THIS WEEK IN PRO CYCLING
Tech Report: Slipstream visits the wind tunnel
David Zabriskie is the only guy in the world who has earned a very special yellow sticker shaped like the map of France for his top tube. The sticker proclaims “Fastest Time Trial Ever: 54.676kph, David Zabriskie.”
Ever since he won the 19km prologue at Noirmoutier in 2005 at that record speed, beating a stellar Lance Armstrong and donning the yellow jersey, cycling fans have marveled at his great bike position. But despite having a very low, narrow, incredibly aerodynamic position, his first time ever in a wind tunnel was Monday, November 12.
That will probably come as a surprise to many, figuring that a finely tuned position like his would have been at least partially the result of the means and technology of his powerhouse Team CSC investing heavily in wind-tunnel sessions. Cervelo even uses a full-size plastic Zabriskie in the wind tunnel, produced from a 3D body scan of him in his time-trial position. Yet the flesh-and-blood Zabriskie had not been in the tunnel before. And when asked whether he’s had any coaching on his time-trial position, he replied, “So far, it’s been just low bars and stuff.”
Wind-tunnel testing is part of the Slipstream-Chipotle team camp, and the strong time trialists on the team will all have a go against the wind this week at Fort Collins, Colorado, 50 miles north of the team’s headquarters in Boulder. The Fort Collins facility is “the largest high-precision, low-velocity wind tunnel in North America,” according to Steve Owens of Colorado Premier Training (CPT), which specializes in testing cyclists and triathletes in this tunnel.
DZ’s teammate, David Millar, also a Tour de France prologue winner and yellow-jersey wearer (in 2000), is also a relative wind-tunnel neophyte. He was in the Allied wind tunnel in San Diego once with Saunier Duval, but says “it was a bit of a waste” — too short a stay and none of it devoted to power testing.
In Fort Collins, however, CPT’s Robby Ketchell says, “We do power measurements outside of the wind tunnel on a Position Cycle attached to a Velotron, so once we find a position in the wind tunnel, we can see if the rider can produce power in that position.” Positioned directly under the wind tunnel, which is raised about eight feet off of the floor, the power-testing setup can give immediate position-modification feedback after a data run in the tunnel.
Unlike most wind tunnels used for cycling, this tunnel is neither part of a university or other research institution nor owned by a big corporation. Rather, it was designed and built by engineer and meteorologist Chet Wisner, who, along with his wife, Delores, owns and operates Ambient Air Technologies.
The Wisners built the tunnel four years ago to test airflow around buildings. Hospitals are some of their biggest clients, “so we can avoid having patients breathe air that comes out of neighboring smokestacks.” Much of the Wisners’ work is studying air dispersion over scale models of cities or neighborhoods to see how air exhausted from one building might avoid getting drawn into another building’s air intake system. It almost makes one wonder whether bakeries and fast-food restaurants do this kind of research to direct the smell of what’s cooking to the noses of potential customers.
This wind tunnel is not shaped like a rounded metal donut. Rather, built inside a big warehouse, it is shaped like two long éclairs side by side and connected at their ends with short, perpendicular éclairs; it’s a long, rectangular hallway that doubles back on itself to make a complete circuit around four square corners. Wind is directed smoothly around the four 90-degree corners by means of tall, Styrofoam vanes, so the air does not eddy up and build pressure in the corners.
A 14-foot cooling tower fan pushes air around inside. Unlike most wind tunnels, which control airspeed by changing the pitch of the propeller blades, extremely accurate control of the fan motor’s RPM with a fixed pitch on the blades results in a claimed wind-speed accuracy of 1/100th of 1 percent. Most tunnels measure an average wind speed, but theirs is fixed precisely at a set speed.
“And we don’t have to adjust for an averaged velocity through manipulation of the data,” said Delores Wisner.
Furthermore, where accuracy at 30 mph (the standard wind speed used for bicycle studies) is reduced in many wind tunnels by being on the low end of their range, it is right in the middle of the zero-to-60mph range of the Wisner’s tunnel.
CPT has built its own bicycle-supporting balance that mounts onto the rotating disc on the tunnel’s floor in front of the control window. It is like a giant air-hockey puck riding on a nearly frictionless bed of compressed air. An accurate scale measures how hard it is pushed back by the wind.
Slipstream-Chipotle’s regimen, overseen by Slipstream-Chipotle’s sports scientist, coach and physiologist, Allen Lim, starts with a computer analysis of the rider’s position and pedaling style in three dimensions on both his old time-trial bike and his new Felt TT bike. This is done with a system from Retul that is similar to the 3-Dimensional Cycling Analysis video method employed by the Boulder Center for Sports Medicine (BCSM) sports science department but is easier to set up and more portable.
Retul’s Todd Carver is a former BCSM biomechanist who has partnered with two former Colorado Altitude Training (CAT) employees to produce and market this system of pedal-stroke analysis. Instead of having reflective markers on the cyclist’s joints like the BCSM system, Retul uses LED “active markers” that emit light which is picked up by an infrared sensor and sent to the computer to create moving stick-figure diagrams of the rider. The Retul system confirmed how low Zabriskie is: His torso is angled up at only 7 degrees from horizontal, while Millar’s torso angle is 9 degrees.
Each rider’s old bike and his new Felt are analyzed alone in the tunnel for drag. Then the rider goes into the tunnel on his old bike and gets a drag number in his old position. Jim Felt and his employees position the saddle and bars on the new bike to match the old bike, and the rider goes in to get a drag number on it. Finally, Felt and Co. make changes to the bar and saddle position to try to find a faster position, which then can be duplicated on the Position Cycle to see if the rider can produce as much or more power in that position on the Velotron as he could before in his old position.
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