In recent years many time trial bikes have sprouted nose cones and structural fairings to improve aerodynamic, but more recently the UCI has signaled that it intends to crack down on designs that infringe on its ’3:1′ rule, which says frame and components can’t be more than three times wider than they are tall; a 1-centimeter-tall handlebar, for example, can’t be more than 3 centimeters wide from front to back.
The enforcement doesn’t necessarily limit innovation. Trek Bicycle, for example, was already looking beyond nose cones and fairings.
The company’s road product manager, Tyler Pilger, is a car buff familiar with an automotive aerodynanic feature named after the German aerodynamic engineer who first envisioned it: Wunibald Kamm. It’s called the Kamm Tail (See Wikipedia entry).
The Kamm concept is to take an 8:1 aerodynamic foil shape — the optimum ratio in a head wind, according to Pilger — and cut off its tail just past the widest point. The design tricks air into acting as if the tail of the foil is still there, so you get 8:1 aerodynamic efficiency within the 3:1 UCI rule. But it gets better, because the Kamm Tail is even better than an 8:1 foil in crosswinds.
“The air thinks that it’s an 8:1 airfoil, but it can bend at yaw,” he said.
The concept has been proven in automotive use for close to 70 years. Today’s best examples are the Toyota Prius and the Volvo S60, but many historically impressive cars share the design, including Ford’s GT40 and Ferrari’s late ’60s model 365 GTB Daytona. The Kamm design allows shorter, lighter car bodies while preserving aerodynamics.
Trek claims that its new time trial bike separates itself from the previous TTX models (and its competitors) in crosswinds. The bigger the yaw, the faster it gets.
Additionally, the wider cross section of the bike’s airfoil shapes dramatically increases stiffness, according to Pilger.
“We were able to pull out about 200 grams over our current TTX,” he said. “And it’s 17-percent stiffer in laterally. By maintaining the width we get a much stiffer tube than you would with a traditional airfoil.”
Trek has a patent pending on the Kamm tail design for bicycles, which it has coined KVF, or Kamm Tail Virtual Airfoil. The technology is found on the fork legs, downtube, seat tube and seatstays.
Trek isn’t yet publishing aero stats on the frame, but at a recent press presentation, it quickly flashed a graph that separated the Speed Concept bike from the current TTX by 90-grams of drag at about 15-degrees of yaw — a huge number. Pilger said the number only represented the savings of the downtube, not the entire frame.
The crazy black and white swirled paint job found on the earliest prototypes, as used by Alberto Contador at the Dauphine Libere, also borrowed from the auto industry. BMW uses the unconventional camouflage paint to hide details of its new designs from competitors when out on the open road — exactly what Trek wanted.
The design, and the paint job, also does a good job of hiding both brakes and just about all of the cables, except for the rear derailleur housing loop, from the wind and viewers’ eyes. Along with the integrated brakes, the Speed Concept frame uses a custom integrated bar, Trek’s BB90 Net Molded bottom bracket design and the new DuoTrap speed and cadence sensor integrated into the left chainstay.
For now, only the team will use the Speed Concept; it’s not in Trek’s 2010 line.
“Right now, it’s strictly a proof of concept,” said Pilger. “There are certain things like handlebar adjustment and those sorts of things for production that we would want to take to the next level.”