Drop CdA bigger than hoods CdA

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vmi2uscg
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Joined: Sat Dec 10, 2011 6:17 pm

Drop CdA bigger than hoods CdA

Post by vmi2uscg »

This morning I did two cal riders and sets of coast downs; one for riding on the hoods and another for the drops. Test rides were done about 20 minutes apart. Both were on the same course. I did have two turns on each, but I went slow and gentle.

The end result was that I have a CdA of .360 and a Crr of .001 on the drops, while my CdA was .351 and Crr was .0033 for the hoods. I would have thought I the profile in the drops would have had a smaller CdA and the same Crr? I'm concerned something went wrong.

I did a wind and tilt cal before each. I checked the wind cal after each set. There was no change before and after while riding on the hoods. There was a 1mph difference before and after the series in the drops. Could this have something to do with the difference? Can I fix the error with doing a cal check on a solo out and back ride?

I'm riding a Cannodale SS2 with Bont race wheels with 23mm tyres at 100 front and 110 rear.

I've attached both cal files; I'm sure I'm not the only one here. Thanks for the help!

PS: The last coast down was in the TT position. It should be disregarded.
Attachments
Cannondale_drops_7-2012_w_crr_adj.ibcd4m
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Cannondale_7-2012_w_crr_adj.ibcd4m
(513.29 KiB) Downloaded 474 times
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Russ
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Re: Drop CdA bigger than hoods CdA

Post by Russ »

Hi Va Mil Inst to U.S. Coastguard,

I live in VA and was Navy so had to decode your handle :-)

I am leaving the file analysis to the experts but one thing jumps out at me right away.
Your CRR should be same on both.

I suggest you go into the profile editor and set the CRR to best guess fixed (say around .005) and you should see your CDA changes become more realistic. Then with good values in your profile, you can slowly (if you really need to) try to tweek out a more accurate CRR by several arcane methods :-)

Russ
vmi2uscg
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Joined: Sat Dec 10, 2011 6:17 pm

Re: Drop CdA bigger than hoods CdA

Post by vmi2uscg »

Russ,

You're shooting 50/50. You're right on the handle.

I set the Crr to .0055 (value from the manuals) and the CdA dropped to ~.09 and .28, respectively. With values that low, the iBike says I only produce 50W to ride 20mph on the flats.

More than fixing the profile, I want to know how to avoid this when I go to do my TT profile.

Thanks,
Brian
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Russ
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Re: Drop CdA bigger than hoods CdA

Post by Russ »

Brian,

Glad I got some of it :-) I see that the download counters have incremented, not by me.
I am reluctant to try the cda problem on for you as more expert folk (hopefully) are now looking at your cal rides. While I might be able to help there, they have the expertise and experience with identifying and solving the problems.

Russ
rruff
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Re: Drop CdA bigger than hoods CdA

Post by rruff »

Don't know what tires you are using but .0055 is good guess for good tires. Your CdA should come out ~.35 if you do the procedure properly.
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Russ
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Re: Drop CdA bigger than hoods CdA

Post by Russ »

Brian,

I guess no one wanted to critique your files, past the comments made. Well, I was tinkering with my desktop system (usually do iBike on laptop) and decided to install iBike SW on it and used your file to test it out :-).

In tinkering, if you have really excellent tires and were on an absolutely smooth surface, then MAYBE this could be meaningful.... But first I will comment that a flatter road might help a bit.

What I tried, with your two files, was setting the CRR to .004 as the lowest possible CRR (arguably) under perfect conditions (swag for that though). Then I deselected all but the highest value Aero's in each:

Hoods: deselect all Aero below .3 set Crr .004 Cda = .351 (reasonable value)

Drops: deselect all Aero below .2 set Crr .004 Cda = .223 (possibly reasonable)

Also I would average the two wind scaling values of .954 and .917 and go with that.

I realize cherry picking data is not a desirable thing but that was the only sense to be made of your two cal rides, in my opinion. So the deselection and low CRR, again, were selected to get reasonable ballpark Cda's and may mean the values would be starting to fall into place. I would certainly try some more cal rides, which you may have done by now. I wonder how your ride files look?

Regards,
Russ
vmi2uscg
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Joined: Sat Dec 10, 2011 6:17 pm

Re: Drop CdA bigger than hoods CdA

Post by vmi2uscg »

Russ,

Thanks for the reply. And GREAT JOB on the analysis.

I did a new cal ride, after a Tilt and Wind Cal, on a flatter course (WOD trail near Herndon, Va) in the morning when the temps were constant. Of course, I followed up with a set of coast downs. The first half were in the drops, the second half on the hoods.

With holding Crr at .0055, I got the following:

CdA, Hoods: .366
CdA, Drops: .250

I'm ok with the CdA for the Hoods, but is seems a bit low in the drops. Can I really be that Aero?
I also allowed the Crr to be adjusted, and got the following:

CdA, Hoods: .353
Cda, Drops: .264

I've attached the file. Maybe John or Coach Boyd might have some input? I'm lost.
Attachments
2012_07_25_Good_Can_Drops_w_Hoods.ibcd4m
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Velocomp
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Re: Drop CdA bigger than hoods CdA

Post by Velocomp »

I've looked at your file.

If you've really kept everything the same, with the exception of ride position, then your CdA most certainly is dropping by a significant amount.

Some of your actual coast down data shows wind gusts (maybe from cars?), and one of your coast downs was plain wrong. Also, it looks like you're doing a coast down, then immediately starting a new coast down from the point where you just completed the prior coast down. I infer this from the fact that each coast down takes only one minute; that would not seem to provide enough time to ride back to the starting point.

If you really want to nail things down, then I would recommend you repeat your tests, with 10 coast downs in each position (20 total). Do this on a day with calm winds and no traffic. Make sure that you do each coast down starting from the same point (i.e. coast down, ride back to the starting point, do another coast down, etc.)
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Screen Shot 2012-07-26 at 9.27.40 AM.png
Screen Shot 2012-07-26 at 9.27.40 AM.png (30.1 KiB) Viewed 13945 times
John Hamann
vmi2uscg
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Joined: Sat Dec 10, 2011 6:17 pm

Re: Drop CdA bigger than hoods CdA

Post by vmi2uscg »

John,

Thanks for the look. I did start each coast down at the same spot, and stopped pedaling at the same spot. Every time except the error, the iBike reported a good coast before the next one. Your comments make me think I've hitting too steep a hill? Roughly how long (distance wise) should the slow down part take? I would say the CDs I did took a total of less than .1mi.

I didn't have any cars pass me, but I was in an industrial park with a slight cross wind. I'll redo these with a head wind next time.

Can I do a 2 mile out and back on the drops, but then do two sets of coast downs, one hoods and one drop? It appears that the OB is for tilt and wind scaling, and has little to do with position (well, between hoods and drops)?

Thanks for all the support!
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