As the first and last part of the bike to touch the wind, the tire is a crucial ingredient in an aerodynamically efficient bike. Testing conducted by several wheel companies including Zipp, Bontrager and Mavic has revealed the importance of matching an ideal tire to a specific rim. The newest generation of aero wheels with wide brake tracks ranging from 23mm to 28mm typically performs well with either narrow or wide tires, while older rims with narrow 19mm brake tracks perform substantially better with a thin tire.
But aero performance is only half the equation when it comes to tire selection. Rolling characteristics—the way a tire interacts with the pavement—are also crucial to performance and comfort. Counterintuitively, wider road tires create less rolling resistance than narrower ones. Cycling researcher Al Morrison has conducted an ever-growing series of rolling resistance tests on many of the premier race tires since 2006 and has found that the time saved by switching from a slow-rolling tire to a fast one can be equal to the difference between a great aero frame and a mediocre one. Morrison’s tests have shown that a 25mm tire has roughly 15 percent less rolling resistance than an identically constructed 23mm tire.
Tubular gluing technique is another key factor Morrison has discovered. The tests he conducted before 2007 showed clincher tires to be substantially faster than identically constructed tubulars, but it was the gluing method, not the tires themselves, that made the difference. Morrison originally assumed that gluing a tubular with minimal cement would reduce rolling resistance, since the glue itself is elastic. After experimenting with a thicker gluing protocol, Morrison realized that creating a completely solid connection between rim and tire is more important. Now he applies three layers of Vittoria Mastik One cement to the rim and two coats to the tire base tape before adhesion, totaling two tubes of glue per wheel. As repeated tests have demonstrated, thoroughly adhering the tire to the rim is the most efficient—not to mention safe—way to glue a tubular tire.
Racing with a wider tire that reduces rolling drag while improving ride feel without adding aero drag is another no-loss gain. “A lot of new triathlon fames are well designed in the back so the rear tire is well shielded,” says Yu. A frame that blocks the rear tire reduces or eliminates the aero penalty of racing a wide rear tire. “We tested a Specialized Shiv and a Venge with a 21mm, 23mm and 25mm tire in the back, and aerodynamically there was no measurable difference (between the tires), but, in those cases, rolling resistance decreased and comfort improved with the fatter tire.” As a result, a 25mm rear tire is the best race rubber for athletes on aero frames with a rear wheel cutout. These tests measured more aero drag with broad tires mounted on a “modern front race wheel.”
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