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So take this guy and his position:

...and put him on this guy's bike and adjust it so that they fit identical.

Then, let the first guy test both of these "back to back" using the same 404 wheelset with PT SL hub and cover....Any guesses on what the aerodynamic drag differences (if any) one would see?

Using RChung's most excellent methodology described here: Method to the madness

Here's the results for the P2K with the 404 wheels (the wheels on the P3C above):

And here's the results for the P3C:

So...what's the bottom line?

Well...taking the same rider, the same wheels, the same basebars and brake levers, and with the seat and extensions adjusted to deliver the same positions...at basically zero yaw conditions I apparently measured a drag difference of ~.023 m^2 of CxA (or Cda, whichever you prefer - .228 m^2 for the P2K and .205 for the P3C). Using Doc C's "rule of thumb", that basically equates to ~2.5s per km of time savings.

With only an extremely small amount of crosswind however (I could just barely detect it on my skin, not enough to even move leaves on trees), the drag on both setups drops significantly, with the P3C setup dropping more at .190 m^2 vs. the P2K's .220 m^2 for a growing difference of ~.03 m^2. That translates to a difference of ~3 seconds per km...or a full 2 minutes over 40K.

Can you imagine comparing the difference between a P3C (or even the P2K) against a frame that actually increases in CxA with increasing yaw, which is actually fairly common?

So much for Mr. "Toenail"s assertions that the differences between frames are neither detectable nor significant...

edited 29Oct2014 to fix broken image links

http://bikeblather.blogspot.com/
Last edited by: Tom A.: Oct 29, 14 11:33
Interesting. I see a P3C in your future.

Regarding the Chung testing, are your laps only about 500-600m long? I thought they would have to be longer, but this is good news as I might be able to find somewhere like this close to where I live.

------------------------------------------------------------
Searching for the bliss of ultimate exertion.
http://forum.slowtwitch.com/...rain%20bike;#1701847

Check out the info half way down that page. Cervelo measured the drag difference as 15g.

What was your error margin?
In Reply To:
http://forum.slowtwitch.com/...rain%20bike;#1701847

Check out the info half way down that page. Cervelo measured the drag difference as 15g.

What was your error margin?
P2K grasshopper :-)
Oooops!

I never realised that the P2 came in a carbon and ali version!
In Reply To:
Oooops!

I never realised that the P2 came in a carbon and ali version!

this is the old discontinued P2K shown in the top pic. in any case, 0.023 is a huge margin for bike/frame changeout alone. Maybe 25% or more of the bike-only CdA/drag ...
I like the results! My 2007 bike was that very same P2K and my 2008 bike just happens to be a P3C !

Now the pressure to produce a 2min improvement in my 40k time ...... oh my!

---Francis
Nice work (and nice thread title <g>).
I'll ride my P2K for a few more years as there's other (cheaper) things to fix/change, but I'll start saving my pennies. Thanks Tom!
Great info. Now you just need to repeat for a P2C and a P3SL and the Cervelo Mafia will have all the info they need.

Steve
In Reply To:
Interesting. I see a P3C in your future.

Regarding the Chung testing, are your laps only about 500-600m long? I thought they would have to be longer, but this is good news as I might be able to find somewhere like this close to where I live.

No. They're ~1100-1200m long. I'm using an "out and back" course that's in the shape of a "drawn-out half-pipe". In other words, my turns are done at low speed on the upslope side of a hill at either end.

In fact, the venue I did this test on is not the one I usually test at. I used it for logistical reasons (only a couple blocks from home, shorter lap length). The downside is that the elevation difference isn't as great and so the maximum and average speeds are lower. This means (as was seen) that it's more sensitive to cross winds affecting the apparent wind angle.

If I plan on testing more at that course, I may need to consider investing in a portable weather station with data logging :-)

http://bikeblather.blogspot.com/
Last edited by: Tom A.: May 21, 08 17:04
In Reply To:
I like the results! My 2007 bike was that very same P2K and my 2008 bike just happens to be a P3C !

Now the pressure to produce a 2min improvement in my 40k time ...... oh my!

---Francis

No pressure for you Francis for the simple fact that you go so fast that you see much less yaw component than us mere mortals, so you only get the 2 to 2.5 seconds per km gain. Plus, the ROT that gives us those time savings is based on something like 24 or 25 mph average speed. Again, you go a lot faster...so, less gain for you!

I'm thinking that all things equal, you should see at least 1.5 to 2s per km.

BTW folks...when Francis says "that very same P2K", he means THAT VERY SAME P2K. Guess who I bought the frame from :-)

http://bikeblather.blogspot.com/
Last edited by: Tom A.: May 21, 08 7:40
rmur asked me these questions in a PM and I hope he doesn't mind me answering them here so that anyone else with the same questions can see the answers

Quote:
Now for the reality check:
(1) Why such a high Crr? I was figuring 0.004 for a fellow who pays attention to tires.

Well, the simple answer is, they're not my tires/tubes I was wondering if someone was going to notice that.

Seriously, I'm running a VF Record front and a Bonty RXLPro23 on the rear on my wheels (latex tubes, of course)...and last month at the Piru TT I had the perfect opportunity on my outbound leg to nail down a Crr of .0038 to go along with a CdA of .228 m^2. My friend's 404s had Vittoria Open Corsa CX 20s with what I had assumed to be latex tubes inside. I hadn't had time to switch out tires yet so I just went with what he had for the testing. When I got done, and started looking at the data, things weren't matching up with the true elevation difference with the slightly higher Crr I had originally assumed...so I quick popped off the front tire and discovered the -gasp!- butyl tubes inside.

Check this out though...so I took a look at AFM's chart. I looked at the percent difference between my tires and the Vittorias and adjusted the .0038 Crr. I then looked at the percent change from latex to butyl and adjusted it again. That gave me an assumed Crr of .0056. When I put that into the spreadsheet, the calculated CdA for the zero yaw condition worked out to be...ta da! .228 m^2, just like the Piru data. Pretty cool, huh? Shows pretty clearly to me the validity of Al's testing.

Quote:
(2) Re the x-winds, I wouldn't get too hyper over those apparent levels - remember the demonstrated dependency of Wind_Chung.

What I learned is that with the "mini-halfpipe" course I used this time for logistical reasons (closer and shorter, i.e. more laps) is that with the lesser elevation difference as with my original "halfpipe", I don't see as high of max and average speeds...so the testing is much more sensitive to crosswinds since they can have a bigger affect on the apparent wind angle.

Quote:
(3) Everything the same - you didn't tuck any harder etc? I joke about it sometimes but I honestly do believe there could be a small placebo-like effect (in general)

Everything as "same" as I could get it. The funny thing is...if I had to choose which one "felt" faster just from perception alone...I would swear the P2K "felt" faster...go figure.

http://bikeblather.blogspot.com/
In Reply To:
Nice work (and nice thread title <g>).

Thanks...I thought the title was somewhat appropriate, especially considering your thread from last week.

The good news (for me) is that the whole reason I'm borrowing the P3C is for my district TT on Saturday :-)

http://bikeblather.blogspot.com/
In Reply To:
the ROT that gives us those time savings is based on something like 24 or 25 mph average speed. Again, you go a lot faster...so, less gain for you!

30 mph, actually, since that is the standard for wind tunnel testing. However, I also rounded every number to a one or five*, so the effect may be the same (I haven't checked, but take your word for it).

*I did some calculations in my head while riding this weekend, and realized that, at least for me, the same approach also works for Crr. Specifically, at my speed/mass, a change in Crr of 0.0005 equates to a difference in power of ~5 W, and hence also a difference in CdA of ~0.005 m^2, a difference in drag of ~0.1 lbs, and/or a difference in time of ~0.5 s/km. Neat how that works out, huh? :-)
Last edited by: Andrew Coggan: May 21, 08 8:16
In Reply To:
Great info. Now you just need to repeat for a P2C and a P3SL and the Cervelo Mafia will have all the info they need.

Steve

Naah...these numbers match up pretty well with what Andy has posted before and what's been "leaked" out from stuff like the "brain bike" sessions...I think I'll believe the other numbers on the P2C and P3.

I'm more interested in testing out and finally getting some numbers on something like a Transition...or maybe the new Plasma 2...

http://bikeblather.blogspot.com/
In Reply To:
When I got done, and started looking at the data, things weren't matching up with the true elevation difference with the slightly higher Crr I had originally assumed...so I quick popped off the front tire and discovered the -gasp!- butyl tubes inside.
Knowing true elevation difference can be pretty useful for getting an estimate of Crr.
In Reply To:
In Reply To:
When I got done, and started looking at the data, things weren't matching up with the true elevation difference with the slightly higher Crr I had originally assumed...so I quick popped off the front tire and discovered the -gasp!- butyl tubes inside.
Knowing true elevation difference can be pretty useful for getting an estimate of Crr.

Absolutely :-)

http://bikeblather.blogspot.com/
In Reply To:
In Reply To:
In Reply To:
When I got done, and started looking at the data, things weren't matching up with the true elevation difference with the slightly higher Crr I had originally assumed...so I quick popped off the front tire and discovered the -gasp!- butyl tubes inside.
Knowing true elevation difference can be pretty useful for getting an estimate of Crr.

Absolutely :-)
Yeah, I didn't know true elevation difference in my Bois de Vincennes runs so I couldn't pry Crr apart from CdA. Sometimes it's possible to pry them apart in other ways.

I'm agog that there's that much difference between a P2K and a P3C.
The P2K runs seem a little more stable than the P3C runs.
In Reply To:
In Reply To:
In Reply To:
In Reply To:
When I got done, and started looking at the data, things weren't matching up with the true elevation difference with the slightly higher Crr I had originally assumed...so I quick popped off the front tire and discovered the -gasp!- butyl tubes inside.
Knowing true elevation difference can be pretty useful for getting an estimate of Crr.

Absolutely :-)
Yeah, I didn't know true elevation difference in my Bois de Vincennes runs so I couldn't pry Crr apart from CdA. Sometimes it's possible to pry them apart in other ways.

I'm agog that there's that much difference between a P2K and a P3C.

It gogs me up pretty good too ;-)

Based on Andy's reports, I was expecting (hoping?) to see something on the order of a .015 to .020 m^2 difference...they way the drag drops with just an ever-so-slight yaw component was the most amazing thing to me.

I guess Gerard wasn't BS'ing when he says that the amount of coverage of the rear wheel and "shape" of the seat tube really "works"...

http://bikeblather.blogspot.com/
In Reply To:
The P2K runs seem a little more stable than the P3C runs.

I agree with that assessment. I believe that's the downside of my venue selection for this test and it's lower speeds. I'm sure if I had some "real time" weather data that could be improved.

I'm thinking any future frame upgrades for me might have to be accompanied by a portable weather station purchase :-)

http://bikeblather.blogspot.com/
In Reply To:
Based on Andy's reports, I was expecting (hoping?) to see something on the order of a .015 to .020 m^2 difference

Skeptic. ;-) I said in this post that my CdA was 0.020-0.030 m^2 lower on my Hooker than on a track version of the P2k:

http://forum.slowtwitch.com/...post=1344294#1344294

As a package, the Hooker was almost as good as the P3C (at 0 deg of yaw, anyway), but a good part of that was due to the Hooker "aero-or-die" handlebars. Take those away, and there's no comparison...
In Reply To:
In Reply To:
Based on Andy's reports, I was expecting (hoping?) to see something on the order of a .015 to .020 m^2 difference

Skeptic. ;-) I said in this post that my CdA was 0.020-0.030 m^2 lower on my Hooker than on a track version of the P2k:

http://forum.slowtwitch.com/...post=1344294#1344294

As a package, the Hooker was almost as good as the P3C (at 0 deg of yaw, anyway), but a good part of that was due to the Hooker "aero-or-die" handlebars. Take those away, and there's no comparison...

Not skeptical of you...but, skeptical of my ability to realize the same gains as you. There's a difference ;-)

http://bikeblather.blogspot.com/
In Reply To:
rmur asked me these questions in a PM and I hope he doesn't mind me answering them here so that anyone else with the same questions can see the answers

Quote:
Now for the reality check:
(1) Why such a high Crr? I was figuring 0.004 for a fellow who pays attention to tires.

Well, the simple answer is, they're not my tires/tubes I was wondering if someone was going to notice that.

Seriously, I'm running a VF Record front and a Bonty RXLPro23 on the rear on my wheels (latex tubes, of course)...and last month at the Piru TT I had the perfect opportunity on my outbound leg to nail down a Crr of .0038 to go along with a CdA of .228 m^2. My friend's 404s had Vittoria Open Corsa CX 20s with what I had assumed to be latex tubes inside. I hadn't had time to switch out tires yet so I just went with what he had for the testing. When I got done, and started looking at the data, things weren't matching up with the true elevation difference with the slightly higher Crr I had originally assumed...so I quick popped off the front tire and discovered the -gasp!- butyl tubes inside.

Check this out though...so I took a look at AFM's chart. I looked at the percent difference between my tires and the Vittorias and adjusted the .0038 Crr. I then looked at the percent change from latex to butyl and adjusted it again. That gave me an assumed Crr of .0056. When I put that into the spreadsheet, the calculated CdA for the zero yaw condition worked out to be...ta da! .228 m^2, just like the Piru data. Pretty cool, huh? Shows pretty clearly to me the validity of Al's testing.

Quote:
(2) Re the x-winds, I wouldn't get too hyper over those apparent levels - remember the demonstrated dependency of Wind_Chung.

What I learned is that with the "mini-halfpipe" course I used this time for logistical reasons (closer and shorter, i.e. more laps) is that with the lesser elevation difference as with my original "halfpipe", I don't see as high of max and average speeds...so the testing is much more sensitive to crosswinds since they can have a bigger affect on the apparent wind angle.

Quote:
(3) Everything the same - you didn't tuck any harder etc? I joke about it sometimes but I honestly do believe there could be a small placebo-like effect (in general)

Everything as "same" as I could get it. The funny thing is...if I had to choose which one "felt" faster just from perception alone...I would swear the P2K "felt" faster...go figure.

okay seems like you had the ducks nicely aligned. Sometime it'd be interesting to go out on a plain jane road bike and do this sort of thing! I can see you riding a P3C and drop bars in your crits soon!
Last edited by: rmur: May 21, 08 9:28

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