FLO Cycling – Does Covering the Valve Cutout on a DISC Wheel Matter?


If you’ve ever owned a disc wheel, you will know that the only part of the wheel that is not covered, is the valve cutout.  We are often asked if covering this valve cutout will improve aerodynamics.  To be honest, we didn’t know the answer, so we studied it.

 

If you are interested in our other studies from the A2 Wind Tunnel, please be sure to check them out. 

Does Tire Pressure Change Aerodynamic Drag?
Aerodynamic Difference between Sapim CX-Ray and Round Spokes

How Much Drag Does a Clydesdale Build Add to Cycling Wheel?

What We Tested
In November of 2015, we visited the A2 Wind Tunnel in North Carolina.  We tested a FLO DISC in the wind tunnel with the valve cutout covered, and then repeated the test after we removed the covers.  Here are a few more details.  

1.  The same wheel was used for each test.

2.  We used a Continental GP 4000 S II tire in a 23mm size for both runs.



How We Tested the Wheels
We like to describe all of the details of our testing so the results are as transparent as they can be.  Here is how we tested both wheels at the A2 Wind Tunnel.

1.  Tare was calculated and removed from both tests.

2.  Each test swept the wheel from 0-20 degrees of yaw, in 2.5 degree increments.  The drive side of the wheel was opened to the airflow as the angle increased from 0-20 degrees of yaw.

3.  Each measurement was taken twice and averaged. 


4.  All tires were inflated to 95 psi and calibrated with a digital gauge.

The Results

The results were very similar.  The Yaw vs. Drag graph below shows the FLO DISC w/ Valve Cutout Covered (in blue) vs. a FLO DISC w/ Valve Cutout Not Covered (in green).

 

 
To calculate the time savings for both scenarios, we used our Net Drag Reduction Value formula.  This formula was developed using the 110,000 data points we collected while riding a bike with a custom built data logger.  The formula uses a baseline wheel, which in this case, was a Mavic Open Pro with 32 round spokes.  Please feel free to learn more about our Net Drag Reduction Value formula and Data Logging Process.  
We can see that the difference over a 40km and an Ironman is 2 seconds and 8 seconds respectively.  Those difference are pretty small.   
 
Should You Cover the Valve Cutout?
Before we studied this in the wind tunnel I gave the following answer if someone asked me if they should cover the valve cutout.  

I would cover the valve cutout prior to a race.  If in the middle of the race you flat, I wouldn’t waste my time recovering the valve cutout.  

Today, after studying both scenarios at the A2 Wind Tunnel, my answer stays the same.  However, if you forget to tape the valve cutout closed prior to your race, I wouldn’t worry about it. 

What is the Best way to Cover a Valve Cutout?
You can purchase special stickers to cover your valve cutout, but our personal favorite solution is using 2″ wide electrical tape.  The material is very similar to the covers you can buy, it sticks well to contours, and it’s not impossible to remove.  Another benefit is that a roll should last you several years.  

 
I hope you enjoyed this article.  Please feel free to comment and ask questions below. 
 
Take care,

Jon