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Cycling science: yaw angles explained

Max Glaskin
25 Oct 2016

Modern bikes are designed to function best at specific wind angles, but how do manufacturers know where the wind will be coming from?

Aero frames and wheels are designed to optimise the slipperiness of your bike through the air. Trouble is, the air doesn’t know that. It keeps changing in speed and direction relative to you on your bike, which means one significant factor of aerodynamics is rarely stable for very long – the yaw angle.

Yet manufacturers say they’ve optimised their products for specific ranges of yaw angles, with some even claiming to have created tube and rim shapes that act like sails, driving the bike forward when the wind hits it from the correct angle. But with the speed and direction of both wind and rider being infinitely variable, how can there be an ‘optimal’ yaw angle, and more importantly, what is it?

First, let’s understand yaw. Imagine tying a silk thread to your seatpost, then going for a virtual ride, due north. Assuming it’s a perfectly calm day with no wind, the thread will run straight out behind you, pointing due south, in line with your rear wheel.

But imagine the weather changes suddenly and a wind gusts in from the west. This new force will act upon the silk thread, pushing it eastwards and opening an angle between the thread and the southward-facing line of the rear wheel.

This is the angle of yaw. It’s the result of the force of the natural wind combining with the force of the headwind that you’re creating for yourself by riding forwards.

Narrowing the angles

From this you can now see that even if the wind is coming at you at a right angle, the idea of a pure crosswind is mere hot air.

Your forward movement will always create a draft and that force will, to a greater or lesser extent depending on the speed you’re travelling, counter the wind direction, pushing the thread and effectively closing the yaw angle from the hypothetical right angle to something significantly smaller.

It’s why pro teams never have to ride side-by-side to protect each other when side winds are strong. Instead, they form a diagonal echelon for shelter.

Of course, the wind, your speed and the relative direction of one to the other change constantly throughout a ride. For example, a few miles down the road in your hypothetical ride, the westerly wind could suddenly whip up and push even further to the east to open the yaw angle wider.

But that’s not all. Imagine you start down a steep descent, where your increased speed also increases the effective headwind you’re creating for yourself. This now-stronger force pushes the thread back closer to the due south line of the rear wheel and makes the yaw angle smaller. So speed affects yaw angle too: go faster and the yaw angle gets smaller.

So now our fictitious ride is over, but it still leaves that gale force question: since the speed and direction of riders and the winds they encounter are infinitely variable, how can manufacturers say the sweep of yaw angles they’ve chosen for optimising the aero shape of their frames and wheels is the right one? It’s time to shoot the breeze with the experts.

Working the angles

‘We’ve spent a lot of time testing different athletes – from the casual rider to the pro – in different disciplines and it’s interesting how diverse the range is,’ says Chris Yu, leader of Specialized’s Applied Technology group.

‘If you look at a WorldTour sprinter coming off a wheel in the last 200m of a race, the effective yaw is extraordinarily low – close to 0°. That’s because they’re going really fast, more than 60kmh, and finishing straights are typically well shielded by barriers and crowds, which serve to block any crosswinds.

‘On the other hand, if you go to the Kona Ironman World Championships, they ride up the Hawaiian coast, with the wind blowing in across the water, so for an age-grouper at Kona the effective yaw angles hit up to the 15° range if it’s gusting. Pros will be going a little bit faster, so they’ll see yaw angles of up to 10° or so – maybe low teens,’ says Yu.

On the road

Those figures aren’t just guesswork, they’re the result of fitting instruments to real bikes and getting real cyclists to do what they do best – ride the roads.

Trek’s Mio Suzuki says, ‘We mount a pressure probe on a bike, which sticks out a long way to avoid any “dirty” air from the bike or rider. We’ve sampled air around our headquarters in Wisconsin and the team has also gone to Arizona and to Kona for the Ironman.’ 

These data-gathering efforts allow manufacturers to calculate the probability of a cyclist encountering specific yaw angles, which then informs the design process through the use of computational fluid dynamics software and wind-tunnel tests.

‘We try to narrow it down through experimentation and measurement. For this reasonable yaw angle, the range is between 5° to 15°,’ says Leonard Wong, aerodynamicist at Giant. 

Suzuki tells a similar story: ‘In the real world 2.5° to 12.5° are the most prevalent yaw angles riders encounter.’

Yu at Specialized adds, ‘For an average cyclist, unless you’re riding in extremely windy conditions, the typical angles are less than 10°.’

This slight difference in results is why one aero bike doesn’t look identical to another. Specialized designed the Venge ViAS based on its vision of the perfect range of yaw, while Trek designed the Madone to fit a different range. 

So it seems that if you’re Peter Sagan, driving the peloton along at 50kmh, you want a bike optimised to deal with yaw angles of around 3°-7°, while the rest of us want a bike designed to tackle yaws of up to 10°-12°.

Performance gains

And what about this idea that some designs can harness side winds to generate forward thrust, like a yacht tacking into the wind? Jason Fowler at Zipp Wheels is categorical: ‘We don’t believe so,’ he says. 

Xavier Disley, whose AeroCoach consultancy measures track aerodynamics for WorldTour teams and manufacturers, is equally dismissive: ‘Whenever people have found thrust in the past, it’s tended to be through components like disc wheels. But as part of the entire system of bike and rider any effect would be tiny.’

Max Glaskin’s Cycling Science is out now in paperback. He covers all the angles on Twitter as @cyclingscience1

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