Being driven crazy by numbers - Profile, Power, etc

[MultiRider]

I put a PT wheel on my TT bike, lowered the front end via flipping the stem, widened the elbow pads, and went for a ride. The iBike didn't register any power for the first 5 minutes or so, but I expected that because it thought I was going down hill due to flipping the stem and not doing a Tilt. But, as expected, the iBike figured out that I wasn't going downhill and started showing power numbers.

After the ride, I compared sections of the PT file to the iBike file, found that the iBike was about 3w low at <15mph and about 12w low at speeds above 15mph, so I extracted the profile from the iBike (trying to get the new Tilt value), copied it, changed the aero to be a little lower, and sent it back to the iBike. Unfortunately, I didn't have a chance to test with the new profile before racing with it.

I did a TT race on Thursday and my iBike showed 800w-1700w for the first 5 minutes. I'm not a sprinter and have cracked 1000 watts fewer than 5 times in my life, never for more than a few seconds. It then started showing correct numbers.

I looked at all the "Analysis" tools in the iBike software and can't figure out how to correct it for the race file. The "Analyze Tilt" function shows a pretty dramatic correction to Tilt, but the power is still wacked in the graph, the average power is crazy, my IF and TSS is ridiculous. The "Before' shows a loss of 1200 feet in the first 5 minutes on a pretty flat course. The "After" shows a pretty flat course. But the power file still shows 800-1600 watts in the first 5 minutes.

I would like to correct the file for that race so I have accurate numbers and would like to prevent that from happening again. Help would be appreciated. Please note that I am not an iBike expert, not a Fric / Aero / CdA / CRR expert, and not an expert at using the iBike tools. I was just trying to get a new profile that would accurately display/record power numbers for my TT races. Help should be offered as to a relative novice, not an expert. Prior to this, all I did was coast downs and an out-and-back. I don't really know how to use the tools, but they seemed intuitive. But the results are clearly wrong.

Thanks.

[racerfern]

This is quite easy.
1) Do a tilt calibration with the iBike on the bike. Now the iBike "knows" what flat is and won't have to wait 5 minutes to figure things out.

To make this fix permanent, hook the iBike up to the computer and go to the profiles section. Open the profile you have and click the button on the Advanced Tab just about Tilt Cal that says "Get from iBike". It will get the new tilt calibration from the iBike. Click Accept.

Go back to Edit Profiles and send your profile with the proper calibration back to the iBike.

Doing #1 above, makes it temporary and you will lose this calibration if you do a battery reset. Doing the rest makes the tilt cal part of the profile and makes it permanent.

[MultiRider]

I will do a Tilt and extract the Tilt from the iBike per your recommendation. Thanks!

Can I correct the file for last Thursday's race? Will "Switch Profile After The Ride" fix it? Except I think I would be applying the same profile name - will the software figure out the Tilt is different and re-calculate the power for the ride (particularly the first 5 minutes)?

Does adjusting the Aero number in the iBike software cause the the software to tweak other parameters behind the scenes? Such as Tilt? As described, I extracted the entire profile after the with-PowerTap ride because I was expecting that to result in correct Tilt, but it didn't. But I also tweaked the Aero number in the software after extracting the profile.

My profile numbers seem quite different from what has been judged "reasonable" in a couple other threads:

Aero: 0.204
Wind Scaling: 0.720
Fric: 7.719
Tilt: =0.3
CRR: 0.0056
Weight: 202

I'm 6'2" tall, approx 175 pounds, aero helmet, skinsuit, Specialized Transition with Zipp disc and Zipp 808, Zipp Tangete tubular tires, approx 6.5" drop from top of seat to top of elbow pads.

Thanks for the help!

[racerfern]

That's a lot of questions!

1) Look at the edit profile section and you'll see "hold" buttons. So if you want to adjust one thing, you "hold" another number and the third gets moved. IOW, when one thing changes something else changes also. You should make a copy of a profile so if something goes awry you can get back to your original profile.

2) I don't think changing profiles in any way will fix the first five minutes. Send me a PM with your email.

3) Your profile numbers might be OK and seems to fit pretty well for a TT setup although you didn't say what your CdA is.

[MultiRider]

My current Profile has a Cda of 0.283. Sorry for not including that in the first place because that was kind of the point of my post. Since the PT showed I was 12w low at speeds over about 15mph, I'm wondering if I should increase the CdA value. Hopefully I will go faster with the Zipp disc on the rear in place of the open-spoke, non-deep-dish rim that was on the PT wheel, so that would increase the wattage reading. But I'm wondering if I should bump my CdA up a few points to get closer to "accurate".

I'm very interested in "accurate" as well as "consistent" because I use a SRM on two bikes, the iBike on 2 bikes, and a LeMond trainer that has a USB for exporting the ride file to WKO. I would like for all of these to be measuring my power output as closely as possible. (So I measured the iBike on my TT bike with a borrowed PT just to ensure I covered all the bases!)

I am putting "accurate" in quotes because I realize that all these devices can be off a few percentage points. And I'm not on the Pro Tour trying to make my living at this. So I'm as serious as an amateur can be, but still cognizant of the fact that I will never be a Pro so it is all for fun and fitness.

Racerfern -- I sent a PM with my email address.

[MultiRider]

Oops! Forgot that I found this web site:
http://www.biketechreview.com/tires_old ... g_rev9.pdf

which says a Zipp Tangente tubular (which I'm running) should have a CRR of approx 0.00288. My iBike profile has a Crr of 0.0056. But the PT said the iBike was 3w low in the climbing run that I did. Of course, the PT had a Mich Pro 2 mounted, but the CRR for that tire is 0.278 according to the BikeTechReview site.

Should I worry about the CRR differential to the BikeTechReview site? Not sure how much difference that makes and whether it would help me get closer to "accurate" power numbers.

[racerfern]

OK, fooling around with your numbers gives me negative fric and that's impossible. Do you have .3 tilt or -.3 tilt? On a TT bike either one is possible though positive isn't usually that much.

[MultiRider]

My stem definitely goes up. It is a +8 degree stem.

I tried to attach a couple ride files and a MS Word doc that compares the low-speed efforts and compares high-speed efforts from my Sept 4 ride that had both the iBike and a Powertap rear wheel. I used WKO to find the same segments of the ride and put together a table in a MS Word doc that compares the average watts. max watts, average mph, max mph, and elevation change of the iBike to the PowerTap. It is very interesting.

Unfortunately, when I tried to attach the files, I get "extension WKO not allowed" and "extension DOC not allowed". If you will shoot an email to me, I can send them to you. The DOC file should be particularly interesting - the first high speed run (avg 32mph) resulted in a significant difference in watts (iBike is 32 watts low). The 2nd high speed run resulted in a fairly large difference as well (iBike is 14 watts low).

Some of that is due to running the open-spoke non-deep-dish PowerTap wheel in place of my normal Zipp disc. But probably not 14 watts, right?

The low speed runs are pretty close - iBike is about 4w lower on average in both runs. Zipp Tangente tubular vs Mich Pro rolling resistance?

I would very much like to send the files to you. Please advise how best to do that.

I'm racing again tomorrow, would like my iBike to be accurate! Thanks!

[racerfern]

Check your message box.

Also, I was talking about the tilt calibration in the profile, not the angle of the stem.