Sensitivity analysis of total weight and wheel circumference

[carstenj]

I just started using my new iAero today. I installed it and did all the steps in the calibration procedure. I am thinking about making some sort of sensitivity analysis of the importance of total weight and wheel circumference in the profiles.

Does anyone know how important it is to get these 2 numbers absolute correct? Let's say the weight is off by 1-2 kg and/or wheel circumference is off by 20-50 millimeters. How will this affect the power measurements?

Any feedback is highly appreciated.

[travispape]

I just started using my new iAero today. I installed it and did all the steps in the calibration procedure. I am thinking about making some sort of sensitivity analysis of the importance of total weight and wheel circumference in the profiles.

Does anyone know how important it is to get these 2 numbers absolute correct? Let's say the weight is off by 1-2 kg and/or wheel circumference is off by 20-50 millimeters. How will this affect the power measurements?

Any feedback is highly appreciated.

Actually, there is any easy way for you to get the answer using iBike2 on one of your own rides. After you finish post-processing one of your rides, click the "iBike Settings..." button in the "Detailed" tab on the main window and try changing the parameters. Watch their effect in the window on the right.

Here's the results for one of my recent rides:

This is "baseline":

Code: Select all

Dist:        6.90 mi (0:19:34)
Energy:     272.8 kJ
Cals Burn:  260.8 kcal
Climbing:     356 ft
Braking:      6.5 kJ (2.4%)
          Min   Avg    Max
Power       0  232.3   823  W
Aero        0  185.3   996  W
Rolling    10   41.6    66  W
Gravity -1128   -4.5   709  W
Speed     5.0   21.2  33.8  mi/h
Wind      0.0   21.3  36.0  mi/h
Elev      248    302   349  ft
Slope    -8.9  -0.05   8.4  %
Caden       8   74.9    99  rpm
HR         91  165.1   179  bpm
Aero: 0.363; Fric: 9.10
CdA: 0.340 m^2
Crr: 0.0049
204 lbs; 5/7/2008 6:45 PM
78 degF; 1017 mbar

Add 4 lbs (less than 2 kg):

Code: Select all

Dist:        6.90 mi (0:19:34)
Energy:     273.6 kJ
Cals Burn:  261.5 kcal
Climbing:     356 ft
Braking:      6.7 kJ (2.4%)
          Min   Avg    Max
Power       0  233.0   835  W
Aero        0  185.0  1016  W
Rolling    10   42.5    68  W
Gravity -1150   -4.6   722  W
Speed     5.0   21.2  33.8  mi/h
Wind      0.0   21.3  36.0  mi/h
Elev      248    302   349  ft
Slope    -8.9  -0.05   8.4  %
Caden       8   74.9    99  rpm
HR         91  165.1   179  bpm
Aero: 0.363; Fric: 9.10
CdA: 0.340 m^2
Crr: 0.0049
208 lbs; 5/7/2008 6:45 PM
78 degF; 1017 mbar

Back to the original weight and add 20 mm to wheel circ:

Code: Select all

Dist:        6.96 mi (0:19:34)
Energy:     275.4 kJ
Cals Burn:  263.3 kcal
Climbing:     356 ft
Braking:      6.6 kJ (2.4%)
          Min   Avg    Max
Power       0  234.6   832  W
Aero        0  187.0  1008  W
Rolling    10   42.0    67  W
Gravity -1137   -4.5   717  W
Speed     5.0   21.4  34.1  mi/h
Wind      0.0   21.3  36.0  mi/h
Elev      248    302   349  ft
Slope    -8.8  -0.05   8.4  %
Caden       8   74.9    99  rpm
HR         91  165.1   179  bpm
Aero: 0.363; Fric: 9.10
CdA: 0.340 m^2
Crr: 0.0049
204 lbs; 5/7/2008 6:45 PM
78 degF; 1017 mbar

So the answer in both cases is not too much effect for this particular ride, but that is partly because I wasn't doing a big climb nor lots of accelerations so the weight mostly didn't matter.

By the way, that 20 - 50 mm of error on the wheel circumference would be an uncessary shame. Go ahead and measure it once by rolling it out. Day to day variations due to pressure will be much smaller than 20 mm.

Travis

[carstenj]

Thanks a lot for getting me in the right direction. It is almost so easy to the kind of sensitivity analysis that I was considering