arm angle as long the the crank arms are rotating at the same speed. Is there a way for the rider to know when the arms are at 180 degrees?
Thanks for the clarification about how PC operate.

Jay Kosta
Endwell NY USA
  #1059  
Old 02-01-13, 18:06
FrankDay FrankDay is offline
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Quote:
Originally Posted by JayKosta View Post
================================================== ==
Yes, I agree with this

"... all they need do is start moving the foot as fast as the other pedal. As soon as the foot is moving as fast as the other pedal which is applying torque it will lock up ..."

I had a misunderstanding about how the PC operated - I thought that torque could only be applied to both crank arms simultaneously when the arms were at 180 degrees. From your explanation, simultaneous torque can be applied at any crank arm angle as long the the crank arms are rotating at the same speed. Is there a way for the rider to know when the arms are at 180 degrees?
Thanks for the clarification about how PC operate.

Jay Kosta
Endwell NY USA
Yes. the rider knows when the cranks are at 180º when the timing of the knees coming up is 50-50. If off just a little bit the timing turns into a "gallop". Even the smallest "failure" is quickly noticed as the cranks fall apart. If not very close to 50-50 people don't like the feel and fix it quickly. (Edit: the only time you can have "failures" and not notice it is when the failures are repetitive and bilateral. that is say each leg falls 5º behind on each upstroke. Because each leg is doing the same thing it won't be felt because the timing will still feel 50-50. This is very unlikely as it is extremely unusual for riders to have perfectly balanced legs.)

You aren't the first person to not understand how the cranks work.
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Last edited by FrankDay; 02-01-13 at 18:12.
  #1060  
Old 02-01-13, 20:30
coapman coapman is offline
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Originally Posted by FrankDay View Post

Conclusions: DC size is a pedaling technique parameter that is closely related to energy consumption. To generate power evenly around the whole pedal, revolution may be an important energy-saving trait."

Why is DC size closely related to energy consumption and what's the point in making that statement about generating power evenly when everyone knows it cannot be done.
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