Newton Compared to Power Tap

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ldmitruk
Posts: 78
Joined: Tue Jul 08, 2008 12:20 pm

Newton Compared to Power Tap

Post by ldmitruk »

Hi,

I've gone through calibrating my Newton+ with a power tap and I'm interested in interpreting the data available with the power meter comparison tool. The power tap is a loner and I might not have access to it all the time. Anyhow here's a sample from a ride I did last night:

IBIKE STATS
------------------------------------------------------------------------

Ride: Distance: 65.19 km
Duration: 2:07:03
Start Time: 28/06/2012 5:13:42 PM
Avg Temper: 25 degC
Start Pres: 1007.2 mbar
Avg CdA: 0.364 m^2
Crr: 0.00400
Speed: Avg: 30.78 km/h
Max: 50.5 km/h
Wind: Avg: 27.97 km/h (dist-based)
Max: 43.5 km/h
wrt Road: Min: -26.2 km/h (- is tail; + is head)
wrt Road: Avg: -4.34 km/h (dist-based)
wrt Road: Max: 11.1 km/h
Slope: Min: -7.03%
Avg: 0.00% (dist-based)
Max: 5.48%
Elevation: Min: 660 m
Avg: 710 m
Max: 766 m
Cadence: Avg: 86.3 rpm (ignoring zeros)
Max: 115 rpm
HR: Avg: 133.6 bpm (ignoring zeros)
Max: 164 bpm

------------------------------------------------------------------------
IBIKE COMPARED TO ANALYTICCYCLING.COM
------------------------------------------------------------------------
Plug the following parameters(*) into the static forces calculator at
http://www.analyticcycling.com/ForcesPower_Page.html

Effective Frontal Area(#) = 0.729 m^2
Drag Coef (keep the default) = 0.5
Air Density = 1.084 kg/m^3
Weight of Rider and Bike = 100.2 kg
Coef of Rolling Resistance = 0.0040
Slope = 0.00002 rise/run
Speed = 8.551 m/s
Pedal Cadence = 81 rev/min
Crank Length(**)
Effective Pedaling Range(**)

Compare the avg power result from Analytic Cycling to the comparable
result from the iBike 3 calculation engine:

Propulsion Power = 157.26 W <---<<< compare to AnalyticCycling(##)
Rider Power = 160.47 W

* The AnalyticCycling calculator assumes perfectly calm wind
conditions. Further, it does not account for any accelerations or
decelerations.
# (Drag Coefficient) x (Effective Frontal Area) = CdA
** Crank Length and Eff Pedaling Range do not factor into the avg
power calculation
## The AnalyticCycling calculator ignores drive train losses.

------------------------------------------------------------------------
POWER METERS COMPARED TO EQUATIONS OF MOTION
------------------------------------------------------------------------
Ride File Min Pow Avg Pow Max Pow
AnalyticCycling.com 154.24 160.47 163.67
Speed Elev 0.00 186.74 792.95
Speed Wind Elev 0.00 153.72 640.23
Speed Wind Slope 0.00 152.96 743.76
Newton+ 0.00 154.03 1041.81
PT ANT+Sport (iB DFP 0.00 191.15 869.00

------------------------------------------------------------------------
R^2
------------------------------------------------------------------------
| Newton+|PT ANT+Sp
Newton+| 1.000| 0.550
PT ANT+Sport (iB | 0.588| 1.000

R^2 calculation is row w.r.t. column.

As you can see there is there are some differences between the power reading of the two especially at the high end. Also what numbers should I be looking for in the R^2 values? My assumption is they should be a lot closer if the calibration of the Newton is good.

From my calibration ride I have the following values:
Cm 1.02
Aero: 0.272
Wind Scale: 0.751
Cda: 0.362
Friction: 9.967
Riding Tilt: -0.6
Crr: 0.0040

I've also attached the calibration file.

Comments and suggestions as to how I can adjust thing to get my Newton readings closer to the power tap reading would be greatly appriciated.

I would like to suggest the software documentation could stand a major update. The file provided is for version 5.x released in 2010 and for a number of the tools and analysis options the documentation is very thin or non-exsitant. I understand a lot of effort has been put into the new hardware but out data documentation for the analysis software is really frustrating and makes it hard to get the most out of the device.

Cheers!
Attachments
IbikeCal20120626.ibp
(446 Bytes) Downloaded 257 times
Velocomp
Velocomp CEO
Posts: 7793
Joined: Fri Jan 11, 2008 8:43 am

Re: Newton Compared to Power Tap

Post by Velocomp »

Please post both your iBike file and your PT file. Note that, if your PT is wireless, your DFPM information should be in your iBike file.
John Hamann
ldmitruk
Posts: 78
Joined: Tue Jul 08, 2008 12:20 pm

Re: Newton Compared to Power Tap

Post by ldmitruk »

Here's the ride file.
Attachments
iBike_06_28_2012_1713_65_km.ibr
(799.53 KiB) Downloaded 269 times
Velocomp
Velocomp CEO
Posts: 7793
Joined: Fri Jan 11, 2008 8:43 am

Re: Newton Compared to Power Tap

Post by Velocomp »

I looked at your ride file. Notice that there are blue "icicles" consistently below your white speed. This is a strong indicator that the Cal Ride was "off".

In fact, using the Analyze/Check Calibration, and assuming that average wind speed was zero for the total ride, your calibration is way off.

Applying the recommendation of the Check Calibration, you'll see there is excellent agreement with your PT.
Attachments
ride file as sent
ride file as sent
Screen Shot 2012-07-01 at 6.39.37 PM.png (68.22 KiB) Viewed 4642 times
check calibration screen
check calibration screen
Screen Shot 2012-07-01 at 6.40.20 PM.png (66.01 KiB) Viewed 4641 times
After correcting with Check calibration
After correcting with Check calibration
Screen Shot 2012-07-01 at 6.40.52 PM.png (169.17 KiB) Viewed 4643 times
John Hamann
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