That Cheetah looks awesome but apparently they don't test well in the tunnel.

I agree. Especially since it is pretty much a rebadged Planet X!

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Fatigue is biochemical, not biomechanical.
- Andrew Coggan, PhD

the point being...it doesn't really matter what you ride, you still have to pedal the damn thing.

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Shawn
TORRE Consulting Services, LLC
http://www.TORREcs.com

But if you bike is 25-30% less aerodynamic than the guy behind you, how much harder do you have to pedal to win?

Chris

The real point being...you have to pedal the damn thing (and you can only pedal so hard), so it really matters what you ride, unless you are dramatically stronger than everyone else.

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http://bikeblather.blogspot.com/

I won't get in on the convo here other than to mention a quick thing about wind tunnel testing.

The Specialized data is from the A2 Wind Tunnel and the same test at the University of Washington (UWAL) showed almost the same exact graph. The Cervelo data is from San Diego. As all of the data is "good" and looks to show deltas between bikes, the raw values are not exactly the same. There are variations in where the yaw data kicks up or drops down based upon the tunnel.

We're looking at very small drag values in a localized yaw range. Most bike alone data is "good" to +/- 3-5 grams but each with tunnel will show a characteristic drag curve for a bike because of their apparatus, interaction with the bike, etc. Additionally, the Specialized data did not subtract out the apparatus tare so the shape might be cause by the apparatus.

I just caution people for taking wind tunnel data and comparing "local trends" between wind tunnels. Anyone on this thread that has tested the same exact model at different facilities knows what I'm talking about. If you want to compare trends, run the data all the way from 0-90 degrees of yaw and you'll see bike axis drag goes to 0 and bike axis side force goes to a maximum.

Additionally, the trends are very reliant on the wheels used (if aero wheels are used). Think of a bike like a wing and then put it in the wind tunnel. While each tube will have a local stall point, the wheels (especially discs) almost always dominate the bike's stall point.

Myself and a few other engineers here have taken our "control bikes" (2 different bikes) to five different wind tunnels over the past six months. The graphs (with and without tares) do not directly line up.

This topic could take an entire additional thread to really discuss, but these are just some notes to consider.

Mark

We tested with a Zipp 900 rear disc (flat). Additionally, the A2 tunnel seems to show a little kick up in drag at 10 degrees. I don't know what causes this but it is distinctive.

Mark

Mark...I'm glad you brought this up. Can you tell us what disc was used on your control bikes? It's hard to tell from the pics since they're spinning...it's obvious the front is the Roval Star Fusee, but not so obvious for the rear. Thanks!

edit: Doh! You answered before I could even ask...

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http://bikeblather.blogspot.com/

Ironically, the woman whose performance you use as evidence against the importance of aerodynamics has spent a lot more time than most trying to lower her (already low) drag. That she used the bike provided by her team merely reflects the sort of compromise often required of professionals, not a belief that "...it doesn't really matter what you ride...".

...as demonstrated by the results of the VeloNews sponsored (or at least reported) aero bike "shoot out" back in the mid 1990s.

Thanks for the info Mark. Since you have been so responsive, I was going to get around to asking you the question about the "shape" of the drag curves. In the meantime I wanted to try to get "donm" to do a little thinking for himself rather than blindly spouting off "x is faster!"

It does still seem somewhat striking to me to see the stall after 10 degrees in the Cervelo data and contined decreasing drag in your data after 10 degrees (and, as you mentioned, the slight bump up at 10 degrees). You mentioned the quirks of differenct windtunnels, the types of wheels used, not subtracting the the apparatus tare, etc. as possible explanations.

If the testing procedure didn't include sweeping back down the yaw angles couldn't that also be an explanation? I don't know what your procedures are, so you well may be doing that. It seems subtracting for apparatus tare at the different yaw angles could have some large implications for the overall shape of the drag curve, but that's just guessing on my part.


Somebody should start that thread!

Rik

if you think about it....

"every" bike looks very aero but aparently there is always a test that sais its as aero as a pig.

Every bike except the cervelos of course :P

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S�rgio Marques
When it hurts is when it feels good ;-)
Sergio-Marques.com

Not the whole bike -- just the handle bars -- i.e. there are none. She rode with only Aerobar extentions.

Rik

There are two major things that can affect drag data by sweeping up and down yaw angles:

1. Mechanical: yaw table backlash from the motor gearing; loss of pre-loading connection to a loadcell
2. Aerodynamic: separation bubbles that form on the downwind side of tubes, wheels, etc.

1. is not a problem at A2 because there is no motorized yaw table. The table is simply set to yaw and locked down. UWAL also does not have a problem with this from my past testing (anyone else?). We sometimes had issues with this at MIT but fixed it in my later years there.

For 2. aerodynamically, sweeping up and down yaw angles is actually different. i.e. 15 degrees should look slightly different whether the bike is "swept up" to it or "swept down" from higher yaw angles. Generally, you still see the same stall points and if adequate settling time is given before taking data, the bubbles usually settle.

1. is more of an issue than 2. but the best way to know if there's hysteresis in the measurement is to sweep up and down. I usually only do this once or twice at a wind tunnel and then decide on a sweep direction to do all runs at. I generally sweep from driveside to non-driveside and usually note if done using a motorized yaw table.

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Second note:

The A2/Specialized data has a very smooth curve from 0 decreasing as yaw increases. This is what one should expect from doing a bike wind tunnel test. Theoretically, take a flat plate and yaw it to the wind -- look at "plate/BIKE" axis drag. It's highest at 0 degrees and decreases to no force at 90 degrees. There will be some small local changes due to the aerodynamic shapes, especially from 10-20 degrees as this is where most tubes will see stall. BUT if you plot the whole graph from 0-90 degrees, the fluctuations should be small. This is what is seen in the A2/Specialized data.

When testing without a disc, this bike-axis drag decrease still exists but goes much slower because of the decrease in side force (sail affect).

Mark

Dear "Rik",

If there are any more numbered lists of questions you'd like answered, you just let me know!

Don

I didn't see your responses, so how about you just answer the first list again? Or maybe just one of them if three makes your head hurt. Or not. The rest of us are interested in actually having a discussion and exchanging ideas - you do not appear to be.

And you can call me "rik" (Slowtwitch name) or Rik (real name). But "Rik" confuses me.

Cheers,
Rik

Dear Rik,

1) Does your livelihood depend on the aerodynamics of your bike frame?

2) Would your livelihood be affected by the differences in drag between top-tier bike frames in the range that are commonly observed?

3) If your answers to 1) and 2) are "no", why exactly do you take this so seriously that you can't be cordial in a discussion about it?

Thanks,
Don

Don,

I thought I was being cordial (initially). I answered your questions and then provided some of my own to you in the same vein and tone of yours.

Then I responded in a bit less cordial fashion because of my perception of your uncordiality towards me.

Clearly, we have gotten off on the wrong track. I suggest we drop it and let the discussion go on...

Cheers,
Rik

I think that the front of the bike can be a little cleaner if you used a Hooker front caliper instead of the nose cone

Now that sounds fair enough. To be honest I guess I was trying to be controversial with the "Transition is fastest" comment. They're all fast frames and, unfortunately, most of us only have access to data produced by the various manufacturers so it's difficult to know which is fastest.