The Kestrel Airfoil has been the brand’s marquee triathlon bike for well over a decade and every version of it has shared one key feature—no seat tube. This unique characteristic reduced the Airfoil’s aerodynamic drag and allowed the bike to absorb road vibration instead of sending it up to the rider. At first glance, the Kestrel 4000 looks a lot like an Airfoil with a seat tube, but it is built around an entirely different design philosophy and its ride experience—not to mention aerodynamic characteristics—reflects the next step in Kestrel’s evolution.