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Wheel vs Pedal/Crank Power Readings

 
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JaRok2300




Joined: 01 May 2014
Posts: 377
Location: Worcester, UK

PostPosted: Fri May 11, 2018 2:44 pm    Post subject: Wheel vs Pedal/Crank Power Readings Reply with quote

I've been using a Powertap wheel for the last year or so but recently picked up a P2M crank based set-up so I could use any wheels on my TT bike. The Powertap will now permanently reside on my road bike.

I'm getting slightly higher readings from the P2M than equivalent sessions/HR on the Powertap and wondered how much of that might be down to losses in the drivetrain or just accuracy issues between them.

I intend to fit them both on the TT bike at some point and record the same ride for comparison purposes but wondered if anyone had a similar set-up or done a similar test.

Any idea of a rule of thumb for expected differences given their relative positions in the drivetrain?
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kevb




Joined: 11 Aug 2010
Posts: 789

PostPosted: Fri May 11, 2018 2:48 pm    Post subject: Reply with quote

Have a look at the DCRainmaker site, some great comparisons and graphs of power meters against each other. he always rides with about 3 on his bike to be able to directly compare.
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iwaters




Joined: 06 Sep 2016
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PostPosted: Sun May 13, 2018 12:28 pm    Post subject: Reply with quote

I think, and I am no expert, but the hub should already be accounting for drive train loss. Unless your chain is filthy or very stretched I don't think you should be getting different readings.

Depending on what model P2M you have but the advertised accuracy values are different. +/- 1.5% for the Powertap and either +/- 1 or +/- 2%

Also you said you haven't tried them at the same time and are comparing HR values and watts. That's not a good test for something so accurate. Also are the tyres different on the wheels? Differing CRR values are going to affect your outputs.
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TriSam




Joined: 26 Aug 2011
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Location: Tunbridge Wells

PostPosted: Mon May 14, 2018 8:22 am    Post subject: Reply with quote

My P2M is quite skewed by oval chainrings, something to think about if you also use those
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JaRok2300




Joined: 01 May 2014
Posts: 377
Location: Worcester, UK

PostPosted: Mon May 14, 2018 8:49 am    Post subject: Reply with quote

Thanks for the feedback. The oval rings comment in particular is interesting as I've never used them before but got a set with the P2M and Rotor crankset so I am running them at the moment.

I did a short ride yesterday with both PMs. Powertap linked to my old 705 and P2M to my Fenix 2.

I'll try and work out how I can put both files on the same graph but in the meantime I did a basic comparison using Strava segments and the overall AP & NP. Given both meters could be 1/2% out so upto 4% if those conspire against each other the results were about where I'd expect. The P2M was slightly higher most of the time which I half expected. DC Rainmaker does mention this in some of his tests/comparisons due to being further up the drivetrain compared to the Powertap wheel.

The exception were the last 2 segments I passed though where the variance went up to nearly 10%. No idea why that should be and makes me wonder if I should try again with a longer ride to see if they continue to diverge.

I'll try and post the graph from my phone as can't do that on this PC.
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JaRok2300




Joined: 01 May 2014
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Location: Worcester, UK

PostPosted: Mon May 14, 2018 9:14 am    Post subject: Reply with quote

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moonmonkey02




Joined: 30 Sep 2008
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PostPosted: Mon May 14, 2018 1:45 pm    Post subject: Reply with quote

Powertap/ hub should read lower than crank based pm all things being equal.

Power in (crank) v power out (hub) due to drive chain loss: often quoted around 10w.

I use a a PT and 4iiii and I see this: regularly do rides with both pm's and the 4iiii consistently reads higher.

Not a problem if you know the difference: the 4iiii has the ability via their app to make adjustments to the power reading either higher or lower.
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JaRok2300




Joined: 01 May 2014
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Location: Worcester, UK

PostPosted: Tue May 15, 2018 8:15 pm    Post subject: Reply with quote



Not really sure what, if anything, this is telling me (other than I have too much time on my hands) but shows some differences between the 2 meters under certain circumstances, although they track pretty close the majority of the time.
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Chrace




Joined: 28 Apr 2010
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Location: Eating a Yorkie

PostPosted: Wed May 16, 2018 12:14 pm    Post subject: Reply with quote

Guess the question is, if you are only riding with one are they close enough to each other for the reading to be usable? If you train on one and race on the other, is the data usable?

I can't ride consistently within 5w (or 10w) even on the flat so variance wouldn't matter unless they are way out.

Otherwise you're probably just curious and/or have too much time on your hands. Which is fine as well of course. Smile
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TriSam




Joined: 26 Aug 2011
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Location: Tunbridge Wells

PostPosted: Wed May 16, 2018 1:07 pm    Post subject: Reply with quote

JaRok2300 wrote:


Not really sure what, if anything, this is telling me (other than I have too much time on my hands) but shows some differences between the 2 meters under certain circumstances, although they track pretty close the majority of the time.


Good images, shows the tracking nicely. Might be worth doing some very gentle/very hard efforts to see if the tracking goes awry at different levels of stree
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JaRok2300




Joined: 01 May 2014
Posts: 377
Location: Worcester, UK

PostPosted: Wed May 16, 2018 1:23 pm    Post subject: Reply with quote

More curiosity really, at my level I'm happy enough to take them at face value as they're clearly not miles apart which was what the exercise was to establish.

Now to find a suitable road and do some aero testing. Gotta be better than losing weight and training harder!
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