General Discussion Triathlon Talk » relationship between watts and speed Rss Feed  
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2015-07-25 5:02 PM

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Subject: relationship between watts and speed
This may be a dumb question but I am curios. Lets assume a reasonably flat course and the pace options can be sustained. You can ride it one of three ways.

1. Gear A - 100 rpm
2. Gear B, one gear higher - 90 rpm
3. Gear C, one more gear higher - 80 rpm

If all three produce the same watts, would there be any difference in speed? If yes, which would be faster?

Thanks in advance.



Edited by Stuartap 2015-07-25 5:03 PM


2015-07-25 5:13 PM
in reply to: Stuartap

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Subject: RE: relationship between watts and speed
The drivetrain will be slightly more efficient when the chain has a larger radius so if all three cadences are on the same chainring, the higher cadence will be slightly faster. However, the differences are so small that in practice, there would likely be no measurable difference.

Shane
2015-07-26 7:34 AM
in reply to: gsmacleod

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Subject: RE: relationship between watts and speed
The watts being applied to the rear hub are the same despite the cadence of the front chain ring. The speed is the same.
2015-07-26 7:49 AM
in reply to: Calvin386

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Subject: RE: relationship between watts and speed
Originally posted by Calvin386

The watts being applied to the rear hub are the same despite the cadence of the front chain ring. The speed is the same.


Assuming you are measuring power at the hub, you are correct.

Shane
2015-07-26 4:01 PM
in reply to: Stuartap


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Subject: RE: relationship between watts and speed
Same speed but they feel quite different. 100 rpm's tends to put less stress on the legs but more on the lungs. 80 rpm's is the exact opposite. It's important to know when is the right time to use each.
2015-07-26 5:40 PM
in reply to: ziggie204

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Subject: RE: relationship between watts and speed
Originally posted by ziggie204

Same speed but they feel quite different. 100 rpm's tends to put less stress on the legs but more on the lungs. 80 rpm's is the exact opposite. It's important to know when is the right time to use each.

This is the issue that prompted the question. I joined a cycling group that I ride with on Saturdays. Most of them are faster than me so it has been good for me. But I have noticed that they all tend to ride a gear lower but at a higher cadence. I find it easier to keep pace with a higher gear and lower cadence.

I think you hit on the issue. I get gassed pretty quickly trying to hang at their cadence. I do pretty well on the flats and rollers. It doesn't help that I still have at least 10# to lose.

I can say that riding with them just once a week for the past three months has made me a stronger rider. I will keep at it until I can keep up with them on the hills, my nemesis.

I appreciate the feedback.


2015-07-26 7:35 PM
in reply to: Stuartap


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Subject: RE: relationship between watts and speed
Yup, it's a balancing act and that's why it's good to be able to ride a wide range of cadences. But one thing is certain, riding with them is going to make you stronger so keep at it.

2015-07-27 7:52 AM
in reply to: gsmacleod

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Subject: RE: relationship between watts and speed
Originally posted by gsmacleod

Originally posted by Calvin386

The watts being applied to the rear hub are the same despite the cadence of the front chain ring. The speed is the same.


Assuming you are measuring power at the hub, you are correct.

Shane


Agreed. I was assuming hub based power meter like I use. However, despite where you measure the watts (crank or hub), the speed is the same.
2015-07-27 8:36 AM
in reply to: gsmacleod

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Subject: RE: relationship between watts and speed
Originally posted by gsmacleod

The drivetrain will be slightly more efficient when the chain has a larger radius so if all three cadences are on the same chainring, the higher cadence will be slightly faster. However, the differences are so small that in practice, there would likely be no measurable difference.

Shane


Why?
2015-07-27 8:37 AM
in reply to: Calvin386

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Subject: RE: relationship between watts and speed
Originally posted by Calvin386

Agreed. I was assuming hub based power meter like I use. However, despite where you measure the watts (crank or hub), the speed is the same.


There are speed differences for a given power depending on where you measure power. If at the hub, that's basically power delivered by rider to road while if you measure at the hub, you have power delivered to the draintrain and therefore draintrain losses must be considered.

Shane
2015-07-27 8:38 AM
in reply to: mike761

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Subject: RE: relationship between watts and speed
Originally posted by mike761

Why?


Why no measurable difference or why more efficient with larger chainrings/cogs?

Shane


2015-07-27 9:50 AM
in reply to: gsmacleod

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Subject: RE: relationship between watts and speed
Originally posted by gsmacleod

Originally posted by mike761

Why?


Why no measurable difference or why more efficient with larger chainrings/cogs?

Shane


Why are you saying larger chain rings are more efficient? I could see arguments both ways, but never did any calculations on it myself.
2015-07-27 10:15 AM
in reply to: mike761

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Subject: RE: relationship between watts and speed
Originally posted by mike761

Why are you saying larger chain rings are more efficient? I could see arguments both ways, but never did any calculations on it myself.


I forget where I read it but I recall reading it several years ago - the the small the diameter of the chainring/cog, the more losses there would be. However, the losses would be minor and not really a consideration worth worrying about.

As to why, I suspect that it is simply due to the chain becoming more circular the larger the radius of the circle.

Shane
2015-07-27 10:34 AM
in reply to: gsmacleod

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Subject: RE: relationship between watts and speed
Originally posted by gsmacleod

Originally posted by mike761

Why are you saying larger chain rings are more efficient? I could see arguments both ways, but never did any calculations on it myself.


I forget where I read it but I recall reading it several years ago - the the small the diameter of the chainring/cog, the more losses there would be. However, the losses would be minor and not really a consideration worth worrying about.

As to why, I suspect that it is simply due to the chain becoming more circular the larger the radius of the circle.

Shane


I realize the difference would be very minimal. Just curious when you made the statement.
I see the difference as the frictional forces of the chain against more teeth in a larger gear vs the frictional forces of the chain against itself as it flexes more when on a smaller chain. Not sure which one it really is, and if it is consistently the same or do they vary non-linear as the cross chaining goes up? Which one of these forces are greater with more cross chaining?
From gear data I have seen in the past it would suggest that smaller gears are slightly more efficient but this was with fixed sets of gears that are well aligned like motor driven equipment, unlike what we have on a bike.

Really just an academic question, I would like to read any good testing on the subject.
2015-07-27 10:42 AM
in reply to: mike761

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Subject: RE: relationship between watts and speed
Originally posted by mike761

I realize the difference would be very minimal. Just curious when you made the statement.
I see the difference as the frictional forces of the chain against more teeth in a larger gear vs the frictional forces of the chain against itself as it flexes more when on a smaller chain. Not sure which one it really is, and if it is consistently the same or do they vary non-linear as the cross chaining goes up? Which one of these forces are greater with more cross chaining?
From gear data I have seen in the past it would suggest that smaller gears are slightly more efficient but this was with fixed sets of gears that are well aligned like motor driven equipment, unlike what we have on a bike.

Really just an academic question, I would like to read any good testing on the subject.


This isn't where I originally read the information but here a post that talks about it:

http://www.cyclingpowerlab.com/DrivetrainEfficiency.aspx

Shane
2015-07-27 2:36 PM
in reply to: gsmacleod

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Subject: RE: relationship between watts and speed
Shane that was a very informative article. Thanks for posting it.



2015-07-27 3:07 PM
in reply to: gsmacleod

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Subject: RE: relationship between watts and speed
Originally posted by gsmacleod

Originally posted by mike761

I realize the difference would be very minimal. Just curious when you made the statement.
I see the difference as the frictional forces of the chain against more teeth in a larger gear vs the frictional forces of the chain against itself as it flexes more when on a smaller chain. Not sure which one it really is, and if it is consistently the same or do they vary non-linear as the cross chaining goes up? Which one of these forces are greater with more cross chaining?
From gear data I have seen in the past it would suggest that smaller gears are slightly more efficient but this was with fixed sets of gears that are well aligned like motor driven equipment, unlike what we have on a bike.

Really just an academic question, I would like to read any good testing on the subject.


This isn't where I originally read the information but here a post that talks about it:

http://www.cyclingpowerlab.com/DrivetrainEfficiency.aspx

Shane


So they basically are saying that the chain friction is greater than the friction of the chain engaging with addition teeth, therefore being in a larger diameter gear is more efficient. So I believe that. They also say that cross chaining has huge amount of frictional force, and on a bike we are almost always cross chaining to some extent. I would think that the most efficient gear ratio is actually whatever lines up the best. So if you 13 lines up with you 53 better than the 15 does than the 13 would most likely be more efficient. The alignment of gears would be different for each bike
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