The benefits of wind tunnel testing

The fan behind draws air over the rider to test CdA at different wind speeds

When cycling, the biggest force that our body has to overcome at speeds above 20kph on flat terrain is air resistance, with the faster we go the more of a force it is we have to face. Even up climbs air resistance is still a very significant force that we have to overcome. For example, when cycling at 30kph on a flat road, the resistive forces we have to overcome are rolling resistance (~20%) and air resistance (~80%). As we increase to 40kph this changes to a ~10/90 ratio! The way we can reduce the impact of air resistance is to decrease our coefficient of aerodynamic drag (CdA). Cd itself is dimensionless and represents the friction/texture of the shape, while A is the frontal area (measured in m^2), and CdA is also presented in m^2 as a measurement. A 1cm wide cylindrical tube on a bike would have a lower drag than a 2cm wide cylindrical tube, but probably a higher drag than an aero profiled 2cm wide tube. So although generally a smaller Area equals reduced drag, the shape can alter this a lot so even with a large Area, the Cd may be lower. Likewise in certain scenarios a trip like structure can reduce drag with the correct placement.

So what does this mean for a cyclist? Well if a ride can reduce their CdA, then the speed that they can travel at any given power output will increase. It’s a complicated process to determine exactly by how much as it is a combination of air pressure, temperature, speed, CdA, weight and rolling resistance. But in practical terms, reducing CdA will make you faster. The faster you go, the more watts you will save for a reduction in CdA. However as air resistance/Drag increases at an exponential rate as speed increases, the speed gain you get will be smaller at higher speeds. The formula for Drag is below.

F_{D}=\frac{1}{2} \rho v^{2} C_{D} A

So FD = Drag, 𝜌 = density of air, 𝑣 = velocity, CDA = coefficient of drag. As you can see, as Velocity increases the drag increase by a squared factor. So if velocity was 2 then drag would be 4, if velocity goes up to 4 then drag is 16. It’s not quite this simple, but it gives you an idea of how drag increases so much. Think how easy it is to go from 10kph to 30kph on the road compared to 30kph to 50kph.

Sounds simple then, reduce drag, increase speed, easy peasy. Easy ways to do that generally include reducing frontal area by getting lower or narrower. However this needs to be balanced with capacity to output power while maintaining the position. Additionally, aerodynamics is a bit of an art and what may be traditionally seen as something that would reduce drag, does not necessarily mean that it will. To do that you need to measure it.

Position and equipment can be tweaked and CdA measured to see what’s fastest

How wind tunnel testing can help you

The wind tunnel is one place where we can actually test our coefficient of drag in different positions on the bike and get a reading of how many watts a change in position may save. In general, testing of this sorts can regularly help people save between 15-40 watts on their position, sometimes significantly more depending on the starting point.

The way the wind tunnel works is by drawing air over the rider by using a large fan behind them. The speed of the air can be increased all the way up to 60mph, however for each individual rider the wind speed can be tailored to the speed at which they will be competing. The Yaw, or angle to direction of the wind, can also be changed up to 30 degrees, to simulate crosswinds. The faster a rider goes, the less Yaw they will experience generally. A time triallist travelling at 50kph will experience a lower Yaw angle in a 5kph crosswind than a ride travelling at 40kph. However, testing different Yaw angles is very useful as it helps ensure that any positional changes will be faster in a range of environmental situations such as cross winds.

Yaw angles are particularly useful for those wanting to improve their position on their road bike. Generally speaking, velocity will be lower on a road bike than a TT bike, so the likelihood of experiencing greater Yaw angles is higher. Additionally although TTs are traditionally where people look to find aerodynamic gains, if you can be more aero on your road bike you will be able to travel faster for the same effort when out of the bunch such as in a breakaway. You will also be able to conserve more energy for the finale of the race, and you can also increase your sprinting velocity.

More details about how wind tunnels work can be found here.

The 3D capture to ensure position consistency between runs when testing equipment

Equipement testing

Another way a wind tunnel can be used is to test different equipment. It’s all well and good seeing that Helmet A saves 10 watts over Helmet B, but that doesn’t tell us very much. We don’t know what position the helmets were tested in, what speed necessarily, along with the fact that different morphologies can result in very different outcomes and drag from different pieces of equipment. What may work well for one rider may be slower for another. Wind tunnel testing allows you to select equipment with confidence knowing that it will result in your travelling faster for fewer watts.

A quick change of kit to see how one jersey or skin suit compares to another

How we test you

Our testing is all carried out at the Silverstone Sports Engineering Hub, however the process begins before you arrive there and doesn’t finish till well afterwards.

To start with, we have an initial consultation with you to determine what it is we want to test and how we will go about doing that. Let’s say Joe Bloggs is wanting to improve their time trial times for 10 and 25miles. Their speed for these ranges from 40-45kph and they want to get to a speed of 50kph for them. All their course are flat out and back. So straight away we know that the wind speed we want to test at is probably between 35-55kph to allow for headwinds and tailwinds on course altering air speed. Ona sporting course we may test lower and higher for speed on hills and downhills. We will also do several biomechanical and morphological assessments to determine if there will be any positions on the bike that will not be sustainable even with appropriate S&C work.

Next we will look at your position on the bike. Doing this allows us to assess any obvious changes that could be made and tested in the tunnel. Ideally these images will be from in a race so we can see if your position changes under duress or with fatigue. It’s all well and good finding the ideal position but it needs to be sustainable. We will also discuss with you if there are any equipment items you wish to test out, say helmets or skin suits. Fortunately the SSE Hub does have access to many different helmets so you can test which will work best for you.

Once we get to the tunnel itself, the bike will be set up and the testing programme will have been drawn up to ensure that everything goes as smoothly as possible and you get as much testing for your time as possible. Once the bike is set up, an initial run will be done and a capture of your position on the bike will be taken. This ensures that during other tests you can maintain position if testing equipment, or show you how the position has changed from run to run.

After the testing has been completed, we will then sit down and have a full debrief. We will talk you through what positional changes result in the lowest drag, while also being mindful of your biomechanical limitations. Any equipment tests done will also show us what the fastest pieces are on you. Even once you have gone home, the testing doesn’t stop. For those that are coached by us, we will implement specific sessions on and off the bike to ensure that power output and comfort in the new position are optimised. For those not coached by us, we will still give you recommendations of what to include as part of your training to optimise power and comfort.

Front could be lower, but a hamstring injury prevents that. We work within biomechanical constraints to produce a sustainable position

The bottom line

There are no two ways about it, wind tunnel testing is expensive. However the benefits from it can help save money in the long run. Being able to test and compare equipment helps prevent you from purchasing items which won’t make you faster. Additionally the drag reduction gains you can make will likely far outweigh the cost per watts saved than say a new frame or even set of wheels.

If wind tunnel testing is something you are interested in, please get in touch with us and we can get you booked into the tunnel and saving watts for a faster 2023 season!

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