Science Interviews


Sun, 8th Aug 2010

Aerodynamics of the Tour de France

Robby Ketchell, Aerodynamicist, Garmin-Transitions

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Meera -    Now the cyclists on the tour wouldnít be able to perform as well as they do without their support teams of doctors, nutritionists, engineers, and more.  And a factor increasingly playing a key role in winning points in stages is an understanding aerodynamics.  With cyclists reaching speeds of up to 100 kilometres an hour on descents down mountains, and wanting to preserve their energy whilst also trying to ride fast on flat stages, the ability to cut through the wind and reduce its resistance is crucial.  One man trying to improve this is the team aerodynamicist, Robbie Ketchell.

Robbie -   Aerodynamics is hugely  important for time trials and road races.  In time trials wind resistance is the number one resistance on a cyclist.  For road races, mainly flat stages, itís the same thing.  For climbing - then gravity plays a larger role.  So in time trials, they use different equipment that would be more aerodynamic and is just specific for that event, and they also adopt different positions to try to reduce aerodynamic drag, Pelatonwhereas in road races, they draft behind each other ,and ride in packs, and try to help different riders on the team be blocked from the wind.

Meera -   So as you've just mentioned say, in a road race, when cyclists cycle, they like to stay together in a pack.  How much of a difference does this actually make and what effect is it having on their cycling in terms of the energy they use?

Robbie -   So depending on where they are in the pack and whoís around them, they can save anywhere from 25 to 40% of the energy that they would have to produce in order to go that fast alone.  And so, itís significantly important for them to conserve energy in say, a 5-hour, 6-hour race in which they don't want to be expending that energy or else it will hurt them towards the finish.  Generally, especially for the first week of the Tour de France, youíll see most general classification contenders hiding within the pack and thatís just to conserve energy.

Meera -   I guess this is an important aspect then of team work - how the team all work together in order to protect their key rider.

Robbie -   Exactly.  In order to protect your team leader, there are times that you'll see the entire team surrounding that leader, and you'll also see that in sprints, in which you obviously can't sprint from 10 kilometres to the finish.  And so - most teams will do whatís called a lead out and they will put four or five riders in front of their team sprinter in order to put them on the front, and allow them to ride towards the front of the pack, while conserving energy for them.

Meera -   So youíve said that 25 to 30% of energy can be saved when riding in a pack - why exactly does that happen?

Robbie -   Basically, what happens when you ride behind somebody is you are blocking that wind - so it basically directs wind around the first rider that hits the wind.  The wind goes around that body and it creates a bubble that they can sit in, and thatís called ďdraftingĒ when they are reducing that wind on the second rider.

Meera -   And what about as more riders get involved?

Robbie -   Riding in the middle of a pack, with people surrounding you on the left and right side, and in front, is different than riding in single file and that's why they ride like that.  The problem with drafting in single file is that wind doesnít always come from head-on, and so sometimes you'll see people ride in whatís called an ďEchelonĒ and they're just off set to the right or left side of the rider in front of them, and that's to draft from the direction that the wind is coming from.

Meera -   So thereís really a lot for a cyclists to be thinking about as they're going along these courses?

Robbie -   Yes - itís funny you asked that-  because drafting, when you're in a bike race, itís almost like a natural occurrence.  You can literally just feel that the windís coming a little from the left and you know to ride a little to the right of the rider in front of you.

Meera -   Now you've mentioned the importance of drafting, but just how close do the riders actually have to be for this?

Robbie -   The riders can actually feel an effect anywhere from 2 feet behind the rider in front of them, but you get a largerTour stage 9 effect the closer you are to the rider in front of you.  So, you try to get as close as possible.

Meera -   Well that touches on road racing, but another aspect is time trials.  What role does aerodynamics play here?

Robbie -   In individual time trials they obviously can't draft off another rider because they start by themselves.  Itís actually illegal to draft, so if they were to catch a rider that started in front of them, they wouldn't be able to use their slipstream to draft off of.  So what we do in that case is we adapt;  we use different helmets, different clothing, different shoes, different bikes, and different positions.

Meera -   So letís go into positioning first.  How does a riderís position affect speed in a time trial and how does aerodynamics play a role?

Robbie -   The position can contribute up to 90% of the aerodynamic drag - So even though we spend a lot of time on different equipment, position is the number one factor.  But in order to reposition somebody to be more aerodynamic, they also have to be physiologically efficient - they have to produce power to actually move the bike. And so, we can't just put somebody in a superman position flat out and expect them to still have the same performance even though it significantly reduces aerodynamic drags.  You have to have a balance between the two and thatís why you see so many riders with different positions out there.  Thereís not one position that works for all riders.

Meera -   Well what about the gear?

Robbie -   There are a number of things that you can do just with the equipment.  So for instance, all teams use different time trial bikes.  Road bikes generally have round tubings to them because they are trying to make them light and better to go over hills, and also really stiff, whereas time trial bikes have airfoil-shaped tubing to them, to make them more aerodynamic.  All riders wear aerodynamic helmets -they have a teardrop shape to them.  For clothing in road races -  rider wear shorts and a jersey ,whereas in time trials they wear whatís called a skin-suit and itís a one piece tight fitting skin-suit.

Meera -   So these skin-suits and helmets, what are they actually doing to reduce the drag?

Robbie -   The biggest thing that they're doing is reducing turbulence around the body.contador  For instance, if you were to stand outside in a windy day and you had a rain jacket on, you can hear it flapping.  And that similar effect happens when you have a jersey on that's loose fitting, which would be better for climbing, itís a little bit cooler, a little bit maybe lightweight, and not as restrictive. Whereas these skin-suits are tight and reduce those wrinkles, and reduce the turbulence around the body.

Meera -   You've  just touched on climbing there as well.  So what role does aerodynamics play if you're a climber?

Robby -   A much smaller role - generally, when you get anywhere below 20 kilometres per hour, aerodynamics does not play as much of a role.  Aerodynamic drag actually increases exponentially to the velocity that you're going.  So anytime that you're going at a slower speed, itís not going to contribute as much.

Meera -   Lastly, how would you summarise then the importance of this to cyclists? How much of a priority is it compared to all the different things they have to consider?

Robbie -   For events where it makes a difference, it is the most important thing that you can try to manipulate.

Meera -   That was Robby Ketchell, Aerodynamicist for team Garmin-Transitions.



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