Note: This piece is a portion of our extensive three-part wheel guide that also includes our look at the new trend in wheel width (and why you might need it) and an expert rated look at six of the latest and greatest carbon hoops.
In the world of triathlon gear, bike frames often get the most attention. We assume the frame is responsible for everything from aerodynamics to ride quality to acceleration and more. And while bike frames are a big part of the equation, many triathletes don’t pay as much attention to the spinning things underneath them: the humble wheel.
While each component of a wheel is important—hubs, spokes, and even the tires and tubes that go on them—thanks to the proliferation of carbon and, more recently, disc brakes, triathletes have more choices than ever before when it comes to rims. And rims are a huge player in the aerodynamics/ride quality/acceleration jigsaw puzzle.
The biggest variety of wheel choices comes, even within a certain brand or model, via the depth of the wheel’s rim. Traditionally, the deeper a rim the more aerodynamic it is when the wind is hitting it at certain angles. “With a deeper cross section, the airflow stays attached to the rim longer and separates later, minimizing the size of that low pressure region behind it,” said Luisa Grappone, the engineering and product manager for the Rider Firm, the company that owns Hunt Wheels, among others. “By minimizing the size of that low pressure region, the magnitude of the turbulence is smaller and the drag force is lower.” In other words, a deeper rim creates less drag, which is a good thing—but there’s more to a rim than just drag.
First, a deeper rim is typically heavier—which isn’t quite as important in triathlon as in cycling, since you don’t generally need to attack suddenly or do long, steep climbs. But a deeper wheel also presents a bigger side profile to the wind and gets more side forces. “Those forces will be the same for both a heavier and lighter cyclist, but a heavier cyclist—assuming he has the same riding skills as the lighter rider—will better oppose the force of the wind trying to push the wheel perpendicular to the direction of travel,” Grappone added. “A deeper wheel is more difficult to handle or, at least, requires better riding skills, as it’s subjected to higher side forces and steering moments.”
A deeper rim is also stiffer, for better or for worse. This means that if everything else is equal (and it’s important to note that hub flange size and spokes also play big roles here), a deeper rim will ride rougher than a shallower counterpart with the same construction. A deeper rim will also have greater lateral, side-to-side, stiffness due to the bracing angle of the spokes. So which is best? Our chart/graphic below can help you choose the right rim depth. Finally, check out our expert guide on this year’s top wheel choices.
NOTE: A lighter rider will require exponentially greater skill with more wind on a deeper front rim in particular.