Beginners Power Meter questions

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Beginners Power Meter questions

Postby nickobec » Fri Mar 25, 2011 7:01 pm

So I went an bought a PowerTap Elite+ hub (actually prebuilt wheel) paired it to my Garmin 705 (real easy) and have started collecting data.

I also purchased "Training and Racing with a Power Meter" 2nd edition by Hunter Allen & Andrew Coggan on recommendation by Alex Simmons in another post. Can not recommend this book highly enough. It just makes so much sense to me.

My main question is displayed the "power" data on the Garmin as mentioned by Alex in another post:

Alex Simmons/RST wrote:1. on the road, power output is highly variable, and is the first thing people notice when they see it on a bike computer for the first time (or when they inspect a power meter file)

2. there is a mathematical artefact of the way the Powertap calculates data, which exacerbates the variability in the second to second data. It isn't an accuracy problem (on average). The issue is caused by the fixed duration sampling of the torque data, which results in an effect known as "aliasing". It is more noticeable at certain cadences over others. I can explain more if anyone is interested.

3. to help reduce the "signal noise", it is useful to set the display to show a rolling average power, usually something like a 5-second rolling average tends to reduce the variability in the second to second data somewhat. over time, if you are using the meter to guide say an interval effort on the trainer, then you get pretty good at doing a running integral in your head and not concern yourself with the ups and down of the more instantaneous data but rather focus on the overall effort level.


The Garmin 705 gives you three options:
1: Current Power
2: Power averaged over 3 secs
3: Power averaged over 30 secs

Current Power as Alex said, bounces all over the place one second 200w next 400w.

Averaged over 3 seconds is not much better still bounces up and down.

The next time I ride, I am going to try 30 sec average, because I want to concentrate on overall effort and not what power I am generating right now. I don't want to know how much a power I am generating during a fifteen section sprint, otherwise I am not working hard enough. I rather try and keep riding in a certain range over a much longer time span.

So how do other people manage this?
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by BNA » Fri Mar 25, 2011 7:13 pm

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Re: Beginners Power Meter questions

Postby toolonglegs » Fri Mar 25, 2011 7:13 pm

You can start an interval (say 5 minutes) and have the average power showing just for that interval...but then you wouldn't know exactly what you were doing at that exact moment.I prefered keeping it on 1 sec...yes it jumps around but you get to grips with it pretty quickly...IMO 30 secs display would be pointless...hit a slight rise and go from 300w to 360w and not see a change in your average till 30 secs later?.
Best for you maybe is to keep it on current...but push the interval button at the start of a 15 minute (for example) interval...then every so often flick across to average power for that interval then back to average...then you are looking at average for the interval and not the whole ride.
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Re: Beginners Power Meter questions

Postby nickobec » Fri Mar 25, 2011 7:24 pm

toolonglegs wrote:hit a slight rise and go from 300w to 360w and not see a change in your average till 30 secs later?.

What would happen is over next 30 seconds (assuming flat unchanging power) it would read 302w, 304w, 306w, 308w, 310w etc until you hit 360w.

Problem you crest the ride and start going downhill and you slack off and drop to 180w. The readings would go 354w, 348w, 342w etc until hitting 180w.

Can display both 3 & 30 second or 3sec and interval
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Re: Beginners Power Meter questions

Postby toolonglegs » Fri Mar 25, 2011 7:41 pm

True I am thinking powertap (old style) head...still I would be sticking with current power,it is only for the first week or so that it is strange.
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Re: Beginners Power Meter questions

Postby Alex Simmons/RST » Fri Mar 25, 2011 8:05 pm

Go with the 3-second rolling average for the display.

Keep the effort level smooth during intervals, and if you notice more often than not it's below target, then lift your effort just a little until it's mostly within the range you are looking for.

Likewise, if it's above the desired range more often than not, then a very gradual easing of effort, especially early on in an interval, or for the first of several intervals. Of course if you ultimately realise you can go harder (because you are fitter than last time), then it's OK to lift the power to a level you think you can sustain.

The difference in effort between a hard tempo, threshold and supra threshold effort can be quite subtle early on in an interval, but our ability to sustain such power levels varies considerable.

The trap to avoid is constantly moderately pedal pressure based on the instantaneous readout fluctuations and hence avoid playing a game of "chasing your power tail".
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Re: Beginners Power Meter questions

Postby Leeroy » Fri Apr 01, 2011 9:22 am

+1 - When I first started using power the biggest surprise was how "easy" a threshold of VO2 effort felt......for the first few minutes. Then the pain started :o :D

It was immediately clear to me that when I was previously going by HR and RPE alone I was initially going far too hard for shorter interval type efforts (and interestingly) probably not hard enough for longer efforts.
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