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Stupid Powermeter question
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Suppose you foolishly route your chain so that it goes on the wrong side of the guide between the two derailleur pulleys (significant additional drivetrain friction) Then you ride around and do a bunch of Chung tests with it like that. Does power loss show up:

1) On a crank-based powermeter (SRM)
2) A chainring powermeter (Powertap C)
3) A hub powermeter (Powertap)
4) None of the above -- Only your legs notice and you told them to shut up.

I think I know the answer. But I'm just checking. And yes, I was stupid enough to do what I described. That's the amazing thing about a waxed chain, you actually don't notice that much.

My latest book: "Out of the Melting Pot, Into the Fire" is on sale on Amazon and at other online and local booksellers
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Re: Stupid Powermeter question [jens] [ In reply to ]
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Based on what you said, the friction should be parasitic to the force applied by the chain to the rear casette/hub, so the crank and chain meters should pick up how much power your legs are putting out while the hub won't.

So while you can still pick up output power, you sadly can't go as fast :(

I like analyzing things - http://engineeringfitness.org
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Re: Stupid Powermeter question [jens] [ In reply to ]
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I did this once before a TT...I was surprised how fast I was for how little power put out. I was beginning to wonder if there was something wrong with the PowerTap...then I saw the chain...:)

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Re: Stupid Powermeter question [stumpyx13] [ In reply to ]
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stumpyx13 wrote:
Based on what you said, the friction should be parasitic to the force applied by the chain to the rear casette/hub, so the crank and chain meters should pick up how much power your legs are putting out while the hub won't.(


Well that's what I thought too a while back.

But when you really look at the system, force is applied from the top of the chainring through the top half of the chain to the cassette and from there to the hub. The derailleur is downstream from all that. IOW, the force has already been applied before it gets to the derailleur. The additional drag only occurs on the return.

So I would guess that all of the devices actually reflect the power generated by your legs. None of them reflect the extra derailleur drag. You only feel it in your legs.

My latest book: "Out of the Melting Pot, Into the Fire" is on sale on Amazon and at other online and local booksellers
Last edited by: jens: Aug 18, 17 17:35
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Re: Stupid Powermeter question [jens] [ In reply to ]
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I don't think I am clear on what you mean by "reflect the extra derailleur drag. Do you mean that the power meter should show a lower power (power to the wheel) or do you mean the power meter should show your actual exerted power (power from the leg).

To me, reflecting the extra drag means to show the actual power from your leg and count power not exerted to the wheel as well.


jens wrote:


So I would guess that all of the devices actually reflect the power generated by your legs. None of them reflect the extra derailleur drag. You only feel it in your legs.
Last edited by: bloodyshogun: Aug 18, 17 19:58
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Re: Stupid Powermeter question [bloodyshogun] [ In reply to ]
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bloodyshogun wrote:
I don't think I am clear on what you mean by "reflect the extra derailleur drag. Do you mean that the power meter should show a lower power (power to the wheel) or do you mean the power meter should show your actual exerted power (power from the leg).

To me, reflecting the extra drag means to show the actual power from your leg and count power not exerted to the wheel as well.


Yeah, I wasn't clear on that. My speculation is that all of the power-meters will show close to the actual power from your legs -- even the Powertap at the hub. They will not show the reduced power from the increased chain drag.

Suppose your legs generate 200 watts. The powermeters would read like this:

1) Pedal powermeter: 200 watts
2) Crank powermeter : 200 watts minus some pedal spindle flexion. So maybe 199.9 watts
3) Chainring powermeter: 200 watts minus pedal and crank flexion. Maybe 199.8 watts.
4) Hub powermeter: 200 watts minus pedal&crank flexion and minus friction from upper half of drivetrain (chainring to cassette). Maybe 195 watts.

I'm guessing the actual loss from the mis-routed chain was something like 10 watts. But you wouldn't know that from any of the powermeter readings. The only way to know is because of your slow speed or the extra perceived effort.

My latest book: "Out of the Melting Pot, Into the Fire" is on sale on Amazon and at other online and local booksellers
Last edited by: jens: Aug 18, 17 21:21
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Re: Stupid Powermeter question [jens] [ In reply to ]
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If you think about which path the chain takes (clockwise) you have to pull it through the derailleur before it reaches the cassette so I'm pretty sure the derailleur drag/friction shows up in the figures of the hub based power meter.
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Re: Stupid Powermeter question [jens] [ In reply to ]
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No, the power lost from the chain drag shows up on the crank and spider based meters only. Think of the chain as a rope in tug of war, you (the crank) are pulling against one other person the (cassette/freehub/wheel), suddenly another person (chain drag on the pulley plate tab) grabs the rope on the other end and starts pulling. You now have that additional force to overcome.

rich
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Re: Stupid Powermeter question [jens] [ In reply to ]
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The powermeters won't reveal your issue from their data alone.
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Re: Stupid Powermeter question [jens] [ In reply to ]
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jens wrote:
stumpyx13 wrote:
Based on what you said, the friction should be parasitic to the force applied by the chain to the rear casette/hub, so the crank and chain meters should pick up how much power your legs are putting out while the hub won't.(


Well that's what I thought too a while back.

But when you really look at the system, force is applied from the top of the chainring through the top half of the chain to the cassette and from there to the hub. The derailleur is downstream from all that. IOW, the force has already been applied before it gets to the derailleur. The additional drag only occurs on the return.

So I would guess that all of the devices actually reflect the power generated by your legs. None of them reflect the extra derailleur drag. You only feel it in your legs.

Stumpy 1
jens 0

Everything but the hub based shows the power to rotate the chain.
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Re: Stupid Powermeter question [lyrrad] [ In reply to ]
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lyrrad wrote:

Stumpy 1
jens 0

Everything but the hub based shows the power to rotate the chain.

I didn't know we were keeping score. ;-) The important thing to me is that my painstaking Chung test is still valid (based on hub reading) despite the significant additional drag.

My latest book: "Out of the Melting Pot, Into the Fire" is on sale on Amazon and at other online and local booksellers
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Re: Stupid Powermeter question [jens] [ In reply to ]
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Should be ok, with a Powertap hub. After all they're the only ones where Eta=1. :)

Developing aero, fit and other fun stuff at Red is Faster
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