• Increase font size
  • Default font size
  • Decrease font size
Home Aerodynamics Misc Big Fat Helmet

Big Fat Helmet

E-mail Print PDF

Big Fat Helmet™

Can someone please tell Robbie McEwen to retire the Big Fat Helmet™ that he and his teammates wore during the opening Tour de France prologue?  It is physically painful for me to look at that thing.  Not only is his helmet just plain dorky looking, there isn’t a doubt in my mind that it is an extremely slow helmet and ultimately is a detriment to his time-trial performance.  What would make me say such a thing? 

Well, I was recently lucky enough to spend some time in the San Diego Airs and Space Technology Center wind tunnel where I got to tunnel test some one-piece aero bars.  During that entry I also got to stick my long-legged and gangly self in the test section and quantify my aerodynamic properties. 

Figure  1. In the tunnel at Allied Aerospace.

As part of my aerodynamic positioning analysis, I took the time to verify that my tunnel results were reflective of real-world results.  I did this by conducting a field-test using a power meter, an anemometer (wind speed measurement device), and a known piece of roadway. 

As one should expect, the “real-world” test showed that my tunnel numbers and field numbers were in agreement to within a few percent.  As a final step in documenting my current position, I also measured my frontal area using a makeshift photo-studio in my living room.  The image below illustrates the importance of making sure your pets - in this case my dog Lucho (named after the late 80’s Columbian climber Luis Herrera) – are kept confined.  This confinement, of course, is necessary in the name of science…

Figure  2. The photo studio and Lucho.

Thanks to the miracle of a modern digital photo manipulation software package, it was possible to remove Lucho and my bicycle from the original photos in an effort to improve consistency in the results.  What was left, then, was a bunch of black and white pixels that could be counted in order to determine a frontal area measurement (hint: the inclusion of a known reference area - that black piece of construction paper hanging on the ruler to the left – is what allows for the final correlation of pixels to square meters).  The images below shows some examples: my standard TT position with three separate helmet configurations.

Figure  3. L to R: no helmet, 1994 Specialized, 1998 Giro.

Figure  4. L to R: no helmet, 1994 Specialized, 1998 Giro.

When this exercise was all said and done, the frontal area analysis for the middle image above (my standard position and aero helmet) correlated well with tunnel measurements using some standard assumptions defined in the scientific literature with regards to my body shape.  After my position investigation was finished, I had convinced myself that measuring frontal area was meaningful.

Back to the helmet thing, though:  the black and white images above should show that the conventional ro helmet presents a slightly larger frontal area (as the distinct bulge between my shoulders shows in the right-most image above).  I would speculate that the head fairings that pro’s in Europe used last year did not significantly increase the frontal area of a well-positioned rider.  Furthermore, the streamlined nature of those fairings should have improved rider performance due to the more aerodynamically slippery shape that resulted.

With new safety standards for TT helmets in place this year, helmets necessarily got bigger in frontal area.  From the looks of it, they are nearly as big as a standard helmet, but are, for the most part, more streamlined.  If one makes the precarious assumption that the only thing that changed in the aerodynamic properties of the helmets from last year to this year was the frontal area, we should be able to ballpark just how much slower these new “safe” helmets are during a 40k TT.

The following table shows the results for the three helmet configurations I took data on:

Based on a frontal area analysis, it is reasonable to estimate that the wrong aero helmet can impact TT performance by around 20 seconds or so.  In Robbie McEwen’s case, though, I think the penalty is much greater:

Figure  5. Big helmet on the left.  Big Fat Helmet™ on the right.

…and the updated table:

So, who is going to tell Robbie the bad news about his Big Fat Helmet™ – you, or me?

Last Updated on Sunday, 24 January 2010 04:53