iBike DFPM comparison anomaly

[mullerrj]

After downloading my ride file last night I noticed an anomaly of sorts that I can't explain. I was wondering if any of you guys had an explanation. I was doing an interval workout, alternating paceline, and got dropped. After I got dropped I continued riding. I noticed on the downloaded file that there was a huge delta (labled Delta 1) between my iBike Power and Power Tap file between mile markers 12 and 18...riding solo. Then, on the return trip..where I jumped back on the pace-line, and drafted for the most part, (mile markers 18 to 24) I realized that there was hardly any Delta (labled Delta 2). Wouldn't you think that riding solo in a clear/open windstream void of any cars/wind/etc. that there would be less of a delta between the iBike and DFPM? And, similarly, wouldn't you think that riding in a draft there would be more delta Power between the two? Just seemed odd to me.

Here are the particulars:
iBike Aero w/ latest firmware
Power Tap SL+ recently calibrated
Accurate Tilt, Wind Offset, temp adjustment prior to ride
Little to NO wind/calm day
Little to NO traffic on route
Temp: 53 deg F
Recent calibration file
Aero: 0.398, Fric 0.719
CdA: .437
Crr: .0052

[coachboyd]

Can you post the file. . .I'll take a look at it and see if there's anything I can figure out.

[mullerrj]

Thanks Coach Boyd..really appreciate you taking the time out of your busy schedule to take a look. File attached. Cheers Rob

[mullerrj]

Hey Coach Boyd..I was just thinking...in an effort to catch back up to the guys that dropped me, I could have gone from my hoods to the drops...to get more aero to try to go faster. Since the calibration ride was done in the hoods, could that be the reason for the big delta from mile 12-18? I remember on the way back, I stayed on the hoods which is maybe why less delta. Yes/No?

[Ratman]

Hi, Mullerrj,
From the snapshot of your ride file above, there seems to be a larger than normal discrepancy between the iBike and the PowerTap from miles 4.25 to 6, too, and in the same direction and approximate magnitude as the solo section of the ride after you got dropped by the pack. If you can think back to what might have been going on at that time and add that to the "mix," perhaps you'll be able to to solve the mystery??

[Velocomp]

This is a very interesting thread!

When you went from the hoods into the drops between miles 12 and 18 you would go faster because of your more aero position. Since the iBike doesn't know that you changed ride positions it would determine that the cause of your faster speed was more power--just as the ride file shows!

In fact, you can see this in the CdA Analysis: between miles 12 and 18 your CdA dropped from 0.42 to about 0.3. The Time Advantage analysis shows that you picked up about 4 minutes by staying in the tuck.

Also, if you went into a tuck on the downhill between miles 5 and 6, you'd get the same higher watts, for the same reason.

Note that where you're riding in the hoods you get excellent agreement with your PT.

Also, you did a fair amount of drafting in this ride. Since you have new firmware 308 in your iAero you can see that your iAero is very accurate in the draft. Between miles 18 and 24 you were riding in the draft. Your PT measured 210W average and your iAero 207W.

[mullerrj]

Here's another file from tonight. You can clearly see the difference in Power readings starting at the 4.5 mile mark up to the 8 mile mark. I went back into Google Earth and re-traced my route via GPS to take a BETTER look at the landscape..and the only thing I remember was there was a STRONG sidewind coming from the right...enough to blow me from side-to-side a foot or so along the way. Nothing but wide open sod farm fields. Anyway, here is the comparison graph..and file attached. Thanks guys..really appreciate you trying to help me identify the reason.

Oh, my position was on the hoods..the same as it was calibrated. Also did a new tilt, wind offset, acclimated the head unit to the temp, etc. BEFORE the ride.

[coachboyd]

OK, I looked at it a bit and tweaked your profile. It matches VERY well with the whole ride that you just did on the 18th. For the ride on the 17th I could either get the watts to match when you were by yourself or with the pack. It was weird seeing how different your watts were when you were drafting. You by chance don't feather the brakes when you are riding in a pack do you? The other thing I can think of is you sit in a more upright position in the pack in order to see ahead for what's coming and this could be costing you a lot of watts and wasting energy in the pack.

I have the attached profile here. You can save this profile and then import it through the software. You can try switching the profile after the ride to see what kind of results you get and if you like them. If you do, go ahead and send this profile to your iBike and you'll be good to go.

[R Mc]

Comment deleted due to user stupidity in not reading the graph correctly ;')

[mullerrj]

Coach Boyd..thanks for tweaking the profile for me. Nope, I never feather the brakes when I'm in the draft. I always pop-out to control my speed. Funny that you mention sitting up. You're right, I think I do sit more upright when I'm in a draft. I don't do it consciously though. I probably do it because of the herniated disc I have in my back..just more comfortable. You're right, I also do it (sit up more) to see around the people in front of me (since I don't trust them 100%) because this is pothole season in Pennsylvania and I don't want to ruin a tire/wheel or worse- go down. Thanks so much for bringing this up. I've got a race in Philly next weekend and you probably saved me a lot of energy. Thanks again for your time..much appreciated. Cheers Rob

[mullerrj]

Yikes..I went back and looked at my Profiles and here I thought that my Profile 2 calibration ride was with my hands on the hoods and actually it was with my hands on the handlebar tops. The two rides above were both ridden with my hands on the hoods...NOT the Profile 2 that was setup on the iBike. Sorry guys..but could this be the REAL reason for the power discrepancy? Also, Coach Boyd, can I still use the new edited profile you made up for me? Wow, so sorry for the error on my part guys. I'm glad I made sure I put a description in the Edit Profile box...to confirm. Thanks again. Rob