Interestingly, a well-designed wheel’s aerodynamics can be negated by the tire that is installed on it. A slow tire will hurt the performance of any wheel; on the other hand, some of the fastest rolling tires are so fragile that users run a significant risk of puncturing during the course of an event. Clearly, compromises need to be evaluated between a tire’s low rolling resistance and its overall durability.
There are many factors that impact a tire’s rolling resistance: casing construction; tread thickness/design; width; and air pressure. There are other variables that come into play, such as the transition between a tire’s casing and the rim, but for the sake of this discussion, we’ll focus on the largest and easiest to quantify factors that shape tire performance.
A good tire is a supple tire—its construction normally is typified by high thread counts in its casing and a relatively thin (and smooth) tread. A supple tire better conforms to road surface variations; a supple tire also enables a user to run lower pressures, which yield a more comfortable ride. Here’s a simple way of looking at it: supple tires (tires with lots of compliance) generally role faster and are more comfortable than tires with less compliance. What makes a tire less compliant? Stiff casings (casings with low thread counts or molded casings, such as TUFOs), thick tread, and puncture-resistant belts. Interestingly, under this model, wider tires (23mm) provide excellent rolling resistance under less pressure than narrow (19-20mm) tires at the same pressure; to achieve similar rolling resistance in a narrow tire, higher pressures must be used (which decreases comfort accordingly).
So does this mean that you should immediately put 23-25mm tires on your aero wheels? Not necessarily—it’s important that the width of your tire matches the width of your wheel’s rim. If you’re running a narrow rim, such as what’s found on a Hed 3 or a Blackwell/Planet-X, you’ll need to run a 19mm tire in order to aerodynamically match the wheel’s design. If you run a wider tire, then you will lose some of the wheel’s designed aerodynamic advantage. On the other hand, the new models of Hed and Zipp are designed around a 23 mm rim, which means that you can run wider tires (and at a lower pressure), with minimal impact on overall wheel aerodynamics. In theory, then, the new generation of wide rim wheels from Zipp and Hed promise greater rider comfort without the aerodynamic penalty. And remember, because a wider tire can be run a lower pressures, it also should be more compliant than its narrow counterpart, which means that rolling resistance should be lower.
This is not to say that you should immediate go out and sell your narrow rim aero wheels, though—you still have to do the calculus to determine whether the increased aerodynamic profile of the new generation of aero wheels is mitigated by the lower rolling resistance associated with wide tires. Again, tire-wheel dynamics are extremely complex and riders should carefully consider the type (and importance) of the events in which they are competing and select tires to maximize their chances for success.
What do I use? I have a very strong bias toward narrow rims wheels, which means that I generally run 19-20mm tires, which match well to them. As there is little statistical difference in rolling resistance among top performing tubular and clincher tires (assuming that the tubular is glued correctly), then I’ll choose a fast rolling tire that has some degree of puncture resistance, unless I’m riding on a smooth and clean surface. Here are some examples of tires and pressures that I use, assuming a disc in back (clincher disc is a Mavic Comete/Michelin ProRace2 20mm; tubular disc is a Zipp/Continental Competition 19mm):
Average road TT, smooth roads:
Average road TT, mixed roads (some chip seal, cracks, etc.):
Smooth and clean road/track, high priority/highly competitive
For interesting discussions and test data, visit:
Rouse Artisanales (http://www.rouesartisanales.over-blog.com/article-1503651.html)
BikeTech Review (http://www.biketechreview.com/tires/rolling-resistance/475-roller-data)
Rolling Resistance of Tires (http://www.terrymorse.com/bike/rolres.html)
Tires and wheels for timed cycling events (http://www.slowtwitch.com/Tech/Tires_and_wheels_for_timed_cycling_events_226.html)
Rolling Resistance (http://www.slowtwitch.com/Tech/Rolling_resistance_234.html)
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