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Back 4.25.2016

How Zipp NSW Achieves Aero Balance

A primer on how Zipp NSW wheels achieve superior aero efficiency and crosswind stability. Hint, dimples really do matter…

Imagine a bike ride in a strong gusty wind that keeps both hands firmly on the bars. In this situation, your wheel could see wind yaw angles above 20 degrees, as opposed to the sub-teen angles experienced in average or calm wind conditions. This gusty wind scenario is precisely where you want a wheelset that’s as controllable as it is aero.

Pairing aerodynamic efficiency with crosswind stability was a cornerstone innovation of Zipp’s industry changing Firecrest rim technology introduced in 2010. Building on that legacy, Zipp’s new halo-level NSW Series wheelsets (404 and 808 NSW Carbon Clinchers available now) take this approach to a new level. With NSW wheels, Zipp engineers were able to achieve lower drag force while maintaining the same side force (which contributes to crosswind stability) as Firecrest from 0 to 15 degrees yaw. But beyond 15 degrees yaw, the side force produced by the NSW rim design plummets by over 30 percent compared with Firecrest. This is important because higher yaw angles are where bike handling is most affected by the wind.

 Photos by Wil Matthews

To better explain how the NSW design does this, it helps to understand what Zipp engineers call Aero Balance™, which is comprised of three key components influencing wheel stability: center of pressure, vortex shedding, and side force.

Center of Pressure
This is a similar concept to center of gravity. This is the single point representing where the aerodynamic force is acting. But unlike gravity’s consistent force, center of pressure can shift as wind velocity and direction change. Ideally, a cyclist wants the center of pressure to remain as close to the steering axis as possible – just behind the hub.

Vortex Shedding
Picture a flag fluttering in the wind – the movement of the flag is caused by small vortices shedding off the back of the flagpole. Lower frequency shedding produces larger, more powerful vortices. In cycling terms, this unstable situation is often referred to as “buffeting.” A higher frequency of shedding produces a greater number of smaller, less powerful vortices leading to greater wheel stability.

Side Force
Side force on a wheel increases with yaw angle and wind speed. It also is affected by the wheel’s surface area exposed to the wind. Riders must compensate for side force by leaning into the wind, especially in strong winds.

In wheel design, achieving low drag and high stability (or Aero Balance) is a difficult task. Improved aero efficiency typically results in higher side force, and vice versa. But what Zipp has been able to do through NSW technology is achieve low drag and low side force at yaw angles greater than 15 degrees. One of the key ways that is achieved with NSW rims is through control of the airflow boundary layer. This control prevents the boundary layer from separating and causing both buffeting and increased drag.

Zipp’s famous dimples, known technically as ABLC™ (Aerodynamic Boundary Layer Control), have long helped reduce wind drag on Zipp wheels. With NSW, the dimple pattern has been modified with the ABLC Sawtooth™ Technology. This design consists of 12 nodes that are clocked to induce small sheet vortices that shed behind the front half of the wheel at a lower magnitude and a higher frequency, thereby improving crosswind stability in gusty wind conditions and improved aerodynamics in any weather condition.

Photo by Sean Robinson

Finding New Speed

When engineered together, these technologies allow riders to select a deeper section wheel than the competition and gain an aerodynamic advantage without sacrificing control. Built on the aerodynamic foundation laid by Firecrest, NSW now takes it all to the next level.

Learn more about:

Zipp NSW approach

Zipp 303 NSW Carbon Clincher

Zipp 404 NSW Carbon Clincher

Zipp 808 NSW Carbon Clincher