Note: This piece is a portion of our extensive three-part 2021 summer wheel guide that also includes a handy at-a-glance wheel rim depth chart and an expert rated look at six of the latest and greatest carbon hoops.
When it comes to bicycle tires and wheels, we now know: wider is better. Of course, old-school thinking was just the opposite—and many veteran triathletes might remember gluing 19mm tires onto their tubular rims. But tires have been steadily getting wider over the years, and if 25mm is currently the new 23mm, 28mm will soon be the new 25mm. The reality seems simple: The ride quality of a bike equipped with wider tires is notably better. Wide equals smooth and smooth equals fast, but there’s more to it than that: Wider also means less flats and a better-handling bike.
The Tire/Ground Connection
When it contacts the road, a tire slightly flattens—a section of tire referred to as the “contact patch.” Tire pressure determines the total area of a contact patch, and if tire pressure is equal, a wider tire will have a wider and shorter contact patch than a narrow one, due to the shape of the rubber meeting the road, even though the net area is the same. If pumped to the same pressure, the difference in contact patch shape between a wider tire and a narrower one creates less rolling resistance on the wider tire. However, it’s common to pump wider tires to lower air pressures, since there are other benefits such as better suspension and reduced chance of a pinch flat with a lower tire pressure. Using slightly less air pressure in a wider tire, e.g., about 10-15psi, will generally increase the rolling resistance, making it roll about as efficiently as a narrower tire at higher air pressure. That means that with the lower pressure and wider tire, you’ll still get the boost in comfort and speed without any rolling resistance penalty—compared to a narrow tire at a higher psi.
The Tire/Wheel Connection
As tires became wider over the past few years, there was one downside: aerodynamics. The mismatch in tire width to rim width created an aerodynamically inefficient shape. According to Nathan Barry, a design engineer at Cannondale, your rim needs to be wider than your tire for optimum aerodynamics. Josh Poertner, formerly of Zipp, coined the term “the rule of 105%,” which means that the rim should be 105% the width of the tire in order to allow for a smooth transition of air between the tire and the rim. Wider bicycle wheels for road and triathlon are now mainstream, with virtually every manufacturer embracing the wider tire trend and offering much wider rims than they used to. A tire mounted on a wheel with a wider internal width will inflate to a wider width and larger diameter than on a narrower inner wheel width (all else being equal), so riders with a wide wheel and wide tire are effectively doubling down. Considering that for maximum aerodynamic efficiency your rim should be slightly wider than your tire and that there are hundreds of tires to choose from, it can become tricky to figure out which wheel and tire combination will yield you the fastest overall bike split.
Of course, bike engineers aren’t going to stop at widening tires, the whole tire/wheel interface is constantly evolving—just like it did with width. “Hookless” rims, such as the new Zipp 303S and Enve’s Foundation series wheels, are compatible with tubeless tires only, and they feature a special smooth and aerodynamically optimized interface between the tire and the rim. This technology has carried over from the mountain bike world, retaining a lot of the original tubeless tire benefits while allowing for a near-seamless aerodynamic transition from tire to rim with appropriately sized tires. Much in the same way that rims had to adapt to the “wider is better” philosophy to take full aero advantage, hookless rims could do the same. Hookless rims are relatively new to wheels designed for aerodynamics, and they may change the game to some extent in the future in the same way that rethinking width paved the way for faster wheel/tire combos in recent years.