How are carbon fibre wheels designed and tested?

Carbon fibre wheels are one of the best bike upgrades to improve speed and reduce weight on your bike, but how are they designed and tested? We sent Ollie Bridgewood to the Reynolds bike wheels testing facility in Germany to get to the bottom of carbon wheel engineering, including the use of CdA (coefficient of aerodynamic drag) and real-world testing.

Aerodynamics is an area that has seen unparalleled growth in the cycling industry over the last few years and the amount of time and investment that goes into aero research and development has seen the products we use change significantly.

What's more, some of the things we once thought we knew about aerodynamics have since been proven completely wrong. For example, narrower tyres were considered better than wider tyres. However, with new rim profiles, tyres of up to 28mm wide are now considered the fastest configuration for a road bike.

• Read more: Past, present and future of road bike tech: Aerodynamics with Cervélo co-founder Gerard Vroomen

We also saw sharp-edged rim profiles from a lot of brands a decade or so ago. However - once again through further research and testing - these were proven to be slower than blunter, more rounded profiles.

How does wind tunnel testing work for brands?

Only a few select brands have full-time access to a wind tunnel. The rest have to rent commercially available wind tunnels. This means that any time in the wind tunnel needs to be purposeful, and ultimately, more wind tunnel time costs the brand more money that will then be passed on to the customer.

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Most brands will develop wheels using computer modelling before renting a wind tunnel to verify its performance

To find out exactly what goes on when a brand brings its latest and greatest products to test in a wind tunnel, Ollie catches up with Todd Tanner, Reynolds’ director of wheel development.

What information does wind tunnel testing provide?

For Reynolds, wind tunnel testing is seen as one of the final checks in the development stage for a wheel. That is where the wind tunnel comes in as it allows for real-world testing to verify the computational predictions of performance.

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Wind tunnel testing is an opportunity to test the wheels in real-world environments in an incredibly controlled way

Something else that wind tunnel testing provides is benchmark data for future products to be compared against in order to measure improvements.

How has product development evolved over the years?

In the past, the product development cycle relied heavily on prototyping and testing. A brand might come up with a handful of different solutions that would be designed to meet the performance criteria set out. These would then be tested in the wind tunnel with the closest design to the requirements being selected for further development. Although this worked, it meant that progress was slow and expensive as it essentially relied on trial and error of physical products.

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CFD modelling provides far greater research capabilities to design engineers, allowing for thousands of variations to be tested

Now brands have access to computational fluid dynamics (CFD) which acts as a virtual wind tunnel. Using CFD software, hundreds or thousands of design variations can be tested over a few months to find an optimised profile before the product is prototyped. One of the main benefits of CFD is that it allows for the cost of design to be greatly reduced as it removes the need of intensive wind tunnel research and can all be done by a computer.

• Read more: Cycling aerodynamics: The benefits of body fairings and aero base layers

Something else that has been a consideration in the latest generation of aero wheelsets is how the airflow leaving the wheel is received by a bike and rider. It is possible for a wheel to be tested in isolation to perform really well but when tested with a bike and rider a lot of the performance gains are lost. Complete system testing in a wind tunnel is a way of ensuring that the wheels are not just fast on their own but as a complete system.

Has the way wheels get tested changed?

The answer to this one is yes. When aero wheels were in their infancy and they were taken to the wind tunnel for testing it was common for them to be only tested with the wind coming head-on. This is what is known as a zero-degree yaw angle.

However most of the time the airflow will have some level of diagonal direction when you're riding outside. The more the airflow becomes a crosswind the greater the yaw angle, and this is where the real gains are made in wheel design.

• Read more: The huge problem with modern tyres and wheels

Testing wheels across a range of yaw angles is what has led to the blunt and rounded rim profiles we are now seeing. Although a V-shaped rim is faster at zero degrees yaw, as soon as the air comes from an increased yaw angle it suffers massively and creates very dirty air as it leaves the rim, which in turn creates drag, slowing the rider down.

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Real world riding means riding in inconsistent turbulent airflow that puts side pressure on the wheels something that wasn't originally tested for

The understanding of airflow at higher yaw angles allows for wheels to have tuned characteristics that can increase a wheel's stability, its performance in crosswinds and its overall characteristics in a whole bike and rider system. Changing the rim's dimensions and the location of the widest point on the profile gives designers control of how the wheel will behave.

Are there any limitations to wind tunnel testing?

Wind tunnels are fantastic at simulating steady-state environments. However, one element of riding they struggle to emulate is that of sudden gusts of wind. For anyone who has ridden deep-section carbon wheels, how they handle a sudden gust can be the difference between an enjoyable ride or an absolutely terrifying one.

To test this, there is no better way than throwing a leg over the bike and taking the wheels out for a spin. This introduces the human element of how a wheelset actually feels on a ride. How it performs in the tunnel is one thing but if it feels twitchy, harsh or difficult to control it fails its usability test.

• Read more: Lidl-Trek announce partnership with Aerosensor in the hunt for aero gains

Ollie also takes a look at what the differences in rim depth mean for different riders and how you can pick the best style of wheel for your riding. Make sure to check out the video linked at the top if you are in the market for a new set of carbon hoops.

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