All samples described below were tested at the Oran W. Nicks low speed wind tunnel at Texas A&M University using dedicated cycling fixtures in January of 2003. Data is presented as axial force (which is equivalent to drag at 0 degree beta/yaw) at 30 mph. Relative ordering will be the same at lower speeds, but the magnitudes will be decreased.
Chester Kyle has previously indicated that the tire interface with the rim is an important detail to be managed when trying to minimize axial force. Details about this tire effect and its magnitude are shady. In an attempt to quantify whether or not tire size even matters, I executed a test schedule that would shed some light on the issue.
Two distinctly different aero wheel constructions (the DuPont Three-spoke clincher, and the Zipp 404 clincher) were tested. The same wheels were tested with different size tires (700x20c and 700x25c), but of the same tire brand and model. The tire used was a Contintental Ultra 2000 kevlar beaded tire inflated to 110 psi. The wheels in the test are shown below:
A bonus data point that is included from this same tunnel entry was a Hed 3 tubular. The tire on this wheel was marked as a Continental 700x19mm tubular and was also inflated to 110 psi.
A Ritchey standard clincher wheel with a 700x20c Continental Ultra 2000 tire is included as a reference.
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The above graph clearly shows that tire size affects the aerodynamic performance of bicycle wheels. The 20c tires have on average about 0.1 lbs less axial force at 30 mph than the 25c tires. This benefit appears to be independent of wheel construction. The same 0.1 lbs reduction is seen for the traditionally spoked Zipp as is seen for the composite spoked DuPont Three Spoke.
It is also interesting to see that the labeled 700x19c tubular Continental tire performed very similar to the 25c clincher. This tubular comparison is not very rigorous though, since more than one variable was changed between the two samples tested (wheel construction, tire construction, tire size). This data point does provide an interesting comparison however for tubular enthusiasts.
What kind of time savings might one expect from using a FRONT narrow tire versus a wider tire (20c vs 25c)? The table below summarizes typical time savings for a flat 40k TT. It seems reasonable, then, to expect roughly half this effect when comparing a 20c tire vs a 23c tire.
These data suggest that the narrower the tire, the faster the wheel/tire combination from an aerodynamic perspective. There are some potential compromises with this narrow clincher tire approach, however - clinchers may tend to pinch flat more often than tubulars. Running higher pressure will help to alleviate this problem though. As a rule of thumb, one should put on as narrow of a tire as possible to improve wheel aerodynamics.