Aerodynamic Frame Design - What is important?
Published in Ontario Association of Triathletes Transitions Magazine
June 1996 Issue - by Gerard Vroomen, Technical Director, Cervélo Cycles

Introduction
In today's world of triathlon and cycling, few topics are more misunderstood than aerodynamics. It is probably more difficult to find two people with the same opinion on this topic than it is to make drafting legal in the Ironman!

At normal triathlon speeds, 80-90% of the rider's power is needed to overcome aerodynamic drag. Roughly 2/3 of this resistance is caused by the rider, and 1/3 by the bike. Weight has only a minor effect, and only on hilly or twisty courses. Therefore, the best approach to improving performance would be to find the rider's optimal aerodynamic position - without sacrificing biomechanical efficiency - and a bike which works well with the rider in this position, and has minimal drag itself. Gains of 3-5 minutes over 40 kilometers are quite feasible through changes in position and equipment.

Aerodynamic Positioning
Good aerodynamics requires that the rider's back be as horizontal as possible, and good biomechanics requires that the hip angle is not compressed. For most people who are riding in a conventional position, both objectives can be achieved by rotating their entire body around the bottom bracket. This requires that the saddle be in a forward position (steep seat tube angle) and that the aerobars be positioned fairly low, which a 24 or 26 inch front wheel allows. However, some bikes combine a 26 inch front wheel with an extra long head tube, to keep the frame sizes (center to center) equivalent to conventional sizes. This not only robs you of the possibility of lowering the aerobars, it also increases drag, because a head tube has a horrible shape aerodynamically and is a lot wider than a stem. This is also the reason why some tests show 26 inch wheeled bikes with higher drag than the same designs with 27 inch wheels.

Tube Shapes
The non-horizontal tubes (especially the down tube and seat tube) should have an aero shape to minimize drag. Round and square shapes are about as bad as it gets, oval is better and a real NACA (National Advisory Council for Aeronautics) - airfoil shape is best with a drag reduction of 80% compared to a round tube. Also, narrow tubes are more aerodynamic than wide ones. Unfortunately, the width also affects the stiffness, which is why few aero bikes have adequate stiffness. Stiffness can only be achieved by using tubes with a reasonable width and some internal reinforcement or differential butting.

The Tubes
Earlier, we discussed the advantages of a steep seat tube angle and a short head tube. Now, let's take a look at the rest of the frame. Any frame, even a monocoque, can be divided into a number of tubes. By reviewing each tube, we can determine their importance for the structure and aerodynamics and decide how each should be treated in an ideal design.