KVF (Kammtail Virtual Foil) is an unconventional aerodynamic shape designed to employ the advantages of airfoils in a cycling-specific platform that is light, stiff, and performs well in crosswinds. KVF is the result of a nine-month engineering project dedicated to the development of low-speed airfoil technology. Airfoils are designed to reduce the amount of drag a rider experiences by employing a streamlined shape to frames and components. In contrast to a traditional teardrop-shaped airfoil, KVF is a unique truncated-tail design. This design greatly improves not only aerodynamic drag but also increases lateral stiffness, reduces weight, adds stability in crosswinds, and provides a more comfortable ride with additional vertical compliance.
What is the benefit?
Designing airfoils for bicycles requires a great deal of creativity, as the traditional designs utilized by airplanes and automobiles do not consider the unique properties of cycling aerodynamics. The relatively low speed and forward thrust of bikes makes them far more vulnerable to environmental factors such as crosswinds, and requires technology that focuses on more than improving headwind aerodynamics. Bicycle tube shapes inherently exhibit a great deal of curvature and combined flow interactions, further complicating the process of creating a truly aerodynamic design. While airfoils typically extend to a pointed end in a teardrop-type shape, Kammtail features a truncated, square end that increases stiffness and mimics the performance of a much longer, wing-like foil. Airfoils are defined in terms of aspect ratio, or the relative surface area between the total width and profile of a frame or component. As aspect ratio increases, drag decreases. Yet, as with all things in life, there is a tradeoff to high aspect ratios. Stability, weight, and stiffness suffer proportionally as the aspect ratio grows. Professional cycling’s governing body, Union Cycliste Internationale (UCI), places significant limits on how extreme this ratio can be, limiting the maximum to minimum transverse dimension ratio to no more than 3:1.
How Trek's engineers solved this problem
Trek’s engineers set out to achieve the aero performance of a high aspect ratio within a compact, light profile that would be stiff, stable and comply with UCI regulations. Veteran engineer Doug Cusack teamed up with new addition Paul Harder to investigate platforms for a cycling-specific aerofoil. As Paul explains, “at the time, bicycle aerofoils (industry-wide) were being designed using an old system for defining aeroplane wing aerofoils. I saw this as a major disconnect and decided to study aerofoils as they truly apply to bicycles as my first “personal” R&D project.”. In order to test as many designs as possible, the team created a new approach to computational fluid dynamics (CFD) that acts as a virtual wind tunnel capable of testing 3D drawings. This more efficient approach allowed Trek to test more than eighty different shapes, something that would have never been possible in the wind tunnel. Doug had a hunch that truncating an aerofoil could lead to a breakthrough in aerodynamic efficiency, and the CFD results the team saw with such designs exhibited unprecedented results. KVF was born.The new design solved a fundamental problem with previous platforms. Bike aerofoils have very high curvature compared to a more stretched out aeroplane wing or stabiliser type of aerofoil, and bikes also experience much higher yaw, or a larger angle between apparent wind (often inconsistent and coming from the side) and the direction of bicycle motion. The larger this angle is, the more unwanted lateral movement the bike will experience. As a result, air has a very difficult time staying attached to the aerofoil wall and tends to separate, causing large amounts of drag and reduced stability. The truncated design of KVF solves this curvature problem by using the portion of a high aspect ratio that works the hardest, (the front), and doing away with the pointed rear that is less important in cycling-specific applications. Winds flow around the front of the foil like a traditional design, and stay on this trajectory because truncating the shape reduces its curvature. As yaw and crosswinds increase, so does the performance advantage of KVF.
KVF performs like an 8:1 aspect ratio foil in headwinds, but complies with UCI’s 3:1 regulation. The shape can be applied to virtually every surface on the bike, including fork legs, downtube, seat tube, seatstays and handlebar. As a bonus, KVF is an inherently wider shape than a traditional teardrop aerofoil, and provides improved bike handling as a result of increased lateral stiffness.
KVF just keeps getting better
KVF was initially developed as part of a larger project to build the world’s fastest bicycle. Once the truncated design of KVF reached proof-of-concept in CFD analysis, our engineers constructed a complete prototype and took it in for extensive testing at the A2 wind tunnel in Mooresville, North Carolina and later at the San Diego Wind Tunnel in California. They were impressed with the results, and this bike soon became the original Speed Concept, Trek’s wildly successful triathlon platform. The benefits of this bike to triathletes were so immediately apparent that the engineering team soon began to expand the application of KVF to traditional road racing bike designs.KVF was easier to integrate into the smaller surface area of traditional road frames than previous aerofoil designs, allowing new conceptions of aero road bikes to come to life. Madone, Trek’s high-performance aero race bike, was brought into the fold with KVF technology in 2013. Speed Concept has since been revamped and is faster than ever, thanks in large part to a revised KVF with less frontal area and reduced drag at all yaw angles. The 2016 Madone also features an improved design in which KFV is an integral element. Trek’s engineers continue to search for more applications for this amazing technology, and have seen great success expanding it to more components such as handlebars
Is it proven?
KVF’s design has been widely replicated in the years since Trek’s engineers first unveiled it. Though we’re flattered that so much of the industry has adopted KVF-like design, we still push the limits of what KVF as a subtle technology can help a rider accomplish. As Paul says, “by nature, the KVF design must be highly-engineered, and casual knowledge of Kamm-style aerofoil construction and theory is not enough to design a high-performing KVF.” That, it seems, can only be left to those who discovered and perfected it.