Focusing on the Rear--Leading Edges and Aerodynamics

posted Aug 16, 2009, 2:11 PM by Donald Vescio   [ updated Sep 12, 2011, 8:11 AM ]

I've been thinking a good amount about frame aerodynamics as of late and am especially interested in the design of the rear end of the bicycle.  When I started racing in the mid-eighties, my time trial bikes where of double-diamond design and were constructed out of standard round steel tubing.  While disk wheels, cowhorn handlebars, and eventually clip-on aerobars became part of my aerodynamic calculations, the frames themselves remained relatively consistent.

State of the Art Schwinn Time Trial Bike, Southern California Desert, Circa 1990 or so. 
My start was at 6.10 A.M., local time; the temperature was already approaching
100 degrees.

In the late eighties and early ninties, aerodynamic frames became available to the public, first in the form of hand-sculpted and flattened tube sets like Hooker's Elite, which were ridden to many national championships in the U.S.

Soon after, developers realized that there actually are two ways to decrease the aerodynamic drag of a bicycle frame: one can make the frame members as narrow as possible (Hooker's approach): or one could remove frame members altogether, thus giving rise to Zipp's and Softride's beam bikes. (NB: the third way is to optimize the shape and airfoils of the tube sets, but this topic is for a different article.)

Zipp's 2001 Frames (2001 was the name of the model, not the year
in which it was made)

Softride's Frame; I raced a Softride Power V for two seasons in the early nineties

The Hooker, Zipp, and Softride frames all shared a central design philosophy: the best way to reduce bicycle frame aerodynamic drag is to manage airflow.  While each frame tried to realize this goal through different principles, all attempted to smooth the passage of air over the frame with a minimum amount of disruption.  (Remember: airflow disruption = turbulence = drag = slower times.) The concept, actually, is pretty simple: the more the air is disrupted as it travels over a frame, the greater the aerodynamic drag.  

Within the context of the rear portion of a frame, airflow potentially is disrupted twice as it first hits the seat tube, and then the leading edge of the rear wheel.  The space between the leading edge of the rear wheel and the seat tube, then, becomes a site of turbulence, which creates drag:

Airflow represented by green horizontal lines.  Note turbulence in the space between the leading edge of the rear wheel and the seat tube.

Unlike a traditional double-diamond frame that presents two leading edges (rear wheel and seat tube), a beam bike offers only one leading edge, the front of the rear wheel, which (in theory) minimizes the disruption of the airflow and enables the air to pass smoothly along the surface of the rear wheel (a rear disk was necessary for proper performance of a beam bike, for its smooth surface facilitated airflow):


Another way to minimize the number of leading edges located at the rear of the bike is to move the rear wheel into the seat tube itself, thus enabling the seat tube (if properly designed) to function as a fairing for the rear wheel.

Note the curved seat tube in orange--Cervelo (and others) use this design in their UCI-restricted framesets.

Now, there are lots of different forms of aerodynamic drag that impact bicycle performance and what recently has attracted my attention were some of the results that Look gained from their wind tunnel testing.  According to Look, the problem with tight rear wheel frame cutouts (like Cervelo's) is that a forward spinning wheel forces airflow between the tire and seat tube.  Because clearance is so tight, this forward moving air gets trapped, forming an air dam.  This, in turn, increases aerodynamic drag.  Whether this drag is significant currently is under debate; according to Cervelo, a well-designed tight rear wheel cutout will work much better than a poorly designed rear wheel cutout; Cervelo then goes on to say that it's better to have a large rear wheel cutout than a tight and poorly designed cutout (their argument is that Look's emphasis on space in front of the rear wheel compensates for poor design).

Cervelo P3C--Note tight rear wheel cutout

Look's 596--note the ample space between the seat tube and rear wheel.

I've intentionally tried to keep this discussion as simple as possible by eliminating from consideration that impact that tube shapes have on a frame's back-end performance; what I'm mostly interested in right now are the number and position of the rear frame members and how different configurations might make a qualitative impact on aerodynamics.  I rode a beam bike in the early nineties with mixed results--while the bike was very fast, the frames failed frequently (in one season, I went through three separate frames, each breaking in a different location) and the oscillation of the beam itself was disconcerting.  Because the material science at the time could not meet the demands of the frame's design, I went back to double-diamond frames.  For the past four years, I've been racing Cervelo's P3C with great success.

But I still like the design elegance of beam bikes and I am sympathetic to Look's argument about rotating wheels and air dams--though I don't think that I've ever actually noticed any impact like this when riding my Cervelos.  But I still cannot help but wonder if a hybrid design might possible offer the best of Look's, Cervelo's, and the pioneers of the nineties' arguments.

Double-diamond frame, with seat tube completely eliminated.

In the example above, a single leading edge for the rear of the frame is established by completely eliminating the seat tube.  The presence of seat stays still offers a degree of triangulation that should preserve ride performance; ideally, they would be placed far enough away from the rotating rear wheel so that their impact on airflow can be minimized.  In such a design, a rear disk would be necessary for optimal aerodynamics; even with a disk, the rear tire would need to be carefully matched to the rim's width to avoid any aerodynamic discontinuities.

Such a frame does exist.  Kestrel's Airfoil has been around for several years and its tube profiles have been updated significantly for increased aerodynamic performance in 2009.

So, the big experiment for me in 2010 is to see if all of this discussion of leading edges actually holds water and I'll be taking delivery of an Airfoil Pro very shortly to test it out in comparison with my Cervelo P3C.  I'll share data and results are they become available.  For now, what I'm doing is working out my thought experiments in anticipation of actual physical testing.

Of course, though, we all know that it is the FRONT of the bike that is most critical for aerodynamic performance, which is a whole set of conversations for another day ;)