FLO Gravel Wheel Design #11 Can Gravel Wheels Be Aero?

We at FLO have a founding belief that aerodynamic cycling wheels for road and triathlon can be affordable. Since aerodynamics are part of our roots, one of the first questions we asked when designing the gravel wheels was, “Can gravel wheels be aerodynamic?” We observed gravel events like the Dirty Kanza 200 and 350 prime. We saw that riders cared about their time, and we knew gravel riders would definitely benefit from aerodynamics.  

Ideally, for a wheel to be aerodynamic, you want a smooth transition of airflow from the tire to the rim.  However, some argue that the aggressive tread patterns of gravel tires makes that smooth transition impossible, believing that aerodynamics is useless on a gravel rim.  

We wanted to show that aerodynamics do matter, and our process started with CFD (computational fluid dynamics) while taking into account gravel tires.

You can see from the photos below the substantial tread difference when comparing a gravel tire and a road tire.

We use computational fluid dynamics (CFD) software to design rim shapes. Standard CFD is a time-consuming process. You draft a shape, create an environment for the object to be tested in, mesh the shape so that the computer can calculate a simulated fluid movement around the object, and then you let the software calculate the coefficient of drag (CdA) for the object. Ideally, you are looking for the object shape that creates the lowest amount of drag. However, there is no way to solve an equation to determine which shape will produce the lowest drag. So, you have to draft and test many different shapes until you determine your best guess at which shape is the fastest. This is similar to the wind tunnel approach of designing wheels.

We are not standard. Instead, we use an optimization algorithm. How do we know what to put into the algorithm? We collected more than 100,000 data points on road to understand the yaw angles and relative velocities of cyclists. Based on that data, we developed an optimization algorithm that automatically iterates through hundreds of rims shapes and narrows down the fastest resulting rim profile. We start with an initial rim profile and allow the algorithm to solve for a predefined range of parameters. For example: rim height = 50mm - 60 mm; rim width = 30mm - 33mm; etc. Then, we have the ability to solve for the best profile and we’re not just guessing.

A huge benefit to CFD is that it allows us to model the tire while designing the rim. This technology has progressed greatly overtime. Back in 2009, we started with photographing tire profiles for overlays. Then in 2015, we molded tires, and today, we are using a high quality 3D scan. The picture below shows the scanner scanning the WTB Riddler 37c.

Once the scanning process is complete, the scans are imported in a CAD package, like Autodesk Inventor, for post processing. The tire is converted to a CAD model and then the tire and initial rim profile are drafted.  Below are images of our starting rim profiles and tire models.

I'll admit, we had no idea what to expect from the gravel wheel CFD results. When we received the results, we were really surprised and excited about the large drag reduction over a standard box section rim. This was big news! This data verified that a gravel wheel could be aero. See the results in the table below:

CFD always produces cool visualizations that help you wrap your head around how air moves around a wheel. The images below show the velocity of the air as it moves around the rim profile.  

We use CFD as a tool for optimization. We choose to only run rim profiles and do not include spokes and hubs. Including these adds complexities to the model and in our opinion, it confuses what you are trying to determine. CFD allows us to solve a problem and it provides the comfort needed for us to open a mold. CFD does not replace the nuances of real life, which is why visiting a wind tunnel is an important step for us. Once we produce a rim, we build it with carefully designed spokes and hubs , and then the wheel is tested in the wind tunnel and verified as a final product. We are also able to determine the best tires, tire pressures, etc., but that’s for another article.

Stay tuned for Part 12 of the FLO Gravel Wheel Design Journey where we talk about the wind tunnel and the test results.