I was just going through some "what ifs"...and I think you might have miscalculated your ROT with respect to Crr.

According to Robert's presentation, a 1% change in Crr is ~equivalent to a 0.3% change in CdA. That would mean with a base Crr of .0040, a change of Crr of .0005 would be a 12.5% change. That would represent a (12.5 x 0.3% = 3.75%) 3.75% change in CdA. If your Cda is .200 -.250 m^2, that would be a change of ~.008-.010 m^2.

Shouldn't the ROT be more like .0005 Crr = .010 m^2 CdA? = 10W = 1 s/km?

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http://bikeblather.blogspot.com/

Not according to me. :-)

Consider a bike+rider with total mass of 76 kg traveling at 12.5 m/s: if Crr = 0.0040, then the power required to overcome rolling resistance is 12.5 x 76 x 9.81 x 0.004, or 37.3 W. However, if Crr = 0.0035, it is 12.5 x 76 x 9.81 x 0.0035, or 32.6 W. IOW, a change in Crr of 0.0005 is equivalent to a 4.7 W reduction in power demand.

I have to clarify that. That particular relationship depended on those data. In other tests I've done under different wind and slope conditions, I've gotten different relationships. Basically, Andy's ROT applies at the speeds we often see when TT'ing on flat ground. If you're riding at a different speed you're going to be on a different part of the curve.

    

OK...and I understand. In fact, if I just quickly figured out the "power cost" for incremental changes in Crr or CdA for my own weight and speed, I get basically .0005 of Crr = 5W = .005 m^2 Cda.

However, if I go into one of my test runs and vary the assumed Crr by .0005, I need to "adjust" the CdA by ~.010 to re-level the plot.

I guess what I'm really interested in is if somebody assumes that their Crr is lower or higher by a certain amount, how will that affect the CdA determined using the VE "leveling" approach? ;-)

edit: Never mind...the test run I had to "adjust" by a higher amount was a run I did on my road bike when testing tire pressure changes. CdA is MUCH higher and speeds slower on that. Definitely on a "different part of the curve" for that. When I pulled up my P2K run from last week, your ROT of .0005 Crr ~= .005 CdA held. Sorry 'bout that....

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http://bikeblather.blogspot.com/

Could be that donm decided to engage in a little banter to get a feel for the character of the folks in the room. Whether that was his intent or not, he succeeded. Anyway, congratulations and enjoy the big victory party in Lambda house. ;)
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It doesn't really matter what Phil is saying, the music of his voice is the appropriate soundtrack for a bicycle race. HTupolev

Depends on the course. Here's an example for a climb up a steep hill. The contours tell you the different combos of Crr and CdA that will produce exactly that net change in elevation over the run (in this case, I was pretty sure the true elevation change was between 113 and 115 m over about 2.5 km). The speed was quite slow and it turns out that the total elevation was quite sensitive to Crr but not much to CdA (as we'd expect since the speed was low).

I repeated the climb at a higher speed, and got a different slope between Crr and CdA. Given the true elevation change, it let me solve for a single (Crr, CdA) pair.

I did this because I couldn't find a lead bottle to do the delta mass thing.

Perhaps we should compare the complexity of the two tasks and what the result is. In one instant it is a simple procedure taking a few seconds to a minute requiring no calculations and giving an answer that applies to one individual at one point in time. Sort or like looking down at the power meter and knowing what your power is now.

In the other instant. It required a complex set up to be conducted with great rigor and the result is being applied "across the board".

I don't think the situations are particularly similar.

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Frank,
An original Ironman and the Inventor of PowerCranks

Wait a second. Here you're saying taking a temperature is a simple procedure, while in message #512 you wrote that it was complicated. Are you wrong now or were you wrong then?

It is a "simple" procedure that has many ways errors can be made if one is not attentive to details.

What Tom did was a complex procedure. The chances for error are increased substantially, even if one is attentive to details.

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Frank,
An original Ironman and the Inventor of PowerCranks

Hmmm. On second thought, continuing in the theme of simple answers to simple questions, I believe the correct answer is "both."

Doesn't Crr vary some with the slope, since the normal force varies with the slope? I suspect such variations might be very small for the slopes we normally encounter but does this affect your calculations or what Crr might be on the flat? Not trying to be contrary here but to simply ask the question.

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Frank,
An original Ironman and the Inventor of PowerCranks

Not until it's so steep you're slipping off.

Still lost in all of this discussion is something that would assuage some of the concerns about Tom's position- namely photos of him on the P3C and on the P2K. Remember, it was he who posited that his position was "exactly the same" for the different runs. Unless Tom measured himself while he was underway, that dog don't hunt.

Does this mean you're offering to get up at 5:30 to come over and take pics the next time I go out testing? :-)

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http://bikeblather.blogspot.com/

Well, a long time ago on this thread (seems like a lifetime), I posted two photos of myself on two different frames in a very similar position. Anyone care to say whether they see any significant differences between the two that could account for the large CdA differences I measured between the two setups?

Rik

"Unless Tom measured himself while he was underway, that dog don't hunt."

Hmmm....Taken to its logical conclusion, you're also saying people are wasting $1000s of dollars on wind tunnel sessions too.

Speaking of dogs that don't hunt....

 
Did a 20K TT today (Piru) on the borrowed P3C. Set a PR by 24 seconds in conditions near identical to my previous PR (temp, wind direction and strength) set last year. Did it on 8 less watts to the rear wheel as compared to last year. My tires have the same rolling resistance as last year.

Anybody want to guess how much I apparently reduced my CdA by to accomplish the above? The ROT should make that easy :-)

Damn...that was my last race on that P3C...I have to give it back now :-(

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http://bikeblather.blogspot.com/

How do we know you didn't move your head?

I was at a friend's barbecue this afternoon and some guy I'd never met before was talking to some other guy I'd never met before. I overheard him say the words, "time trial." He was talking about last weekend's SCNCA TT. He said his time was slower than last year and was pretty startled when I started describing the course, conditions, and that last year's times were 30 seconds off.

:-)

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http://bikeblather.blogspot.com/

Oh yeah...there should be pics up in the next few days. We can compare them to the 20K TT I did on the P2K 2 months ago ;-)

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http://bikeblather.blogspot.com/

How do we know the wind and conditions were the same?

You try that 2007/2008 Lake Los Angeles comparison yet? You could do the same thing with these two Piru TTs.

Well...you could either trust my memory ;-)...or we could look at the records of some of the personal weather station recordings nearby. I already did that. Unfortunately, the one closest to the course start (and IMO the most representative of the "on course" conditions) doesn't go back that far. However, the next closest does imply VERY similar conditions over the 2 dates. The weather at this time of year in that area is pretty much controlled by a "sea breeze" condition that sets up in the morning and starts the winds building out of the W/SW starting at 9 am...about when the racers start off. That's how it was last July when I set my previous PR, and that's how it was yesterday.

I haven't done the year over year comparison yet...too busy to dig into that. In fact, when I got home yesterday I spent the afternoon "skim coating" the walls in a room I'm getting ready to paint. The wall surface needs to be repaired because the room used to be wallpapered...I HATE wallpaper :-)

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http://bikeblather.blogspot.com/

Apologies to all for dredging up an old thread, but this past weekend I finally got the chance to do some field tests using my wife's P3C, and thought I'd share the results.

To wit: compared to her old P2T, my CdA came out precisely 0.010 m^2 lower on the P3C, despite the fact that the UCI-legal position I now use is a few centimeters shorter, and hence a shade higher, than my position on the P2T. Not all of this difference can be definitively ascribed to the frameset, as there were other unavoidable differences as well (i.e., Zipp 808/bulged disk vs. Zipp 404/flat disk and LG Rocket vs. Troxel Radius II aero helmet). Nonetheless, these data would appear to add additional support to the conclusion that there is a significant difference between the frames (to the order of ~1 s/km in still air).

At near zero yaw, correct? ;-)

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http://bikeblather.blogspot.com/

Well since you bring it up... ;-)

As I mentioned, the field tests results imply an advantage of ~1 s/km in still air, i.e., at/near 0 deg of yaw. OTOH, based on the non-calm TTs I've done on both bikes, the difference seems to be about twice that.

Another way of putting things in context: in breezy/windy TTs, my CdA on the P3C appears to be comparable to what it was on the Hooker, even though the former has a VT clip-on/basebar system and the latter had their proprietary "aero-or-die" handlebars. That's consistent with the bare-bike wind tunnel data that I/we collected. Now if only I could get my functional threshold power back up to where it was a few years ago...