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PostPosted: Tue Aug 13, 2013 2:29 am 
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Posts: 126
I always read that powertap measures lower than srm because of drivetrain losses . I thought that doesn't make sense as the hub and crank are always connected by the chain. So the srm should see the same drivetrain losses.
Here's my analogy.
Say you hooked up a dynometer to the front of an engine then disconnected or pushed in clutch and measured horsepower. You would get the power of the engine. Then you jacked up car in air put it in gear and tested again so you'd add the drivetrain losses. But on a bike like we where saying everything is always connected so it doesn't matter where it is measured along the drivetrain.


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PostPosted: Tue Aug 13, 2013 2:49 am 
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Just to preface I'm looking for someone to prove me wrong as I Definately might be. Thought it might be an interesting topic.


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Posted: Tue Aug 13, 2013 2:49 am 


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PostPosted: Tue Aug 13, 2013 8:33 am 
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Location: France
Unfortunately you are incorrect.

The crank and hub may be connected by a chain but there are losses in the drive train, i.e. stress in the chain rivets and losses through heat, sound etc. These losses must be over come by the power generated at the crank and so a hub based powermeter will read lower than the power generated at the crank.


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PostPosted: Tue Aug 13, 2013 1:48 pm 
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I think it is correct that there are losses between the crank and the rear hub.
But I don't think they are very much. I suspect it would be lower than the accuracy of either unit.


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PostPosted: Tue Aug 13, 2013 2:07 pm 
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Location: Shetland, Scotland
5W - 10W in transmission (chain) losses?
http://www.friction-facts.com/ultrafast-overview


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PostPosted: Tue Aug 13, 2013 2:58 pm 
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Ah I kept thinking about it and finally came to the conclusion that even though they are linked by a chain. The location of the powermeter is the difference. Everything ahead of powermeter although experienced isn't measured. So for the srm chainrings its measuring everything after the spider. So this would include chain friction. But it would not include pedal bearings or bottom bracket bearings as they are ahead of the spider so if those frictions went up although you would feel it your srm wouldn't be measuring it. Just took me awhile to get it clear in my head.


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PostPosted: Thu Sep 05, 2013 2:25 am 
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I have a quarq and was wondering which way is the best to calibrate the meter? Spinning the crank backwards 5 revolutions or calibrating it from the Garmin?


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PostPosted: Thu Sep 05, 2013 2:31 am 
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Location: Haines, AK - Temporarily
*Best* is through the Garmin. IMO, the question is whether the backpedal is *good enough*.


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PostPosted: Thu Sep 05, 2013 10:24 pm 
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brianwelchde wrote:
I have a quarq and was wondering which way is the best to calibrate the meter? Spinning the crank backwards 5 revolutions or calibrating it from the Garmin?
Agreed with the above and add that neither are actually a calibration, they are setting zero offset. In order to calibrate it you need a very precisely known weight and the ability to connect the "qalvin" android or iphone to your powermeter, or send it back to Quarq.


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PostPosted: Wed Oct 02, 2013 1:55 pm 
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I may be wrong, but I assume that the farther a power measuring device is mechanically from the source of the power will introduce variables such as flex, friction, heat,etc... that will make accuracy more difficult to obtain.


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PostPosted: Mon Dec 16, 2013 10:04 pm 
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Location: Ann Arbor, MI
DC Rainmaker typically sees a loss of 5-10W in his tests. I trust the guy as a guru.


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PostPosted: Tue Jan 21, 2014 3:49 am 
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Accuracy may suffer but you only need precision for proper training.


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PostPosted: Tue Jan 21, 2014 10:50 am 
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Location: Urbana, Illinois
brianwelchde wrote:
I have a quarq and was wondering which way is the best to calibrate the meter? Spinning the crank backwards 5 revolutions or calibrating it from the Garmin?


Answer part 2.

Pedaling backwards or using the Garmin is zeroing the power meter not calibrating it. Zeroing it brings the meter back to zero from drift. Calibrating is tuning the meter to show a given value against a known value of torque on the meter.

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Posted: Tue Jan 21, 2014 10:50 am 


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