Wind Tunnel Test Results

In my previous blog post I described my experience at the Texas A&M wind tunnel with John Cobb. Drum roll please… The results are in. Nathan Lesniewski, who was working with John Cobb for my testing session, sent me two files via email today:

Nico’s Report (MS Excel)

Nico’s Raw Data (PDF)

In the Excel Report there are three lines in the top section under “Comparison”. The “3T” line is my original position and my original equipment. (Pretty good…) The “Base Look” is my original position but with the Look Ergo stem and the ugly duckling bars. (Significantly worse) The “Final Look” is my new position, but with the Look Ergo stem and the ugly duckling bars. So by comparing the “Base Look” with the “Final Look” it appears that I could save 250 to 300 grams of drag by going with the new position and new helmet.

So the questions are:

  1. If I could get my 3T Ventus bars in the same position as the ugly duckling bars, would I save the 250-300 grams of drag from my baseline 3T numbers?
  2. Will I lose power by riding in this new position, hence negating the aero benefits?
  3. How much time does 300 grams of drag, at 30 MPH equal in terms of time saved over 40k? (I produce about 350 watts at sea level for 40k)

As for #2 above, I plan to do some riding with my PowerTap and see if I can produce good power in that position. My instincts say I can, as it felt pretty comfortable, and I’m flexible. But if I can’t, then raising my bars might be the way to go. A good amount of the drag reduction was due to the helmet and the narrowing of my arms, so I could decide to go with the higher position and narrower arms. I don’t feel like the narrower arm position would affect my power or breathing as I’ve raced like that before.

Any other feedback would be welcomed.

10 thoughts on “Wind Tunnel Test Results

  1. On the effect of power on speed, I do that analysis here. The result is the fractional increase in speed associated with a certain fractional increase in power equals the following ratio, where f is the fraction of power going into wind resistance (as opposed to rolling resistance or climbing), where the analysis is good to first order:

    1 / (1 + 2 f)

    So if there’s no climbing, and Crr = 0.4%, then and total system mass = 75 kg (for example), then at 30 mph you have 39.45 W going into rolling resistance. This leaves 88.7% of power from wind resistance. Thus I get:

    fractional change in speed = fractional change in power / 2.775

    300 “grams of force” is really 2.94 newtons. Multiply by 30 mph = 13.4 meters / second and you get 39.45 W. This is 2.86%. So I divide by that number 2.775 and I get a 1.03% increase in speed.

  2. 4% change in speed is huge. My recent 49:52 40k at Moriarty is 2992 seconds. Multiply that by .96 and you get 2872 seconds. That’s 120 seconds faster, or 2 minutes. Something must be wrong with that math because I can’t believe I’d go 2 minutes faster with the same power…. I’d love to believe I could, but I think the 4.06% must be wrong.

  3. hi folks,
    Uhm … using the basic ROT: 0.1lbf ~ 50gf ~0.5s/km and @ say 300 gf reduction in aero drag (at low yaw), I get 3sec/km or 120 seconds over 40km.

    Pretty simple math but it confirms a whomping big change!!

    rick

  4. Don’t worry Francis… While I’d love to believe I’ll be 120 seconds faster, I’m not convinced. But it’s a nice fantasy.

  5. Hey Nico,

    Cool posts.

    Here’s my answers for you:

    1. If I could get my 3T Ventus bars in the same position as the ugly duckling bars, would I save the 250-300 grams of drag from my baseline 3T numbers?:

    Well, probably not. You’ll certainly save something, but I don’t think you’ll get the whole of that.

    2. Will I lose power by riding in this new position, hence negating the aero benefits?:

    Maybe. Ya gotta try it! (Assuming you have a power meter to have a crack at it). I know with narrow arms it gets a bit harder to breathe, which can limit performance a bit. Still it is amazing what you can adapt to when you train specifically to adapt though.

    3. How much time does 300 grams of drag, at 30 MPH equal in terms of time saved over 40k? (I produce about 350 watts at sea level for 40k):

    That depends very much on what starting figure you have in grams/drag. Your average 3T drag over the yaw angles measured is 2518 grams or CdA of 0.226.
    (Aero drag (gf) =62.51*CdA*v^2) or
    (Aero drag (gf)/62.51/v^2=CdA)

    Note v^2 is in metres/sec; at tunnel speed of 30mph this is 13.3 m/s

    At 350w with 75kg, 0.004 Crr and flat course @350w I get a time of 50:53, pretty quick.

    If you can knock ~150 gms off, ie about half what you think, this is a CdA of about 0.211, and now @ 350w the time is down to 49:47, which is 1:06 faster than before, or about 2% faster.

    So the only real question is whether you can get the 3T bars into the same pos, and actually field test it!

    Let us know.

  6. Nico,
    The info here was excellent. Just a follow up. How were your time the past season on the new position? Your riding a 54?? saddle height was 80 cm. Your what, 6 feet even??

  7. I’d say that my TT times were about the same, or maybe even a little slower this year. But it’s really difficult to know whether this is due to lower fitness, worse aerodynamics, age, worse conditions… But I certainly didn’t go faster in 2010. Here’s to a faster 2011.

    Oh, I was also about 5 pounds lighter this year, and won some hill climbs. I think that my build in 2010 was maybe better for going up hill, and not quite as good for time trails.

    I’m just shy of 6′ 1″. And I’m riding a 54cm.

  8. Thanks,
    Its tough to quantify aerodynamic gains from races, there are just so many parameters and I hear you about getting older. I ride a 56 P3 and am 6 ‘ but I struggle to get lower in the front end, especially as UCI legal bars are not as low as the old HEDs or Ventus bars. Maybe a 54 P4 Evo is in my future?

    Good luck in 2011.

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