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PostPosted: Wed Jun 19, 2013 2:13 am 
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Same issue? Climbing when the temp changed or something else related to climbing?


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PostPosted: Wed Jun 19, 2013 3:13 am 
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Both. The temperature changes and the rider never coasts. On rolling terrain there's usually sufficiently frequent coasting that it can auto-zero. When climbing that may not happen for an hour, which allows the temperature change to cause the zero to become invalid.

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Posted: Wed Jun 19, 2013 3:13 am 


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PostPosted: Wed Jun 19, 2013 4:10 am 
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djconnel wrote:
Both. The temperature changes and the rider never coasts. On rolling terrain there's usually sufficiently frequent coasting that it can auto-zero. When climbing that may not happen for an hour, which allows the temperature change to cause the zero to become invalid.



This is your impression based off numbers you expected to see but they were actually different, or you tested this somehow?


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PostPosted: Wed Jun 19, 2013 4:18 am 
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Actually with a Quarq I thought that you have to pedal backwards, not just coast. Honestly that would drive me nuts even on rides where you can easily do it.


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PostPosted: Wed Jun 19, 2013 7:14 am 
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styrrell wrote:
Same issue? Climbing when the temp changed or something else related to climbing?


Never thought of the drift as anything other than tiredness or cardiac drift. But at the bottom of a descent it would give ridiculous wattage figures and wouldn't correct itself for periods despite back pedalling (how many of us really want to backpedal when attacking off a descent?).


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PostPosted: Wed Jun 19, 2013 8:00 am 
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In light of the above findings I thought I'd delve into some of the files from my use of the Quarq in the mountains.

Here is a typical 20 min segment up an alpine col. The average gradient is 9.3%.

Image

Note the decreasing power line (yellow) and increasing HR line (red). I had taken these to be a sign of weakness and riding by power really was dispiriting. Closer inspection potentially reveals a different story.

The first half of the segment is shown below. Note the ave pwr and VAM. Even here we can see the power reading steadily decreasing.

Image

Now for the second segment. Power output almost 10% down overall and HR up. Could I put that down to altitude gain and fatigue? Yes but VAM is actually up by 5%. Now Ave Gradient was also up by 5% so working on Dr Ferrari's calculations, it should have been up by less than 2% for the same power output.

Therefore the conclusion I would draw would be that power output in the second section was actually likely to be higher than in the first (as suggested by increased HR), rather than 10% down as suggested by the Quarq's readings. I'm sure DJ can point out if I am wrong in this assumption - either way, a 10% decrease is looking highly suspicious to me. Hope some find this useful.

Image


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PostPosted: Wed Jun 19, 2013 9:16 am 
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@airwise: a good way to see what's going on is to use a steep long climb without wind to verify the power output. It is very important to know the exact system weight at the time, to have no wind, and to use a barometric altimeter or precisely measured course. I say steep and long because if you use a short climb (say 1km) any small errors in altitude data weigh heavily. Similarly on climbs that aren't very steep (say less than 7%) wind resistance takes a bigger role.

If your unit is drifting heavily then I would expect that the predicted and actual power start differing heavily. If this is not an issue they should be close. A word of caution, though: it is difficult to get valid data because of the limitations I mentioned above.

Best
Nicolas

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PostPosted: Wed Jun 19, 2013 9:21 am 
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The climb gradient also needs to be very consistent in order to use VAM as a means of inferring power output.


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PostPosted: Wed Jun 19, 2013 9:22 am 
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As this thread is titled "new powermeter rumors" I'll provide the news here - I hope the mods will be ok with it (otherwise I will delete it): we have a new crank in our range - the FSA K-Force light. Since this is weight weenies I thought it might be of interest. It's 196g lighter than the FSA Gossamer and 90g lighter than the Rotor 3D, coming to 623g with sensor (compact) / 662g (130 BCD).

The crank is specially produced for us by FSA.

Cheers and happy riding!

Nicolas

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PostPosted: Wed Jun 19, 2013 9:42 am 
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jever98 wrote:
@airwise: a good way to see what's going on is to use a steep long climb without wind to verify the power output. It is very important to know the exact system weight at the time,


Check

Quote:
to have no wind


Check - as much as possible. Slow speed. No headwind to speak of.

Quote:
, and to use a barometric altimeter


Check

Quote:
or precisely measured course. I say steep and long because if you use a short climb (say 1km) any small errors in altitude data weigh heavily. Similarly on climbs that aren't very steep (say less than 7%) wind resistance takes a bigger role.


4km at over 9%. I'd say steep and long so check.

Quote:
If your unit is drifting heavily then I would expect that the predicted and actual power start differing heavily. If this is not an issue they should be close. A word of caution, though: it is difficult to get valid data because of the limitations I mentioned above.

Best
Nicolas


So we are simply supposed to "trust" our power meters are we? I ask because there is a lot of evidence to suggest that many drift enough to make the measurements fairly meaningless - certainly for the purpose of pacing a climb.

maquisard wrote:
The climb gradient also needs to be very consistent in order to use VAM as a means of inferring power output.


Within reason - if averaged over a long enough period these things even out.


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PostPosted: Wed Jun 19, 2013 9:49 am 
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@airwise: I don't mean to criticize your approach to check for drift, I just wanted to give you a pointer how you can quantify the drift beyond looking at the graphs, and a word of caution when you run this kind of approach. The calculation of predicted power using a formula gives you a reference to check your power meter against. I'm very much for verifying the accuracy of power meters (we do it many many times in different ways in our R&D and when checking units) and I agree that drift is not a nice thing, because it undermines trust in your data and hence throws up questions like "did I really perform well that day or was it a data glitch".

This is, incidentally, why our PMs are temperature compensated. I won't elaborate because I don't want to be thrown off the forum for illegal advertising :).

Cheers
nicolas

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PostPosted: Wed Jun 19, 2013 9:54 am 
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@airwise: what sort of variance do you see in your manual zero-offset numbers from day to day?


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PostPosted: Wed Jun 19, 2013 10:06 am 
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jever98 wrote:
@airwise: I don't mean to criticize your approach to check for drift, I just wanted to give you a pointer how you can quantify the drift beyond looking at the graphs,
Cheers
nicolas


No offence taken Nicolas. I posted because the results correspond surprisingly closely with those of DC Rainmaker's vis a vis the Quarq.

FWIW, I use SRM and Powertap because I get virtually no evidence of drift under similar conditions. But I'm increasingly going back to HR and VAM when in the mountains as a reliable metric for performance data.


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PostPosted: Wed Jun 19, 2013 3:39 pm 
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jever98 wrote:
@airwise: a good way to see what's going on is to use a steep long climb without wind to verify the power output. It is very important to know the exact system weight at the time, to have no wind, and to use a barometric altimeter or precisely measured course. I say steep and long because if you use a short climb (say 1km) any small errors in altitude data weigh heavily. Similarly on climbs that aren't very steep (say less than 7%) wind resistance takes a bigger role.

If your unit is drifting heavily then I would expect that the predicted and actual power start differing heavily. If this is not an issue they should be close. A word of caution, though: it is difficult to get valid data because of the limitations I mentioned above.

Best
Nicolas

It's too bad no one has come up with a better way to check power meter drift in the field.


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Posted: Wed Jun 19, 2013 3:39 pm 


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PostPosted: Wed Jun 19, 2013 11:51 pm 
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I've been on a new Sram Red Exogram Quarq for about 2 months now and have seen no evidence of drift on climbs or otherwise. I live in SLC and do many HC climbs in the area ranging from 3-15mi in length and some with ave of 7-8% over 5-7 miles.

Maybe it's because it's a newer model, or it's brand new overall. I don't know. Just reporting what I see. Also, I don't find it too much of a pain to zero the quarq by back spinning a few times in 4-5hr rides. I can spare the 12 total seconds combined. But time is money, so maybe it's draining others earning potential.


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