CFD explained: how the computer designs your bike wheels

If you have recently looked at the spec sheets of high-end wheels such as those from Zipp, DT Swiss or Swiss Side, or at the latest aero frames, you will inevitably have come across a mysterious acronym: CFD. Often brandished as a guarantee of scientific quality and absolute performance, the term nevertheless remains obscure for most cyclists. Is it just marketing or a genuine technological revolution?

Modern cycling is no longer just about weight. It is fought against an invisible but ever-present enemy: the air. At 30 km/h, more than 80% of your energy is spent solely on pushing air molecules out of your way. To overcome this resistance, engineers no longer sculpt material by hand; they use supercomputers to simulate millions of possible scenarios.

In this feature, we dive into the heart of the matrix. We will understand how CFD has transformed equipment design, killed off the “V”-shaped rims of the 2000s and enabled the emergence of wide tyres. Welcome to the era of the virtual wind tunnel.

What is CFD? Definition and principle

The acronym CFD stands for Computational Fluid Dynamics. Put simply, it is a virtual wind tunnel that runs inside a very powerful computer.

Imagine you want to test how a new carbon rim cuts through the air. Instead of making an expensive prototype in a mould, curing it, sanding it and taking it to a physical wind tunnel at the other end of the country, you create a 3D model on your computer. The CFD software then divides the space around this wheel into millions of small cubes (this is called the mesh).

The software then applies complex physical laws, notably the Navier-Stokes equations. These equations, which date from the 19th century, describe the motion of fluids (gases or liquids). They are so complex that it is impossible to solve them perfectly by hand for complicated shapes. This is where computing power comes in: the computer solves these equations for each small cube of the mesh, calculating the pressure, speed and direction of the air at every millimetre of the wheel.

Why use CFD to design wheels?

The cycle industry, and the wheel industry in particular, has switched massively to CFD over the last fifteen years. Here is why this technology has become indispensable, sometimes outdoing designers’ intuition.

1. Speed of iteration

This is the number one advantage. With the traditional method, designing, making and testing a prototype takes weeks, or even months. With CFD, an engineer can test 100 variations of a rim’s curvature in a single day. They can change the rim width by 1 millimetre, rerun the calculation and see the result the next morning. This makes it possible to explore radical ideas without putting a manufacturing budget at risk.

2. Seeing the invisible

In a real wind tunnel, smoke is used to visualise airflow. It looks nice, but it only shows what happens “around” the object. CFD lets you see “everywhere”. The engineer can visualise high-pressure zones (in red) that slow the cyclist down, or low-pressure zones (in blue) at the back that act like a suction cup (drag).

Even more importantly, CFD makes it possible to visualise turbulence and boundary layer separation (the moment when the air can no longer “stick” to the surface of the rim). Understanding where and why the air separates is the key to designing a wheel that stays stable in crosswinds.

3. Optimising the tyre/rim pairing

A wheel never rolls alone; it is fitted with a tyre. For a long time, wheel manufacturers ignored the tyre’s impact. CFD revealed that the transition between the tyre sidewall and the rim is critical. If the tyre forms a “bulb” (lightbulb effect) because it is too wide for the rim, it disturbs the airflow before it even reaches the rim. Thanks to CFD, brands understood that rims had to be widened to align the walls with modern 25mm or 28mm tyres.

CFD vs real wind tunnel: the match

If CFD is so powerful, why do brands such as Swiss Side or Specialized keep spending fortunes renting wind tunnels? The answer is simple: CFD is a design tool, the wind tunnel is a validation tool.

The limits of simulation: “Garbage in, Garbage out”

The computing adage “Garbage in, Garbage out” applies perfectly to CFD. If the engineer sets up the initial parameters badly (for example, if they do not simulate the rotation of the wheel or the roughness of the ground), the results will be wrong. That is why independent experts such as Hambini often criticise brands that publish CFD graphs that are too smooth or unrealistic.

Moreover, simulating a bike is extremely complex because it is a “bluff body” with moving parts (legs, pedals, spokes). Simulating the instability of the air (transient simulation) requires phenomenal computing power that few brands possess. That is why final validation in the wind tunnel remains essential to check that the computer has not got it wrong.

In practical terms, what has CFD changed on our bikes?

The impact of CFD is visible to the naked eye if you compare a wheel from 2010 with a wheel from 2024. Here is the major evolution dictated by the algorithms:

The death of the “V-Shape” and the birth of the “U-Shape”

In the early days of aero, intuition said rims should be pointed, V-shaped, to “cut” through the air. CFD proved this was a mistake for a bike. Why? Because unlike an aircraft, a bike is subject to crosswinds (the yaw angle).

Simulations showed that when the wind comes from the side, a V-shaped rim causes the air to separate abruptly, making the bike unstable and increasing drag. Conversely, CFD made it possible to design U-shaped (or toroidal) profiles with a rounded nose. This shape lets the air “flow around” the rim without separating, even with a crosswind. The result: you go faster and, above all, you no longer get buffeted by gusts.

Who has the best command of this technology?

All brands claim to use CFD, but the level of expertise varies enormously. Some outsource basic analyses, while others employ engineers from aerospace or Formula 1.

Conclusion: is CFD magic?

CFD has unquestionably ushered cycling equipment into a new era of maturity. It has made it possible to understand that aerodynamics is not just about “slicing through the air” head-on, but about managing complex, unstable and lateral flows. Thanks to it, we now ride on wider, more comfortable tyres while being faster than on the ultra-thin tubulars of ten years ago.

However, numerical simulation remains a tool. It does not replace field testing. As Jean-Paul Ballard of Swiss Side often points out, nothing beats the “truth of the road”. CFD lets you rough out 95% of the work, but it is the last 5%, validated in the wind tunnel and on the tarmac, that makes the difference between a good wheel and a legendary wheel.

As a bonus, CFD applied to a peloton: