The Felt IA: A Lesson in Aerodynamics

Have you ever seen a high end triathlon bike; not a UCI legal TT bike but a triathlon specific racing bike? With no limitations in terms of frame design, these bikes incorporate some of those most unique bicycle designs on the market. The reason for this? Any experienced triathlete knows the answer: aerodynamics. While road cyclists are obsessed with weight savings, triathletes obsess over their ability to cut gracefully through the air, saving watts, saving minutes, and saving energy.

There are plenty of good resources discussing aerodynamics and how they affect cycling (Saving Energy With Aerodynamics Aerodynamics: Debunking the Myths) but let’s look at how it works from the perspective of how it goes into the design of a bike. Over flat and rolling terrain at race speed, wind resistance is the greatest force affecting the ability of a rider to move forward. When Cameron Robertson and the Aerovelo team developed the Eta speed bike to break the human powered land speed record, aerodynamics was the primary concern. It was so important to achieving the speeds needed to break the record that even a single bug splatter on their aerodynamic fairing could cause enough drag to prevent them from achieving top speed.

Normal riding, however, doesn’t rely on such a perfect system and there are many points on a rider that cause aerodynamic drag from the buckles on shoes to the shape of the frame. In professional cycling, all these factors are accounted for by riders. They use shoe covers to create better flow over their shoes, they wear skin suits designed to reduce skin surface drag and adjust their position to reduce their frontal drag as much as possible while still being able to produce race winning efforts. Their only limitation is UCI regulations on frame design.

The opposite happens in triathlon. Shoe covers and skin suits are impractical and the time required to take them on and off in transition is more than the savings they create. Where triathletes gain a strong advantage is the position of the rider and the design of the bike and wheels. This is why machines like the Felt IA looks like something straight out of science fiction. It is designed to be as aerodynamically efficient as possible. A lot goes into the design of these bikes so let’s look at some of the considerations that engineers account for when designing bikes.

Most of us are familiar with the airfoil, the tear drop shape of an airplane’s wing. In aviation the tear drop shape is
manipulated to create a low pressure area above the wing and high pressure below to generate upward lift. In bicycle design, the primary focus is cutting through the air with the greatest efficiency, sometimes this can mean creating an airfoil and other times it can mean decreasing the amount of drag created at less than optimal airflow angles. An airplane wing, for example, relies on the forward thrust of the plane to push air over the wing parallel to the wing (air flows over the wing from front to back). An airfoil does not generate lift if the air moves over the wing from top to bottom or bottom to top, in fact, this would create a high degree of drag. Gusts of wind crossing over the airfoil at less than optimal angles is a major contributor to turbulence we experience during flight. Airplane airfoils are designed to generate lift from air moving in a very specific direction (front to back). Bicycles on the other hand, need to be aerodynamically optimal from air flowing over the bike from a variety of different directions. This means that the standard tear drop shape isn’t necessarily the best design for bike tubes.

Felt’s Egg shape design actually creates forward thrust when the wind is coming from certain angles.

Let’s untangle some of the jargon by looking at the Felt IA’s frame shape. If we look at the frame shape of the IA you’ll notice that the down tube (the tube leading from the handlebars to the pedals) is very wide and narrows slightly towards the inside of the frame. This is a very shallow tear drop. It doesn’t come to a point but is more of an egg shape. Older aero bikes and some current models use very sharp points maintaining a true tear drop shape in the down tube. This is efficient for winds coming from directly in front of the rider but creates a large area of drag in crosswinds. These bikes often boast wind tunnel testing with positive results. And they would get positive results if the bike were setup to face the movement of air straight on. The IA’s construction, the egg shape, allows the bike to achieve better aerodynamics from a wide range of angles (called yaw). It sacrifices some performance in direct wind but gains significant performance in cross and cross headwinds. This allows a rider to continue to maintain their aerodynamic advantage in a wide variety of practical conditions on the road.

The depth or width of the tubes help to create a smooth plain for the wind to move over and eliminates the point of separation where the wind can no longer follow the flow of the tube. The earlier the point of separation, the more drag there will be. The IA’s design allows air to flow more smoothly and separate later, reducing the area of drag. See the video below for a more scientific explanation of separation and drag.

Felt is one of the few manufacturers that truly accounts for crosswind and real world riding in their aerodynamic calculations. The list is growing as more manufacturers realize the need for better development in aerodynamics but  still remains small. Felt uses computational fluid dynamics (CFD) software to simulate airflow over their frames much in the same way Aerovelo did with their record breaking bike. Using computer models allows the engineers to develop a near perfect design before ever laying any carbon. Once the design has been optimized by the software, Felt takes their design to the wind tunnel to test in various conditions. Refinements can be made to design at this point and the final aerodynamic profile of the bike is achieved.

Computational Fluid Dynamics (CFD) in action.

The reality of aerodynamics is that it is difficult to know how altering shapes will affect the overall aerodynamic ability of the shape. This is particularly true of bicycles, which have many curves, shapes and bends that can alter the airflow over even the most aerodynamic shapes. When a bike is put in the wind tunnel and its drag coefficient measured, there is no algorithm to determine what needs to change to make the bike more aerodynamic. The process is largely left up to the experience of the engineer and can involve hundreds of tweaks based on educated guesses to get the perfect design. In CFD simulations, changes can be made quickly and engineers have the ability to look at many different scenarios at once. The result of these labour intensive efforts is what we find in the Felt IA, a bike that pushes the boundaries of aerodynamic performance.

The traditional upright riding setup of bicycles that we’re familiar with is one of the most difficult positions to optimize aerodynamically, this is why Aerovelo chose a recumbent position with a fairing to cover the rider. There is no best design that will work in every situation. Every bike has it’s advantages and disadvantages and because of the research and development costs associated with bicycle design, manufacturers often have to pick one or two considerations to spend their time optimizing. This can mean designs that are excellent in cross winds but poor in calm conditions, or vice versa. The best designs are those that account for every possible variable, a near impossibility. Manufacturers like Felt, who devote a substantial portion of their R&D specifically on triathlon bike design, come very close to accounting for many of these variables. There a lot of great bike manufacturers out there who have a similar passion for aerodynamics (see Argon 18 and Cervelo) but the Felt IA stands near the pinnacle of bicycle design with respect to aerodynamics.