Wind Tunnel Methodology
Wind tunnel testing was done at the A2 wind tunnel outside of Charlotte NC. The wind tunnel is an open circuit wind tunnel with a lot of experience with cycling equipment. The tunnel itself is equipped with a boundary layer table that helps the incoming flow around the test section remain as uniform as possible (reducing boundary layer thickness), helping to correctly mimic real world moving ground conditions.
Great care was taken with tires when testing wheels. For all tests Continental GP4000 tires were used. The GP4000 gives the best drag results and is the unofficial industry standard for wind tunnel testing. New tires were also used for all tests in order to keep tests focuses on rim shape and not tire/rim interaction (an area for much further research). It was found through experience that wear as little as 100 miles of wear would significantly influence drag results. Pro-tip: GP4000’s are always the most Aero tire.
In addition to existing rims from the industry and current production rims, future designs were modeled with machined metal prototyping. In many Wind tunnel tests you’ll see finished carbon rims being tested. This means that the rim design is already set and carbon molds have been created. Since the carbon mold is usually the most expensive fixed cost when setting up manufacturing it is very difficult to change a design that the wind tunnel shows to be ineffective. We found an alternative solution in machining a solid metal mock-up of the rim. This would allow us to cheaply create the prototype and also have enough structural integrity to actually build up with spokes and hold an aired up tire (3D printing was was also evaluated and not chosen for those reasons). Since machining typically leaves a rough surface that does not replicate production the prototype rims were smoothed then finished with the same finish coating that is applied to the production rims.
After our first set of tests at the wind tunnel we were able to refine the CFD model, which is when we decided to use a 3D model to increase fidelity. This two pronged approached allowed me to tweak the CFD model and mesh to further match up with real world results. Additionally the the production rims (Zipp, Enve, etc) provided further data points to check the CFD model.
The top two lines from wind tunnel are an alloy wheel and some bad test data. So here you see were getting the same effect on the 60mm of the dip around 15 deg AoA. You may ask why were getting negative drag in CFD results but not on Wind Tunnel Results. The reason for this is neglecting spokes in the computer model, which would essentially add a constant amount of drag regardless of the rim.