Aero Testing at Home - A Step by Step Guide

Wind tunnels in cycling are nothing new and for anybody who loves tech, we would love to go test all of our gear.  The problem with the wind tunnel is that you are looking at about $1,000/hr and the need for an experienced engineer to help turn the data into usable information for the majority of us.  

For those two reasons testing which helmet we should wear, or where you put your nutrition is often a decision based around eyeballing it.  We now have affordable power meters, continuous glucose monitors, and seemingly an endless stream of data at our disposal, yet aero and rolling resistance data has been difficult to to get.  

Robert Chung made a post on a blog 20 years ago about a way to field test aerodynamics and rolling resistance, and it is still the gold standard today.  I sat down with Robert to get his tips on how to do all four of his protocols and when each might be best.

For all protocols there are a few important tips:

1. Find a safe course that you can make turns safely out of traffic.
2. Start and Finish at the same spot so net elevation is the same.
3. Find a course with minimal traffic, which includes cars, dogs, or anything that might cause you to stop the test.
4. Pick a day with minimal wind
5. Hold position constant
6. Avoid hitting the brakes

The virtual elevation method of testing works due to the exponential relationship of power losses to aerodynamic drag.  That is to say power required to overcome aerodynamic drag is relative to the cube of speed.  In much easier terms, to go 2x as fast, you create 4x the drag, which requires 8x the power.  

Rolling resistance on the other hand is constant and the power required to overcome rolling resistance is linear to speed.  To go 2x the speed, it requires 2x the power to overcome the rolling resistance losses.  

When you do these tests, you can plug the results into a program such as Golden Cheetah and make estimates at your CdA and Crr until the "virtual elevation" and actual elevation profiles are horizontal and line up. This is because the speed changes lined up with your power data will show if the differences are linear or exponential which separates the rolling resistance and aerodynamics.  

The classic Chung Method approach involves finding a closed loop with turns that you don't need to brake.  If you can find one with a little bit of elevation gain that is ideal.  The loop should be 30 seconds to no more than 5 minutes.  You need to do at least three laps and ideally 5 or 6, but anymore laps than that is unnecessary. 

Varying your power throughout also gives you the best data.  The variety of power helps identify the drag as aerodynamic drag or rolling resistance.  

The couple tips for this protocol is to hold position constant, avoid wind, and don't hit the brakes.  These can muddy the data and make it harder to isolate aero vs rolling resistance losses. 

For this method you do need a dedicated speed sensor, an altimeter, and a power meter.  The best data is going to come from a dual sided power meter but you can get most of the way with a single sided power meter.  

The Shen Method is a great alternative protocol when a loop isn't available.  This is essentially an out and back.  If you put a cone or some sort of marker out on the road it gives you a good place to make the turn around.  It is ideal to find a small bump in the road so you can slow enough to make the turn without hitting the brakes.  If you don't have the ability to make the turn without the brakes, as soon as you get to the turn around slam the brakes hard.  This will show up in the data as a massive climb and you can go back and snip that portion out of the ride.  

When you get going again, hold position constant and get back to the start.  Do this protocol three times with a slow, medium, and faster run.  The plotting of this will be a little more difficult as you would need to export the data points and plot them on your own as Golden Cheetah does not currently have a mirroring function.

The equipment needed for the Shen Method is the same as above.  A dedicated speed sensor, power meter that measures total power is ideal, and an altimeter.  

This is likely one of the easiest protocols to find a course for most.  You just need to find a 90 second to 5 minute long climb.  The key here is to do a really slow effort, a medium effort, and another around your threshold or even higher.  

This will give you three virtual elevation profiles to plug into Golden Cheetah.  If you then use the CdA and Crr estimation tools until the three lines match up.  This will give you an accurate estimate of your CdA and Crr for that given surface.  This protocol is likely best when you are looking for a consistent surface for gravel testing since it might be difficult to find a short enough loop with a consistent surface.  A single climb is much more likely to have the same surface as a loop. 

Coast downs are a great option if you don't have a power meter because you aren't adding any power to the pedals so we know that power is zero.  You pick a start and end point and hit the start point at three different speeds.  First start at almost a dead stop, then walking speed, then a faster run.

This method can then be plugged into Golden Cheetah just the same but the speed sensor is absolutely critical.  Since we don't have power to include in the data, the speed needs to be very accurate.  This protocol can still be just as accurate as the others but it does require some snipping of the ride file to make sure the start and end points are the exact same. 

If you want to hear more from the inventory of virtual elevation testing, check out my interview with him below: