High interval values

[jkallo]

Hi all,

I have a first gen iBike on loan from a friend, and I've been running it alongside my powertap for comparison. I've decided that if I find the results solid enough to train on, I'll probably switch -- I love the additional data the iBike provides and I'm kind of a wheel junkie so being tied to a single rear wheel drives me nuts. So far, the results have been great with overall ride averages within a few watts on a variety of rides over ~150 miles or so. There is one exception to this: in interval-like efforts, the iBike consistently reports an average of 30 or so watts higher than the PT.

I've attached files from my ride this morning in which you can see this happening: at 20 minutes and again at 27. What's odd is that it's been pretty consistently right around 30 watts high for a minute effort -- kinda makes me wonder if the issue might be with the PT reading low.

Any thoughts would be appreciated.

[coachboyd]

These are great results for a Gen 1. There are a few things that could lead to the power being slightly higher for intervals. One of the huge improvements in the Gen 3 is the on-the-fly tilt correction so the numbers that you see on the road are the numbers you are doing.

If you get more aero when you are doing the intervals, this can lead to faster speeds which can lead to higher watts. This is a great way to test your cda and find out how much faster you can be in different positions. Overall it looks like you are getting great results and a Gen3 would work for you just as well as any other power meter, without having a big bulky back wheel.

[jkallo]

Yeah, I have to say I have been really surprised as well. And I am using the first and only profile I made -- 4 coast downs and a 4 mile ride. I wondered if tilt might be it as well. I really like my modolo bars, but they're a bit flexy.

The variance is so consistent that I could probably use this as my sole PM as it stands right now -- the on bike numbers are close enough to allow me to hit my target interval numbers. The only problem is that the skewed high-end numbers mess with normalized power in WKO and this messes with TSS which messes with all the PMC functionality etc etc. So while average power is actually identical for this ride at 152 (using the first 55 minutes which is the time I was actually riding, not sitting on the bike waiting to send my daughter off to camp etc) the NP is 210 (ibike) vs 192 (pt). Not a huge difference, but over a long ride that variance makes a big difference in TSS etc.

[coachboyd]

That higher NP can be attributed to the Gen1 having a few false spikes. This issue has been completely resolved and I am getting NP readings very close during all my rides. In general, the iBike will report slightly lower NP readings due to the slight "smoothing" of the data being calculated. If you look at the DFPM data vs. the iBike data, the trends follow each other great with 5 second smoothing, but you can see where the DFPM's power line updated quicker than the iBikes when you turn off the smoothing. This variability due to faster updating will tend to give a slightly higher NP.

Here you can see the same 5 mile section (in a group ride) where I have shown the power meter comparison with 5 second smoothing and with no smoothing