TECH: IN THE WIND TUNNEL
Held aloft as the ultimate go-faster tool for cyclists, wind tunnel use has exploded recently as gains become harder to ind. Procycling talks to industry brains about the latest test methods and what the future holds
How professional teams and equipment companies are harnessing aero tech
The foe that grows stronger with your every strike, wind is a cyclist’s reallife nightmare. If you want to go twice as fast, your enemy the wind pushes back four times as hard and you will need eight times the power to match it. Drag increases to velocity squared, power to velocity cubed. Becoming a stronger rider is the most diminishing of returns. But the wind tunnel is how we fight back. We can’t change the strength of the wind but we can give it less to grab hold of, and wind tunnels are how teams and the cycling industry test, experiment, develop and measure their efforts.
From rudimentary beginnings in the 1980s, wind tunnel use in cycling has come a long way. World and Olympic champion Chris Boardman, more recently one of the brains behind British Cycling’s success before founding his own bike brand, recalls his first experience in late 1991: “I went to the tunnel at MIRA [Motor Industry Research Association, Warwickshire, UK] with Mike Burrows. It was made for testing out trucks, but it was the first time we’d been able to test positions to get real numbers. We came out with more questions than answers, but it was a start.”
Cycling is one of the relatively more trivial uses of a wind tunnel. As with so much technology, this early 20th century invention went through accelerated development during the two world wars and through the advent of the jet era, leading to tunnels capable
of supersonic speeds and able to accommodate fullsize aircraft. NASA has the world’s biggest wind tunnel at the Ames Research Centre in California, USA, with a vast test area cross-section of 80x120ft.
Motorsport – especially Formula One, with its high budgets and fierce pace of development – was the conduit for this technology moving from military usage to cycling. It has also given cycling some of its best brains, including Simon Smart of Drag2Zero, a consulting aerodynamicist for Enve wheels and Scott Bikes, among others. He’s arguably the foremost TT positioning expert in the world – six of the top 10 on GC at the last Tour de France had been in his tunnel to hone their positions.
“In F1, the goal was for a 10 per cent improvement per year. We had three or four groups of engineers working 24 hours, six and even seven days per week. That’s the dirty way of finding performance, making every shape you can think of and testing and testing,” Smart remembers “When I started in cycling, at first I thought, ‘This is easy, it’s only a cyclist’ but you have a much lower drag force, around 3kg at 30mph compared to 100kg at 100mph for an F1 car, and such chaotic flow because a cyclist is a bluff body and always moving. It’s certainly a challenging business, but we’re now testing repeatably to within 1 Watt, which I never thought would be possible with a rider. At the same time, all of the low-hanging fruit – the frame shapes, wheels – has gone, so now we’re into second order stuff such as skinsuit fabrics and performance in oscillating winds. When you look at all of it, those second-order elements can add up to first-order gains.”
For all of the progress since then, it can sometimes seem as if the main thing that’s been learned is just how little is known. “We’re still in the infancy of our understanding of aero in wind tunnels,” says Kevin Quan, designer of Knight wheels and the radical new Diamondback Andean tri bike. “Most people in the industry are not PhDs, with the exception of the guys at Specialized. We’re engineers; we’re applying the tech, not creating it. The tunnel generates numbers; it doesn’t explain why something is faster or slower.”
Getting beyond trial and error and into some real understanding is the goal of every aerodynamicist, but that goal is also incredibly time consuming, as Boardman recalls: “In one Olympic cycle we tested 10,000 materials on a tube in a small tunnel, testing fabric tension, grain direction… We tested sequins, fur, all sorts of weird and wonderful things, just to see. Sometimes you do one test and it gives you
“In one Olympic cycle we tested 10,000 materials on a tube in a small tunnel. We tested sequins, fur, all sorts”
a whole other direction to look at. We found some clothing in the second Olympic cycle that worked extremely well, but we didn’t know why. So we spent the next four years working to understand it.”
UK Sport’s budget dwarfs that of most cycling brands, so that level of research was out of reach to them for a long time. However, now the need to find second-order gains is pushing brands into making a bigger commitment.
“The commercial pressures of getting out a product necessitated a trial-and-error approach,” says Quan. “Now some companies are moving towards research – such as Specialized and the Euroregional Bike Valley hub in Belgium [shared by Flemish companies Ridley, Lazer Sport and Bioracer] – but that’s a big investment and then you have to make it pay.”
The man responsible for the Specialized facility, dubbed the Win Tunnel, is Dr Chris Yu, director of integrated technologies. Yu told Procycling how their tunnel is designed to be cycling-specific: “We had three main objectives: speed, size and sensitivity. We wanted to be able to cover speeds from 15-110kph [ie up and down mountains] so we could test flapping clothing, cooling etc. The size [16x10x30ft] allows us to test TTTs and lead-out trains.
“Finally, aside from the quality of air flow, the most important element is the force balance. It turns out that cycling is unique in that the resolution required is very high; we need to accurately measure changes of one gram. At the same time, the peak forces are quite high, such as when we have a sprinter riding at maximum power. It’s challenging to design a scale that is both very sensitive and very robust.”
For all of the science, wind tunnels have their idiosyncrasies, too. Each one is a different size and shape. The smaller ‘closed jet’ test areas limit the size of the subjects that can be tested in them. “You don’t want the object to be bigger than five per cent of the total area. The walls can affect aero performance and can distort results,” says Smart. For this reason, ‘open jet’ tunnels are more suited to testing riders on bikes at high yaw angles.
More techy still is the metric of ‘initial turbulence’ – how smooth the air flow is as it reaches the subject, having earlier been chopped and swirled by the fans. “This is diving quite deep into how tunnels work and how different aero phenomena can be affected,” says Yu. “We targeted a free stream turbulence intensity of just under one per cent, which from our studies is appropriate for outdoor cycling. This is controlled by turbulence screens, as well as the contraction at the inlet of the tunnel. Values that are too low or too high can result in misleading conclusions about the performance of different aero shapes, especially when it comes to flow separation and attachment.”
Smart adds a less scientific evaluation of it. “It’s these things that give tunnels their own character,” he says. “But we’ve also solved a couple of those known unknowns, and we’re really excited about the next few years.”
TESTING VERSUS DEVELOPMENT
Many products and bikes are claimed to be wind tunnel ‘tested’ but that’s a world away from being wind tunnel ‘developed’. Only the latter uses data to make improvements and new iterations. If you’re testing a signed-off production model, all you can do is hope that it’s fast. It’s even been known for a brand to book an hour in a wind tunnel just for a catalogue photoshoot. Those marketing people will probably ask for smoke to be blown over the product, too, but the engineers will be rolling their eyes. Quan explains that smoke “works on much bigger objects, but not in cycling, with lower flow speed and smaller airfoils. Cotton strings likewise, and oil mapping just makes a mess. The latest method of flow visualisation is Particle Image Velocimetry (PIV). You flood the tunnel with smoke, then fire a sheet of laser through it to see vortices. If we can get PIV here that would definitely progress our understanding.”
Dr Dave Marshall, who runs the wind tunnel at the University of Southampton (where Procycling tested the wheels in this issue and where British Cycling and Team Sky have done much work) believes finer data resolution is the goal: “We take 20 second averages for each configuration, and it would be good to know how things are changing within that. For an inanimate object such as a race car, not much is happening, so you can take shorter averages and get the same sort
of data quality. But a cyclist is moving, so seeing a time history of where the drag is coming from through a pedal rotation would be good, perhaps in a live readout overlaid with slow-motion video.”
Data measurement is also where the greatest progress has been made over the last century. “This tunnel dates back to the 1930s,” Marshall says. “Quite early on they had very good flow quality and a good understanding of what the air was doing. The difference is in the instrumentation and resolution – we can measure thousands of times per second to very fine accuracy. The balance used to be just that: an analogue balance with weights.”
SIMULATION VERSUS CONTROL
Accurate testing is all about control and repeatability, but that creates a paradox – the more controlled the test, the less like the real world it becomes. For an example of this, look at TT downtubes from 10 years ago. With the front wheel clamped in place, engineers were spoilt by consistent flow, so they designed thin, wheel-hugging downtubes, but on the road the front wheel is rarely aligned so neatly. Modern TT and aero road bikes have more versatile interactions between the two. So which is more important, simulating the real world or creating a perfectly controlled test?
“That’s delving into a debate that has been going for eight years,” says Boardman. “You can choose. If you go with a mannequin you gain control and get very repeatable data, or you can go to the track and get very close to reality, but the data is very noisy.” Smart agrees to an extent, but he also sees another way: “Tunnels have been driven to be as pure as possible for the ultimate fidelity, but then you move away from the real world. Our job as engineers is to find the sweet spot. That’s what the future holds. Outdoor testing is good for validation. Just 10 years ago an anemometer needed a backpack full of data-logging gear. Now you can run it from an iPhone.”
Boardman, Quan, Yu and Smart agree that a wind tunnel may be the best single tool, but it’s at its best when it’s combined and cross-referenced with CFD studies and real-world measurement.
At worst, excessively controlled testing risks skewing R&D efforts, as Marshall explains: “A mannequin test gives you ultimate repeatability, but rider shapes vary, so you create the risk of designing something that only works for certain shape riders.”
A case in point: at the last Winter Olympics the US speed skating team blamed their poor performance on their new Under Armour ‘Mach 39’ skinsuits, which had been heralded as the fastest ever after development but produced poor times and no medals. Likewise, in cycling, POC produced the Tempor TT helmet, which is also incredibly fast but only in the ultra-low position of the pro who helped develop it, and while it’s good at 30mph, above 45mph it starts generating lift like a wing and wobbling around. Such misadventures are retold now as cautionary tales.
WINDS OF CHANGE
While Formula One teams continue to push the tech – their latest innovation is the ‘adaptive wall tunnel’, better simulating outdoor conditions and costing £50m to build – this most elite of performance tools is about to become democratised, at least in the UK.
In November, the Boardman Performance Centre will open its doors. Alongside a concept store and café it’ll house a cutting-edge, cycling-specific tunnel that Boardman promises will be far more affordable than we’re used to. “The concept started over a curry with Rob Lewis, an aerodynamicist I worked with at British Cycling,” he explains. “He said, ‘Wouldn’t it be great if we could run tests for the price of a curry?’ So we started to explore the concept and took away the 150mph fans, the rolling road, the heavy balance for cars, and by the end of the meal we had a plan.
“It’ll be a custom open jet and open return tunnel, housed inside a temperature controlled building. It’s designed to be fast to change a bike, with a balance built to clear up data noise from pedalling. Hundreds of hours of CFD have already gone into this. I’m pretty confident we’ll have the best tunnel in the world.
“My biggest challenge is to get people to see how exciting it is to go to find out stuff and have that eureka moment.”
“A mannequin test gives you ultimate repeatability, but rider shapes vary in the real world”
Inside the Specialized ‘ Win Tunnel’ testing facility, which was designed speci ically for testing bikes
Alex Dowsett has spent hours having his position honed in wind tunnels
Chris Boardman was one of the irst riders to make extensive use of wind tunnel testing
Team Specialized, with performance manager Mark Cote (left) and Dr Yu (right)