Refining calibration numbers

[tbarczak]

So I've been running my iBike for about 6 months now. Works well. I've recently lost some weight and decided to just do another side by side comparison of it with my powertap. I'm close with the numbers consistently, but the iBike keeps reading a slightly harder workout than the PT. For example, a 3hr ride is about 20TSS points higher / 15-20avg watts higher ect. I feel like if I can just 'dial down' the config manually a touch it would work fine.

So I took a few iBike files, did the "Tools / Tweak cda" option and dropped my CDA value a bit and got the files in line with the PT. But what I'm not sure about is how to properly tweak the CDA value in the profile. If I change CDA, then the WindScaling automatically updates. Is that the right approach? To let the Windscaling just be what it wants to be? Or should I be adjusting some other number in order to bring the equation into balance?

Here's what I have today after updating the CDA to be what worked well in the 'tweak' data:

Aero: .300
WindScaling: 0.952
Cda: .315
Fric: 5.958
Cal Weight: 206
Riding Tile: -0.1
Crr: 0.0055


My last Cda value was .342 (and I found the .315 by doing the manual tweak of the file). WindScaling was 0.877 but obviously got 'auto-corrected' to 0.952 as shown above. No other numbers changed.

Also, what is Cal Weight? Because I updated my rider weight and total weight to now be 188, but that one didn't change (so I didn't touch it).

Any advice would be appreciated.

Thanks!

[travispape]

Cal Weight is you weight at the time of last calibration. Congrats on the weight loss.

A part of the reason that you iBike is over-reporting power now is because the rolling resistance force is a little smaller now because you weigh less. Instead of attributing all of the difference to the aerodyamic componenet, I would just drop your CdA to about 0.33 and lower your fric.

Also, the thing the determines what wind scaling should be is that you want average wind (as viewed in the Tools -> Analyze Wind window) to be close to zero for any out & back rides that you do.

Let's assume that your wind scaling & riding tilt values are still good from your old calibration and try this:

Aero: 0.289
Wind Scaling: 0.877
CdA: 0.330
Fric 5.437
Riding Tilt: -0.1%
Crr: 0.0055
Cal Weight: 188 lb

You might want to do another quick cal ride to make sure there have not been changes for your wind scaling & riding tilt. (You don't have an iAero, do you? If so, let me know because you can easily make a good profile based on the PowerTap.)

Travis

[tbarczak]

Hi Travis,

Thank you for the response. That is a huge help. I will give those adjustments a try and see what happens.

Also, you peaked my interest on your last statement. Currently I have an iPro and a PowerTap. If I upgraded to the iAreo are you saying there is some way to use the PowerTap data to build a more effective profile than doing these tweaks? If that's the case, that would make the upgrade worth EVERY penny.

Thanks again,
- Todd

[travispape]

Yes it's easy. With the iAero paired to a wireless PowerTap, all you have to do is a 4 mile calibration ride and iBike4 will create a profile based on the combination of the iAero and PT data. See http://www.ibikesports.com/detail.aspx?ID=75

Travis

[tbarczak]

That's awesome. I'm buying that upgrade for sure now. Thanks!

Regarding the steps to use the iAero to perform that cal, is there an online doc with the details or is it pretty straight forward?

[ghost]

I'm very interested in this as well b/c I haven't been able to achieve a reasonable calibration with my iAero. Hopefully I can borrow a wireless PT from someone and get it worked out if there are instructions...I don't think that there will be much difference between the two if I use the same tire/tube on the borrowed wheel.

[Russ]

Hey Guys,

I don' t have a PT but looks like, to me that you just select the help menu in iBike 4, then select the
"iAero DPFM instructions". Note that the rest of the iBike doc is accessible there too !

Russ