Wind tunnels are the current gold standard for testing a cyclist’s aerodynamic efficiency because they provide repeatable results and a close approximation of real world conditions, but they are still not perfect. Alphamantis, an engineering start-up from Montreal, has created an alternative aero testing tool called the Track Aero System that eliminates the limitations of a tunnel while closely integrating bike fit into the process, making it an incredibly powerful tool for accurately tracking aero drag while a cyclist rides freely. But, despite removing the drawbacks of a wind tunnel, the Track Aero System has one significant restriction of its own.
Sticking a rider on a bike mounted to a fixed platform within a tube-shaped wind tunnel introduces a few elements that are not part of regular cycling on a road. The tunnel itself can create interference, causing air to flow irregularly; instead of leaning side-to-side with each pedal stroke, the bikes stays fixed in an upright position; psychological effects of riding in a tunnel can cause a person to assume a posture they are unlikely to replicate outside of a test. The impact of these variables and others can be tiny, but they are nevertheless important when examining differences in rider position and equipment selection. Plus, tunnel time is very expensive.
The Track Aero System is designed to instantaneously measure aero drag outside the semi-contrived environment of a wind tunnel. Michael Hall, Zipp’s Director of Advanced Development, called it the “third leg to the stool” representing cycling aero testing, the final piece to the puzzle when added to wind tunnel testing and CFD analysis.
Here’s how it works. A cyclist shows up to the LA Velodrome with his/her bike. It must be equipped with an ANT+ compatible power meter and speed sensor. (These tools can be provided for athletes that do not own them.) Combined rider and bike weight is taken and the sensors are paired to the Track Aero System using an ANT+ Bridge, a device slightly larger than a 9-volt battery that coordinates and relays data to the computer controlling the test. Following a fit (more on this later), the rider starts making laps of the track and data flows to the computer. Air temperature, pressure and humidity are input through a Kestrel air flow meter.
The secret sauce to the Track Aero System is the way it uses the information measured directly by the speed and power sensors to back-calculate a rider’s CdA, the figure representing aero efficiency. CdA is displayed on a screen trackside and recorded while the rider pedals. Changes to equipment or position are tested one at a time by alternating between the original condition, the experimental condition then returning back to the original condition.
Drag from an algorithm
Although using lap time and power to estimate CdA is inherently simple, Alphamantis has taken it to an incredible degree of sophistication to achieve accurate results.
The following is a list of all the forces acting on cyclist: pedaling force, tire rolling resistance, bearing and drivetrain friction, gravity and aerodynamic drag.
The Track Aero System uses an algorithm to neutralize the effects of any small changes to these factors, thereby isolating the rider’s aero drag as the only variable. Andy Froncioni, Ph.D., CTO of Alphamantis, says the Track Aero System uses more than 20 calculations to account for the variables influencing resistance in order to drill down to CdA in the most accurate way possible.
Accounting for the changes in gravitational resistance as a rider rounds each bend is one of the complex calculations happening in the background. Velodrome corners are banked dramatically so riding into, around and out of each turn influences the rider’s gravitational resistance. Not only does the rider climb a slight incline, but he leans dramatically, changing the vertical distance climbed by the center of mass of the bike and rider. Tires also spin faster than the rider’s center of mass moves because of the characteristic lean. Froncioni and Alphamantis engineers derived a correction for this factor called Virtual Elevation based on velodrome geometry and rider speed. As the rider rounds the track, this equation estimates the elevation gained and lost by the rider’s center of gravity as he leans through a bend. Although he wouldn’t describe the equation in detail, it is dependent on the rider accurately following a line along the bottom of the track. Calculations for rider lean were checked by photographing cyclists. The calculation for Virtual Elevation is just one of the many running simultaneously during a test. According to Froncioni, aero measurements taken with the Track Aero System are accurate and repeatable to +/- .6 to .8 percent. Five percent error is more typical when using just power and lap time, says Froncioni.
Testing resolution is so fine that the change was obvious and repeatable when the test subject switched his grip position from the top of the bars to the underside.
Another advantage of this system is real-time information. Instead of flying blindly from trial to trial, the Track Aero System spits out a clear, concise result with each lap of the track. Instantaneous data means unsuccessful changes are immediately jettisoned, saving valuable test time.
Fit and aero efficiency
“Coming up with impactful outcomes is not easy,” say Froncioni, and bike fit is the key to translating CdA data into improved cycling performance. Bike fitter Jim Manton of ERO Sports is the key to bridging the gap between Froncioni’s data and improved performance. A test with the Track Aero System begins with a fit in the ERO Sport studio located within the LA Velodrome. During the fit, he comes up with a range of positions an athlete can comfortably maintain, not a single set of coordinates. “For a tri fit, a closed hip angle between 45 and 55 degrees is what we look for, but it’s not set in stone,” says Manton. “Using that range and others, we can get an idea of where to take the rider [during the aero portion of the fit] and what’s too far.”
The fit then moves down to the track, where the athlete starts testing the aerodynamic efficiency of the various positions deemed to be sustainable. The LA Velodrome allows subjects to ride their own bikes—not just track bikes—during the test. Although they have not yet tried every combination of bottom bracket drop and crank length, no subject has yet had a problem with the crank arm digging into the steeply banked corners. 175mm crank arms on a bike with an 8cm bottom bracket drop is fine. Those riding 650c wheels with long crank arms may need to ride faster than other athletes in order to safely circumnavigate the track.
One position or equipment variable is tested at a time. The cyclist does several laps in the baseline condition, and then makes a change for the next set of laps. He then records several laps in the new condition before reverting to the original baseline. Going from baseline to experimental condition back to baseline helps ensure any changes to CdA are in fact due to the variable being tested and not some other factor. Measuring tire rolling resistance is another of the system’s interesting capabilities. Instead of adjusting rider aerodynamics, the Track Aero System can be set to record the coefficient of rolling resistance between various tires and the track.
Aero testing on a velodrome has one glaring limitation: wind angle. A rider circling a track experiences almost perfect headwinds. Zero yaw. Riding outside creates far different conditions, with an average wind angle typically between five and 15 degrees of yaw. The big question of the Track Aero System’s effectiveness as a position testing tool is whether or not testing at zero degrees of yaw yields data that is truly relevant on the roads.
Mat Steinmetz of 51 Speed Shop is performance consultant to Craig Alexander and a host of top pros. He helps administer aerodynamic testing for many athletes and has experienced mixed results with aero testing at zero degrees of yaw. “The theory is that, if [a position] is good or bad at zero degrees of yaw, the trend will continue,” says Steinmetz. “If I’m working with an athlete and the budget is unlimited, I will get as much information as possible and will test at yaw with almost every change… When resources are limited, I’m fine with a rider’s position being tested at zero yaw.”
Trends at zero degrees of yaw do not always translate directly to wider yaw angles, but, as it pertains to position and not equipment development, there is much to be learned from zero degrees. The relative value of testing at yaw in a tunnel balanced against testing while actually riding a bike isn’t yet clear.
A two-hour fit and aero testing session with ERO Sports is $800. The San Diego Low Speed Wind Tunnel charges upwards of $800 per hour of testing and offers no fit services. The rider is responsible for ensuring all positions tested are in fact sustainable. The A2 Wind Tunnel in North Carolina charges an hourly rate below $500, and fitter Jim O’Brien can be scheduled to come perform a fit in conjunction with the aero test. Faster in Scottsdale, Ariz. also integrates fit with aerodynamic testing. Two hours in their fit studio and one hour in the tunnel goes for $750. The LA Velodrome is the first and only facility with a Track Aero System, but ERO Sports hopes to apply the technology at facilities across the country. Each new track will require a new calibration to account for gravity since track geometry influences rider position during a lap.
Alphamantis has another tool called the Aerostick, a device that mounts to the front of a bike to measure the wind speed and angle experienced by the rider. Adding wind angle correction to the preponderance of equations used to calculate CdA allows the system to create data while riding outside. Bringing this technology outside might be the key to creating the ultimate aero testing tool. Testing over a full sweep of yaw angles is critical to product development, so Zipp is taking on the task of testing outdoors using the Alphamantis technology. Michael Hall of Zipp says this type of testing offers data that is exceptionally relevant to actual cycling, but controlling every possible variable is extremely challenging. Rider lean, road surface and other factors further complicate the formula needed to arrive at CdA. While ERO Sports is not yet offering this sort of testing for age groupers, the tools are available. This might become a reality in the near future.
Combining accessibility, accuracy, realism and repeatability is the key to effective aero testing. While the Track Aero System is limited in the wind conditions it can test, it allows the rider to pedal a bike free of a contrived environment while producing incredibly good data.
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