Nope. Perhaps I didn't remember correctly...I'm sure a search on Mark Cote's quotes (MITAerobike) might confirm Damon's statement above.

---

http://bikeblather.blogspot.com/

Well, the admission I was making was that I've been avoiding the terms "positive" and "negative" yaw because I became confused after reading tunnel reports posted here. I think I recall Mark and Damon using the convention in the paragraph above, but the convention used in the Project 96 studies is the airplane convention. So (uncharacteristically) I decided to shut up.

For example:

http://www.slowtwitch.com/..._McLarenHelmet10.jpg

How do I know I have repeatable data? Is it the same CdA # or the wave of the graph looking exactly the same within each session?

Thanks

Well, if you do things right, no matter what the CdA, all the curves will look like the real elevation. Repeatability should be measured by how consistently the same set-up will yield the same CdA.

---

AndyF
bike geek

This is what has me worried. I'm doing a half pipe and I can see the peaks on both sides, but the peaks are not at the exact same elevation each time. If it helps here is a screen shot of my graph:



Thanks for all the help.

You're off by, what, 20cms per 1km? .2% There must've been practically no wind that day. Very nice!

---

AndyF
bike geek

lol. You mean all I had to do was use the scale on the left side of the graph to get some perspective?

Thanks again.

Ummm, yeah. :-)

Congrats on an excellent VE test, BDU.

---

AndyF
bike geek

Yeah, but it looks like the first 3-1/4 laps may have had very (very) slightly higher CdA than the last 2 laps. Anything change?

I didn't change anything in my setup. What are some of the variables that could change this? Wind, not following the exact same line, or different speeds at the turn around?

I'd like to add a sub-point to the "Don't use your brakes". Make sure your brake isn't rubbing! I did some testing where I switched from my H3 to a disc to try to test the difference. The results with the disc were really bad, which was confusing (roughly from .23=>.27). I then noticed that when I spun the wheel freely in my bike stand that it was very slightly rubbing at one point. It was subtle enough that I didn't notice when installing the wheel or while riding (perhaps I shouldn't listen to music while testing). Another day of testing bites the dust :(

OTOH, it shows that the method can pick up pretty small effects.

I've got a bit of a mystery that I'm curious to get some feedback about. I did some field testing today and got some results I don't believe. I'm looking for potential holes in what I did as well as a question of plausibility. My goal was to try to identify whether my desoto liftfoil trisuit or my new cycling team skinsuit was faster. I was expecting that they would be very close and guessed that maybe the trisuit would be a touch faster. I warmed up by doing some testing of two different helmets with so so results, it was hard to distinguish them, but I felt ok about my test setup. I had my race setup, except I was using my powertap rear wheel which has slow tires (it was on the trainer before). I was getting speed and power from the powertap, turning off the gps for speed on my 705 (I have had trouble in the past with my gsc10 sensor and quarq). I did three runs where each run went about two minutes from home to the start of my halfpipe, did two laps of the halfpipe and then went home. I didn't turn off the garmin at home, just put down the bike, changed trisuit for skinsuit, and did the next lap. The results in aerolab look very clean. I was just looking for deltas in CdA, not absolutes, so I just left Crr as 0.005, although I did put in reasonable values for mass and rho.

-) Run 1 and 3: matching CdA of ~.217 in trisuit
-) Run 2: CdA of .192 in skinsuit

I stupidly didn't do a 4th pass in the skinsuit, so I assumed that there must have been some error. After lunch, I decided to give it another shot. This time I got:

-) Run4: CdA of .193 in skinsuit
-) Run5: CdA of .215 in trisuit

The runs also look great in aerolab and match the first runs closely. So is it plausible that there is that big a difference between the skinsuit and my trisuit. Is there some other error that might manifest itself in such a way that my runs match each other so closely? For reference, the skinsuit is the one I'm wearing here. I didn't think it was supposed to be exceptionally fast, it was just the one the cycling team I joined had in stock.

My first instinct is to ask whether you've adjusted your air density according to the time of day. Although they track each other fairly well over the course of the runs (skinsuit vs trisuit relative difference). The absolute CdA you're quoting is probably a ways off, though.

As far as the delta between skinsuit and not...we'll have to let some of the more experienced guys come in. 0.022-0.025 change in CdA is huge, though.

---

The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed all-important.

-Albert J. Nock

Just to be clear, runs 1, 2 and 3 were done over a total duration of 35 minutes, with 31 of those riding (just about 2 minutes per change of clothes).
Run 4 and 5 were done two+ hours later where the temperature had indeed risen by a few degrees, but they were also done within a 23 minute window (21 of those riding).
I'm sure the absolute CdA is inacruate which is why I mentioned that I was just looking for relative results. I also agree that >0.02 CdA change doesn't seem right which is why I redid the test assuming a bug and then decided to ask here after my second test confirmed the results.

I realize I should also have put a picture of the trisuit. This is a slightly older picture of what that looks like. I certainly wouldn't have thought it was a dog. The only thing that comes to mind is that the stitching is pretty raised on it. Otherwise it is nice and tight, etc.

FWIW, when I visited the wind tunnel last year we did some skinsuit comparisons. There were several riders and one brought a team suit that had raised stitching in a creative diagonal pattern that looks similar to your tri suit. It was a full skinsuit but the stitching was similar. It was clearly the highest drag skinsuit of all tested that day and added more than 200 grams of drag relative to faster suits and IIRC nearly 300 grams of drag relative to the Castelli bodypaint suit. The best guess of the testing folks was that those raised seams were in all the wrong places in terms of tripping the air flow and though they look kinda nice they may not be the best thing from an aero standpoint.

BTW, what are you assuming for Crr in your Chung testing? Are you basing it of roller testing or regression testing done in another session or just swagging it?

-Dave

The Crr is just a swag. My front wheel is a Jet9 with a GP4000s and latex. The rear is an American Classic 420 with a powertap and some cheap "strada-k" tire with butyl that I use on my trainer. I've got a Jet disc that I'll use racing, but I was hoping the PT would give nice even results for the field testing.

If I remember my rules of thumb, its 50 grams of drag = 5 watts = .005m^2 CdA @ 30mph. So 200-300 grams would be .02 to .03m^2 CdA. So what you are saying is that it is not on the face of it absurd that there might really be that big a difference between my skinsuit and trisuit?

If my data is right I am sad since I really like my DeSoto stuff and would be sad that it is slow. I've certainly tested it in the pool and it is pretty fast there. I was just assuming it would be fast on the bike too :(

How high does the zipper go on the front of the skinsuit and the trisuit? Is the area around the neck equally tight?

I've never tested a sleeveless trisuit so I don't really know but a difference of .02 m^2 is bigger than I would have thought.

The short answer is: absolutely. We've seen this happen a couple of times. To quote Paul Harder of Trek, "Skin is slow". :-)

---

AndyF
bike geek

I know that the TriTalk podcast did some roll down aero testing of a few tri suits a while back and that of the 3 or 4 tested (not a huge sample or a controlled test, I know) the DeSoto LiftFoil tested the best. I always assumed it was a little faster than other tri suits because the stitching all runs parallel to the direction of the wind coming over the back and there is no pocket sitting perpendicular to the airflow on the low back to trip it up.

So, as far as tri suits go, I think you could do a lot worse than the LiftFoil.

---

__________________________________________________
The plural of anecdote is not data. :-)
- Andrew Coggan

The zipper on the trisuit is not as high and the neck is probably not quite as tight. So I guess it could be a combo of the neck tightness/coverage, the seams and less coverage (skin is slow).

Slightly off topic, but I haven't tried out a skinsuit under a wetsuit before. Any reason to think that won't work well? I don't have to run afterward; I just swim and bike.

Apologies if I sounded overly critical about your protocol--it looks solid.

Your testing makes me wonder about my own clothing selections! I have a baggy team one-piece, a tightly fitting Kiwami, and a new voler skinsuit that requires a second person to put on. Historically, I've just done TT's in bibs + a long sleeve nike pro compression shirt.

What's the pocket situation on your suits? I'm inclined to think that makes a pretty big difference, too.

Seems like it's time for me to test and see where everything shakes out. Hopefully not as big a difference as you've seen. :/ Unless my team suit is fast, then I'm okay :)

---

The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed all-important.

-Albert J. Nock

No problem about being critical. That is what I'm looking for! I was assuming I'd made some dumb mistake but couldn't figure out what it might be. I was hoping someone might poke holes and figure out what I did wrong. On the other hand, if my testing is correct, I ought to be able to get a bunch of PRs since I used the trisuit in all my races last year including TTs.

Neither one of the suits I tested have pockets, so that isn't the issue. As an aside, what does the second person do when putting on a skinsuit ;)