Racing aerodynamically

We visit the wind tunnel which cyclists Wiggins and Froome use to perfect their racing positions...
08 July 2014

Interview with 

Stephen Turnock, Performance Sports Engineering Lab, University of Southampton


Kat in the University of Southampton's wind tunnel


We've had British winners in the Tour for the past two years now and Chris Froome will be trying to make that three this year.

These successes are, at least in part, down to the unique approach taken by Team Sky coach, wind tunnelDave Brailsford. He targets so called marginal gains, saying "If you broke down everything you could think of that goes into riding a bike, and then improved it by one percent, you will get a significant increase when you put them all together,"

One of those one percent's is a riders air resistance and so Kate Lamble visited the University of Southampton to meet Stephen Turnock director of their Performance Sports Engineering Lab, and to have a go in the wind tunnel which the likes of Wiggins and Froome use to perfect their racing position.

Kate - It used to be that who would win the Tour de France was decided on the slopes of a big French mountain like the alpe d'huez. But nowadays, it might be that those decisions are made instead here in a wind tunnel at the University of Southampton as more and more cycling teams are turning to scientists to squeeze every last bit of speed out of their riders. I'm joined here by Stephen Turnock. We're in this big cavernous black space with a very lonely racing bike in the middle of it. Who comes here?

Stephen - We've had all sorts of sportsmen and women in here, mainly through UK sport and the connection with the British cycling team. Primarily, it's about finding their best position on the bicycle, the one that means they need to use the least energy possible to go through the air.

Kate - So, I can tell when I cycle to work every morning that the wind and the wind speed will affect how fast I can go and whether I'm struggling just to get over those miniature hills in Cambridge. How does the wind affect racing cyclists who are doing it properly?

Stephen - It's the simple physical law that the faster you go, the higher wind loading will be and it's basically a square law for drag and a cube law for the power. So, if I go twice as fast, the wind drag I get from my own speed is 8 times the amount of power I need to go that fast. So, if I then get a headwind or a following wind then that's going to either mean I have to work harder or I have to work less to go at the same speed.

Kate - Is it just because I'm having to push the air in front of me out of the way and the faster I do that, the more difficult it is?

Stephen - Effectively, yes. What your body looks like to the wind or the relative wind you get once you're cycling and you've got the wind against you will change what the fluid dynamics does. Hence, what the pressure load is on you and how much force you need to go through.

Kate - So, how does this space help us measure it? We've got a massive turbine fan at one end of the room and what looks like a sort of black hole at the other end of the room, and a very lonely bike in the middle. We obviously can't get the bike in motion. How do we tell how much drag is going on without the bike being able to move?

Stephen - So, the wheels of a bike are spinning on a device that puts a relevant amount of force back to you, so you feel like you're cycling. And that whole system is sitting on a dynamometer which measures the drag force.

Kate - Can we have a go?

Stephen - By all means, yes and see where your elbow should be put.

Kate - The final door has been closed and I'm genuinely a bit terrified about whether I'm going to embarrass myself in front of all the technicians that are staring at me through the window. My drag and my power, and my calories and my time has just appeared in front of me and they're just going to take a zero value before the wind kicks in.

So, you can hear the wind gushing away there. It's been turned on for the first time, certainly getting a new hairdo at least. So, in front of me, it takes a few seconds average of the drag and the power at one time. So, if I sit up large like I'm on one of those sit up and beg bikes, we can see the drag hitting an average of about 30 Newtons, but if I move down onto the drop handle bars like you would on a racing bike, then the drag drops down immediately to under 20. So, that's 10 Newtons difference in drag even though, because I'm getting a bit tired, my power is dropping off. It's actually making quite a big difference how I can feel when I'm pedalling along. 

So, I'll ask the guys how much 10 Newtons in drag difference might change over a long race. 

See, already out of breath!

So, you can hear in my breath, so I'm still slightly recovering from that, but when I was sitting up on one of those sit up and beg bikes, I was on about 30 Newtons of force and when I was on the drop handle bars, even though I'm holding a recorder and microphone, that dropped down to 19. So, that's 10 Newtons of difference.  How much difference would that make for a rider in a race?

Stephen -  So, it's an example. They would unlikely to find such big changes in their posture because most of them have learnt over the years what a really good posture is. However, the beauty of coming in something like a wind tunnel is you can find the small changes. You can't actually perceive them when you're cycling, but when you're over the whole distance of a race, those small changes all add up and by the end of a race, you've got a margin which could take you from outside the medals to inside the medals.

Kate - It's not just tucking in your body that I see cyclist do. You also see them obviously riding in a big group, riding in a peloton. But when there's a sprint, when they want to go really fast, teams tend to line up in a straight line with their sprinter in the middle. How much does riding in a line change things?

Stephen - It changes things a lot because effectively, what you're doing when you're tucking in very close behind a cyclist in front is they're doing all the hard work for you. They're taking away the wind pressure on you and you could be working much, much less. So, when they're all riding in a big peloton, and they're all tucked in there, the lucky guys in the middle are not having to work very hard at all. They're just overcoming effectively the rolling resistance of their wheels.

Kate - I do find when I'm cycling along and I'm sort of stuck behind someone I'm very cocky about like, "I can't believe you're going this slowly" until I overtake and realise how much work it is.

Stephen - Exactly. So, everyone could sort of feel that.

Kate - So, the person behind benefits from the person in front. Does it work the other way around as well?

Stephen - Yes. There was a little bit of gain for the person in front. So actually, having two cyclists working together is actually beneficial for both of them and in an ideal world, they swap around every so often.

Kate - That sort of seems counterintuitive that the person being sheltered from the wind would actually give a hand to the person who's sort of being the shield for them.

Stephen - Effectively, it's because the person behind also changes the flow field on the person in front and that changes the pressure field around them and hence, the drag force on them. Effectively, you look like a larger, more streamlined object if you have a lot of people in a line, the one person on their own.

Kate - All I'm hoping to see is lots of pelotons of cyclists across Cambridge who want to make their commute to work easier.


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