In this week’s “Ask Aaron”, Aaron explains why wide rims can make for a more aerodynamic wheel. Click here to see past editions of “Ask Aaron”. To submit a question to Aaron, email Askaaron@competitorgroup.com.
Q: Hi Aaron,
I saw Bontrager’s new [Aeolus] aero wheels. Deep rims on aero wheels makes sense to me, but this new generation of wide rims is a little confusing. How are these rims more aero than wheels with thinner rims?
A: Hey Robin,
There are two primary reasons rim shapes that do not intuitively appear to be aerodynamic do in fact reduce wind drag.
First, a wheel must be aerodynamic with a tire mounted to it.
Narrow rims intuitively seem to be more aerodynamic than broad rims simply because are smaller, but a rim must create an aerodynamically optimized shape when combined with a tire. When hanging on the wall of a bike shop without a tire mounted, this broader rim style appears to increase frontal surface area, but in reality the difference is quite small.
The standard brake track width is 19mm but most tires are 23mm wide. The constraint of riding a comfortable, durable and well-handling tire means that a rim narrower than 23mm doesn’t actually reduce the wheel’s frontal area since the widest point of the tire protrudes past the sides of the rim anyways. Widening the brake track also blends the tire’s shape with the rim and helps create an aerodynamic profile from the tire/wheel system, not just the wheel itself.
Mounting a 23mm tire to a wheel with a 19mm V-shaped rim fundamentally changes the shape of the wheel/tire combination into something resembling an ice cream cone.