The Aero Advantage

Dramatic black and white image of rider in the wind tunnel

Not everyone has a wind tunnel. We do. We call it the Win Tunnel. That may seem like hyperbole, or death by tiny pun, but the stripped down truth is that, when it comes to cycling, the study of aerodynamics is so crucially important that this room full of fans and brains is a key tool in designing a path to the podium. You can get some of the way with data acquisition, you can make a lot of progress toward your goals with computational fluid dynamics, but when it comes to being able to understand how it is going to work, to realize exactly what your gains will be, where the compromises will show up, in every kind of condition, that’s when you need get in the tunnel. Most people need to book a flight somewhere and hope to eke out a few minutes here and there in someone else’s tunnel. We consider the pursuit of aerodynamic performance so central to our existence that we built our own. The proof of how effective this is can be found in the performance of our road wheels. Model for model, they are the most aero wheels per category that you can buy.

Colorful aerodynamics line graph

Power = ½ air density x velocity cubed x coefficient of drag x area. This is why aerodynamics are so important in cycling. The faster you go, the harder the wind pushes back. As your speed increases, the effort needed to overcome aerodynamic drag increases exponentially. At some point it becomes impossible for you to increase speed, or it becomes impossible for you to sustain speed. Therefore, we study how to beat this.

Closeup of rider's feet on bike with Roval wheels

Our CLX64 is a good case study for the importance of the Win Tunnel. The goal when designing this wheel was to create a deep section road clincher that would be the most aerodynamically slippery wheel on the market, but one that would also avoid the rudder-like crosswind behavior of many deep section rims, AND one that would also realize full aerodynamic benefits when running new-school wider tires. Yep, tires can totally change the way your aero wheels behave. Chris Yu, one of the Win Tunnel’s team of engineers, explains the challenge faced with meeting aerodynamic needs in the real world:

“Our goal is to create wheels that help our athletes get to the finish line as fast as possible. Note, this isn’t necessarily the same goal as making wheels that have the lowest aerodynamic drag possible. Bike riding and racing in the real world is so dynamic that many different factors affect the design and aerodynamic shaping of a wheel. Two big factors are handling and tire rolling resistance. Both of these attributes are heavily impacted by the tire. In recent years, we have seen a trend towards wider tires in racing; this has been supported by anecdotal feedback from athletes as well as data measurements in the field and in the lab.

Multiple Roval wheels leaving against barrier

Traditionally, these benefits of a wider tire are offset by an aerodynamic disadvantage. Aerodynamically speaking, round shapes are very inefficientthey create a lot of drag compared to a more streamlined or airfoil-like shape. A tire is basically a round shape at the very front of the bike where it sees the wind first. The primary task of an aero shaped rim is to turn the round tire shape into a more efficient and streamlined shape by creating a gradual continuation behind the tire. Historically, wheels were designed around much narrower race tire widths, around 21mm. Therefore, much of the aerodynamic benefit of these wheels is lost when modern, 26mm and larger tires are mounted.

Computer graphic of computational fluid dynamics

Given all the advantages of wider tires, we went back to a clean sheet of paper to design rim shapes truly optimized for 26mm and larger tires. This effort is easier said than done -  designing a much wider optimal airfoil shape and keeping weight low are competing technical interests. To be able to maximize the aerodynamic performance around wide tires, while also minimizing weight and maintaining ride quality, our team needed to utilize a range of tools ranging from aero-structural co-optimization simulation to Win Tunnel testing. To take an even more extreme step, the exact shape and interface of the tire was quantified so that it could be accurately incorporated into simulation and rim design. Even though a tire is approximately a round shape, details of how the casing bends around to the bead and texture on the tread can make a difference in how air flows across it and onto the rim. To make sure our rims are optimized to real tires, we created mold castings as well as CT scans of numerous tire-rim interfaces to understand how the tire and rim shapes interact. This even included investigating how our internal bead hook design could influence the shape of the tire, and thus the shape design of the rim.”

We thought about editing this nugget down so that it would appeal more to the non-nerds out there, but this is some potent knowledge that Chris is relaying. Hang in there. Chris continues:

“The Win Tunnel plays a huge role in all this. Even though computer modeling is incredibly complex and powerful, it is still not computationally efficient to accurately simulate the entire dynamic system. Things like how a tire exactly seats on a rim, to how the wheel interacts with the rest of a bike, these are much more easily and accurately captured in the Win Tunnel.

 When we set out to design a new rim, we start out with some technical targets as well as benchmarking of best-in-class at the time. The next step is to characterize/quantify the tire accurately—this is where the 3D casting and CT scanning comes in. By having that to work around, we then start with a base shape (often a prior generation design) and apply shape optimization in simulation to literally morph that shape to the performance targets that we’ve input. Along the way, we 3D print several prototypes to validate and refine in the tunnel.”

From proof-of-concept to final shape fine tuning, the Win Tunnel allows us to rapidly test ideas and define design direction. It speeds up development time and identifies dead ends.

Two thin 1980s bike wheels

Aero research is a foundational principle of Roval. Claude Lanhauer was a civil engineer and avid cyclist who, in the early 1980s, noticed that cyclists used the same wheels for everything, from climbs to time trials. Squared off, box section rims, 32 or 36 three-cross round spokes, about as aerodynamic in modern terms as a cardboard box on a windy day. He developed Maillard hubs that shrouded straight pull spokes inside aero flanges, used bladed spokes, and sourced rims extruded in a more pointed aero profile. 24 spoke radially laced front wheels, 30 spoke rears, 20 on the drive side and 10 on the left, were a dramatic departure from the norm of the day. That was the beginning of Roval, the beginning of modern cycling’s aerodynamic quest. The tools we employ in that pursuit have dramatically changed, but the goal is still the same: Go faster, burn fewer watts.

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