Anyone trying to keep tabs on what’s the most aerodynamic bike these days is likely chasing a fools errand. There’s a lot of science being applied to different brands, tube shapes and bike/wheel/rider “systems”. That said, we’ve ridden some aero bikes and wheels, and they do make a difference. Whether we could honestly tell much improvement from one to the other is unlikely, not just because of our own perceptions, but because conditions are never identical from one ride to the next and our fitness levels are always fluctuating.
That said, it’s always an interesting discussion to see the science and technology behind bicycle aerodynamics, and Parlee is no stranger to making things go faster through a fluid…which is, after all, what air really is. His latest iteration of the TTi, which gets a few updates to the layup and cable/wire routing for 2013, takes a bit different approach than some of his competitors…
Bob Parlee, who got his start building racing boats, used various low-drag NACA sections in combination with his own low-speed sections that are different than “high lift” shapes used elsewhere. A “high lift” shape is a long, skinny tear drop shape that works well on airplanes and boats where you can change the angle of the wing section. On a bike, such a long shape would have to be fixed, which would cause it to “stall”, or create a lot of drag.
On a bike, you’d want a shorter cord length (the distance from leading to trailing edge, generally limited to 80mm due to UCI regulations), fatter tear drop shape with the maximum thickness set farther back, about 45% of the way back, than on a high-lift application, where it would be 30%-35% of the way back. The shape of the trailing edge also tends to be a little fuller with more shape to them, which makes the air want to stay attached as it wraps around the tube, which reduces turbulence, and thus drag. The technical term is that it’s less critical, meaning they work better at higher yaw (crosswind) angles.
Tests are done with competing bikes using the same components and wheels to keep things even. If you recall from recent aero wheel introductions, a big selling point is the ability to perform better in a crosswind, with some parts claiming to reduce drag even to the point of having negative drag at certain yaw angles. The Parlee TTi doesn’t quite get into negative numbers, but its aerodynamics do improve up to about 5º.
The NACA shapes, by the way, are a range of shapes categorized by the National Advisory Committee for Aeronautics. Mavic used two of them on their new Cosmic CXR80 wheels, too.
The fork has evolved quite a bit over the years, the latest iteration spaces the legs and crown wider -farther from the wheel and tire- than what’s typically seen. Parlee says this opens it up and lets the air split better around them, creating less overall turbulence. They’ve tested their bike in the wind tunnel with this fork and a narrower fork and measured about a 5% to 7% improvement from this design.
The frame is mechanical and electronic ready and uses TRP’s aero mini-V brakes.