iBike Av power differs to download

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JOHN WALKER
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iBike Av power differs to download

Post by JOHN WALKER »

Carried out Fitness Test: iBike shows at end of test: 3.78w & Fitnest level 5. FTP shows 280w.
Download to computer and Av Power shows 203.2, Av Aero: 167.8, Av Rolling 34.4. Av Speed: 31.4 km/h

When I do normal rides, again the av power on the IBike is higher than that when downloaded.

Am I not understanding the readings or what?

Please help.

Attached file
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04_13_2012_1434_11_km FITNESS TEST.ibr
FITNESS TEST
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Velocomp
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Re: iBike Av power differs to download

Post by Velocomp »

In your iBike software go to Edit/Preferences/Analysis. Uncheck the two "Auto" boxes, so that rides aren't automatically analyzed.

I used the command "Return to Original Download" to get to your unanalyzed data.

Your watts are much higher on the original ride. However, your ride file shows that you were battling a very high head wind. See image

If this was not an out-and-back ride, and if you were facing a continuous headwind, then your on-the-bike results were correct.

It seems very possible, however, that your wind speed readings could be high. Make sure your iBike is calibrated properly and that you do a Cal Wind, particularly prior to doing a fitness test.
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John Hamann
myuce
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Re: iBike Av power differs to download

Post by myuce »

If you don't mind, can you also look at the attached file? Similar problem, so I figured there is no need for a separate post. I guess what bothers me is that on some rides my wattage is right on the dot, same both as I ride and after the download. But on others, like this one, it usually gives me a different reading. A couple of watts doesn't bother me, but this one is 238 vs. 254. I'm pretty religious when it comes to doing wind & tilt calibration right before I start a ride, and I know I have a good profile that I've been using for at least 3 years now. Any insight would be appreciated. Thank you.
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05_23_2012_1600_8_Miles.ibr
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Mehmet
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Re: iBike Av power differs to download

Post by racerfern »

@myuce Here's my take: I come up with 253 vs 254. Since as you say you are careful about tilt and wind calibration then I suggest you go to preferences and on the analysis tab shut off all the automatic analysis.

Then when doing an analyze since you know your wind is good, simply uncheck all boxes and click on Enter Known Elevation. This will make a minor adjustment for any changes in barometric pressure and with the other boxes unchecked will use your wind data as valid data.

Clicking ride has out & back will tend to balance the wind and that's great if you never set your calibration or didn't do a wind offset. As long as you know that the conditions along a ride such as this one were pretty consistent (which they were) then you can just use the iBike data with the minor correction as noted above.

Finally, if you are doing an official calride be sure to maintain the same avg speed in both directions. Obviously the leg into the wind will have higher power but maintaining the same speed in both directions will yield better results.
Fernando
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Re: iBike Av power differs to download

Post by myuce »

Fernando,

Thank you for taking the time. The only thing that makes me wonder is that when I enter my speed and other parameters (like the 1.3mph tailwind from the analysis during the 10 minute interval) to an online power calculator, I get lower results that are in the 220W range. The reason I kept the auto analyze on was because it was giving a number that's closer to this calculated number. But then it bothered me that what I was seeing during the ride was different than the analysis result, hence the post. Of course the calculator may be off, but as far as you can tell from the ride file, do you think 253 is a reasonable number? I don't have a problem being delightfully surprised that all this time I was actually generating more power than I thought I did, but in the meantime I wouldn't want to deceive myself either, you know...

While I'm at it, maybe you can answer a couple of more questions for me. One is that I use the user interval feature all the time. Sometimes after the download, software chops an individual interval into three pieces or so. I was just wondering what this means and why it happens.

The other thing I'm curious about is what it means to see a non-zero wind offset in the wind analysis dialogue box.

Thanks again.
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Re: iBike Av power differs to download

Post by racerfern »

So... using this ride as a calride and using the check calibration function, your wind scaling actually needs to go from .868 to .791. That's assuming this is a valid out/back ride. Remember I suggested that both legs be the same speed and this calride is longer than it needs to be. Take a look at this ride and consider doing a calride from the .2 mile mark to 1.2 miles then back. You should be able to knock out a couple of these maintaining the same speed in both directions in a short period of time.

Anyways, creating a profile with check calibration brings the segment in question to 235w and that's probably very close to reality. As long as you consistently use the new profile you'll get repeatable results; and that's the most important consideration of all, consistency.

Assuming you have the latest version of the iBike software, you can check this yourself. Le me know how you make out.
Fernando
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Re: iBike Av power differs to download

Post by myuce »

Sorry to be a pest, but just to make sure that I get this right:
Do I also need to do coast downs? And why do more than one out&back ride? If you adjusted the wind scale as you said, can I just not do that and get consistent results?
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Re: iBike Av power differs to download

Post by racerfern »

No coast downs at all! When I do calrides, I tend to do two of them back to back. It gives confirmation that the results of one of the calrides is close to the other one. If not, you need to look at what changed.

Yes, you can just take the numbers and plug them in, but I always like to do the actual calride or two as a confirmation. Besides, if you do a couple of nice hard calrides, it's good exercise.
Fernando
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Re: iBike Av power differs to download

Post by myuce »

I did two cal rides back to back. I don't know if you had anything in mind when you said do these into .2 mile mark to 1.2 miles, but I chose a portion that was rather flat. On the first one I tried harder than I did on the second one. That might've been a mistake, I don't know. I did "check calibration" in software, and adjusted only my weight and height. The two seem to be quite different. Other than the speed I tried to keep, I don't think anything else changed. Your interpretation is appreciated, but I think I will do another set to see how they come out if I try to stay in the same effort zone.

Thank you.
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06_03_2012_1027_2_Miles_CalRide.ibr
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Re: iBike Av power differs to download

Post by racerfern »

@myuce They do vary from your earlier posting. When you have time do another set and we'll nail it down.
Fernando
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Re: iBike Av power differs to download

Post by myuce »

Fernando,

Here's another set of two cal rides. They are done on a different road, but pretty flat and representative of the roads I ride on. Less traffic too. I just replaced my tires with the same kind, and I don't know if it matters enough or not but my original cal ride was done with the same type whey they were new. So, I'm curious to see if the files will make sense to you. By the way, I'm 150lbs and 68in tall if you want to play with those numbers. I don't quite understand the details of the check calibration features, therefore I don't know what to play with, but I'll ask my questions later.

Thank you.
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06_10_2012_1015_2_Miles_CalRide.ibr
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Re: iBike Av power differs to download

Post by racerfern »

So it seems you have a good grasp on creating profiles. I come up with .821 for wind scaling with a CdA of .353. Be sure to set your Crr to .0045 and you should be good to go. Nice job on the cal rides. There's not much to tweak when checking cal rides. For some reason the Crr was artifically low, so we bring it up to normal. From there the software does what it should do, it determines your CdA based on your wind scaling. In your case I took the average of the two rides since they're quite similar.

Simply apply the numbers, send it to your iBike and away you go. You've got numbers and a base to build on. Congratulations!
Fernando
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Re: iBike Av power differs to download

Post by myuce »

I'll see how my numbers come out next time I ride.

Thanks for your time.
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Re: iBike Av power differs to download

Post by myuce »

OK, since it's the first time I'm doing this, I think I need some help. I thought I was just going to re-enter the numbers you gave me to my existing profile, but other things change when I play with the wind scaling factor and crr. One causes the aero coefficient to change, the other causes either the friction or the riding tilt to change. As soon as you tell me how to do this, I will go away forever, I promise :D
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Re: iBike Av power differs to download

Post by racerfern »

LOL, no need to go away. These are good questions and I'm glad you noticed that just like the principle for the iBike for everything you do there is some kind of balancing or opposite reaction.

There are two ways to do this but I'll explain the easiest way which is to use the software "Check Calibration" feature. If you had done only one cal ride this would be the way to go. But since you did two, we are using the wind scaling average of those two cal rides. So... on the bottom section of the "Check Calibration" screen, click the Rolling Resistance Estimated tab so the Crr shows at .0045. Next in the top section make sure light traffic conditions is selected and once again switch to the estimated tab. You should see a CdA of .353. Click "Accept" and a new profile will be created in your list of profiles. IF we were not taking the average of two cal rides we could also send the profile to the iBike and be done with it.

Now click Edit > Edit Profiles and you'll see the profile that just got created at the top of the list. Click on the Advanced tab and take a look at your wind scaling number. In the top right section of the Advanced tab it says Aero xxx, Wind Scaling xxx, CdA xxx. Since your Cda is set make sure the radio button next to Cda is highlighted. That will make it say "Hold". Now you can adjust the wind scaling and the aero will automatically adjust to the proper number. You could also change the aero number to make the wind scaling number become what you want it to be. The Aero number is like a fine tune dial, that can make very minor changes if needed (but usually is not needed).

That's it. With your iBike connected, click Send to iBike and click Accept. Be sure to confirm you have a good tilt calibration so that stays with the profile.

If after a few rides you decide an adjustment to the wind scaling is needed, simply open the profile and keep the radio button next to CdA highlighted. You can adjust your wind scaling up or down slightly as needed.

Make sense?
Fernando
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Re: iBike Av power differs to download

Post by myuce »

Yes, it makes sense when I read it. But when I try to apply the numbers, I don't see what you seem to be seeing. I open "06_10_2012_1006_2_Miles_CalRide", go to the Crr - Estimated tab and change Crr to .0045. So far so good. Then I go to the Aerodynamic Resistance - Estimated tab, but I see a CdA of .332, not .353.

So I open up the second file "06_10_2012_1015_2_Miles_CalRide" and change Crr the same way. Go up to the "estimated" tab, only to see .332 again. However, the "original" tab shows .354. Maybe that's what you meant?

If I assume that's what you meant and accept it, I get my tweaked profile.

But how did you know which cal ride to use? Because if I do the same thing with the other cal ride file, I get a differently tweaked profile. If I use the first tweaked profile to switch the profile in my original ride file (first one I attached - 05_23_2012_1600_8_Miles), I get one wattage. If I use the second one, I get another.
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Re: iBike Av power differs to download

Post by racerfern »

I opened the 1006 file, for safety sake, clicked return to original download then clicked check calibration. I put the bottom and top tabs to estimated and here is what I see.

Since you did two very similar calrides, you are going to use the average wind scaling of the two. They're so close together it almost makes no difference. So let's simplify it. Just open the 1006 calride and you should get the same numbers I have in the screen shot. Click send to iBike and click accept. Later on if your wind scaling seems just a tad high you can tweak the profile ever so slightly.

Let me know if you're OK up to here.
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Re: iBike Av power differs to download

Post by myuce »

If I'm going to use the estimated values, doesn't it make sense to use my real weight and height along with the "drops" option as my ride position?
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Re: iBike Av power differs to download

Post by racerfern »

AHA! Yes of course you need to enter your info. I looked back through this discussion and I can't see where you said your height, weight and riding position was. You can post it here or you can email me at fernando.j.maldonado at gmail.com

I "assumed" the info on the screen was info you had previously entered. SORRY!
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Re: iBike Av power differs to download

Post by myuce »

OK so entered my numbers as you said. I weigh 150 lbs and am 68" tall. With those values in the drops I get a CdA of .317. I think I'm good so far. Do I go to the tweaked profile and adjust the wind scale factor to .821 now, or do I wait? If yes, how do I determine whether or not I should change it and in what direction?

I feel like I'm getting closer!
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Re: iBike Av power differs to download

Post by racerfern »

You're right there. Yes, go to the tweaked profile, set the wind scaling to .821.

You'll figure out in due time if the wind scaling needs a bit of tweaking. It's not a bad idea to do an out/back (mark it as a lap) on one of your rides every once in awhile. Wherever you did these out back rides is a perfect spot.
Fernando
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Re: iBike Av power differs to download

Post by myuce »

I think I got it. I get a reasonable number if I use the tweaked profile in the first file I ever attached in this discussion.

Now hopefully last questions:

You say I'll figure out in due time if the wind scale needs tweaking again. How will I know? And why the out/back ride? What do look for after doing it?
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