Drag acts

Slipstreaming the rider in front reduces drag, save energy and increases speed. Now it’s time to uncover the  more subtle secrets of the draft

It’s stage 13 of the Giro d’Italia and after five hours of hard racing Mark Cavendish stands in the centre of the small town of Cherasco, northern Italy, and gives his now-familiar post-win interview: ‘They gave it 100% until they had no more,’ Cavendish says, ‘then I had to give my last ounce to win.’ The ‘they’ he’s talking about are the team, the domestiques, the men who regularly sacrifice their bodies and minds to control the race, carry water from the team cars and, perhaps most importantly, shelter their star sprinter from the wind.

In your wake

Unless you’re time-trialling, you’re missing out if drafting doesn’t fit neatly into your rider’s repertoire. But why? What’s going on in the world of physics? ‘Bikes and riders aren’t very streamlined and as a result they leave a significant wake behind them,’ says Rob Lewis of TotalSim, an expert in the field of computational fluid dynamics. ‘This wake is effectively air that has been dragged along with the riders and is moving forwards. So when a following rider is close and in this wake, it’s as if someone has turned the headwind down.’

The potential gains of sitting in the draft are significant, with research stating that the trailing rider generates 1% less energy per 1mph (1.6kmh) of velocity. And while any slower than 15kmh doesn’t realise a significant saving, sit in the slipstream of a rider at 55kmh and you’ll expend around 35% less effort.

But just how far back do those rear wheel vortices swirl? If you look at Ironman triathlon racing, where drafting is not allowed, riders must keep 7m back from the rear wheel of another competitor unless they’re overtaking, otherwise they enter the ‘draft zone’. Well, unless triathletes have rewritten the laws of physics, there’s no useful draft effect to be enjoyed with an abyss that deep between you and the rider in front.

In fact, while scientists focus on the micro percentages that can be gained by sitting nanometres from the wheel, feel and experience should be your guide to how close to get. ‘The closer you sit behind, the better. But that distance should be appropriate to your ability,’ says Team Sky stalwart Geraint Thomas. When Thomas says close, he’s talking within 10-15cm, if not closer. But remember, Thomas not only rides on the pro road circuit, he’s a master drafter who’s racked up two Olympic track gold medals in the team pursuit. ‘When you’re on the track just following the black line, you can get within centimetres. On the road, you don’t sit quite as close due to variables like the riders around you and wind.’

Many studies support Thomas’s closer-is-better ideal, and it’s one that’s certainly not a new phenomenon. Back in 1979, scientists studied wind resistance and power output in racing groups, measuring a 47% energy saving when following at a rather improbable 0m, but still a healthy 27% at 2m back. Even at 3m, benefits in energy saving have been noted.

Follow the chubster

Size of rider also affects energy savings. There’s footage online of a cyclist drafting a lorry at 90kmh with no sense of exertion (or common sense – don’t try it at home, kids). Beyond his lunacy, it emphasises the simple fact that a larger rider in front creates a greater barrier to wind than a skinny climber. ‘You don’t necessarily seek out the bigger riders,’ says Thomas. ‘You just tend to shy away from the smaller ones. In the team pursuit, we always take the mick out of Pete [Peter Kennaugh, fellow team pursuit London gold medallist and 64kg in his pants]. He’s got small man syndrome.’

And once you’ve tracked down your cycling behemoth to hide behind, the next question to consider is numbers. Is it more efficient aerodynamically to have several riders in front of you? In a pro peloton, it is often assumed that the best place to sit is right in the middle of the pack, like some queen bee protected by a mass of workers, but large numbers don’t always signify an easier ride.

Bert Blocken is a professor of building physics at Eindhoven University in the Netherlands who specialises in wind flow. ‘We’re working on a publication that saw us test groups of various riding compositions,’ he says. ‘Our results showed that six to eight in a pack is optimum. The reason is that the wake keeps widening until peaking at the sixth rider. However, the sixth also receives an over-pressure boost from the seventh rider.’ Beyond this figure, according to Blocken, additional numbers provide little extra shelter, but increase the potential for pile-ups that inevitably occur when one rider out of 100 loses concentration for a second.

As Professor Blocken suggests, even hardmen such as Jens Voigt and Stuart O’Grady enjoy some benefits from slipstreaming even while driving the pace at the front of the pack, because a rider sitting in the turbulence behind that rear wheel actually smooths the air flow, thus reducing drag, helping to ease the path of the lead rider.

Blocken says, ‘We noticed articles on rally driving that showed if the second car is very close behind the first one, the car in front has a lower fuel consumption [than it would when driving alone]. So we felt that could work in cycling.’ Blocken and his team concluded that with just 1cm between riders there were energy-saving benefits of 2-3% for the front rider. At that distance you are bordering on morphing into a tandem, but Blocken asserts it’s not totally unrealistic. ‘While more akin to track riding, on the road, cyclists hide behind each other in a staggered fashion, so it’s not just one long train linked by centimetre gaps. That said, a more real-world 15cm still sees a 1.5% conservation [for the front rider].’

Deflect the crosswinds

A fluid, cohesive paceline weaving its way around the countryside takes riders with experience, discipline and concentration, but is magical to watch in full flow. But when nature becomes restless and crosswinds blow, it all changes. It’s time to form an echelon. ‘You often see this in pro racing where riders are fanned out in a diagonal formation, especially in windy races such as the Tour of Qatar,’ says Madison Genesis pro team rider Chris Snook.

Practically, you move slightly to the side of the rider in front and just behind their wheel, depending on the wind angle (yaw). If it’s from the left, you move right, and vice versa. And instead of switching the lead by riding through and off, as in a paceline, you rotate in a circular motion across the road, always looking for that little bit of shelter. Setting up an echelon in a crosswind is also an effective way to split up the bunch during a race. As the width of the road is limited, a fight ensues to be in that front echelon, fanned out across the road. If you don’t make it, then you’ll find yourself riding in the gutter, in the wind and in single file. And unless a second or third echelon forms, you’re likely to disappear out of the back.

‘Movistar and Garmin-Sharp both used this tactic to great effect at Challenge Mallorca,’ says Snook, ‘splitting the bunch before we even hit the first climb. People were worrying about that climb, sitting back to save energy, and got caught out long before they reached it.’

The nature of an echelon, where riders are often overlapping wheels, means that it needs only one rider to touch wheels to bring down the entire group like a set of dominoes. This is where practice and attention to your surroundings become vital, although no amount of experience can guarantee a crash-free ride.

‘In the Classics I had a couple of crashes from being in the wrong place at the wrong time,’ says Thomas. ‘In Flanders someone rode into me, then I bounced off them over to another guy and that was it – straight down. The other one was a crash in front of me with nowhere to go. That was Paris-Roubaix. It’s part of racing. Most of the time you’re ok.’ Still, it’s worth knowing a few dos and don’ts about riding in close proximity to others…

Drafting etiquette

Recreational or in a race – either way, there are certain behaviours that’ll either endear you to, or ostracise you from, the masses. Boom time this may be, but ask yourself: does Cavendish take centre stage on the cycle path? No, those Manx pistons only unleash their potential come the race arena. Now, while Strava and its imitators encourage competition with every pedal stroke, racing that chap with the trouser clips who had the temerity to overtake you at the lights, dressed as you are in full Lycra, is bad form. It’s the same when doing training laps of the park.

‘I’ve been drafted recreationally, particularly round Richmond Park, and it can be quite off-putting when it’s someone you don’t know.’ Not the words of Jeff in accounts, but Yanto Barker, pro rider for and winner of two rounds of this year’s Pearl Izumi Tour Series. ‘I don’t race anyone without a number on their back,’ Barker adds. Drafting people you don’t know can be a dangerous affair; and it’s even more dangerous if they don’t know you’re there.

So save drafting for training or the race, and when you do, follow rule number one: don’t be a wheelsucker. You’ll be as unpopular as a pothole in a peloton if you don’t take your turn in the wind and let your clubmates draft you. ‘There’s nothing more annoying than people who sit on for longer than they should and refuse to go through until you’ve spent nearly all your energy,’ says Preston-based club rider Pete Slater. ‘Then they go through like a train so you can’t sit in.’

If you simply haven’t the fitness to take your turn up front, maybe it’s time to find another group or hit the turbo regularly to increase your strength. If, however, you’re on the cusp of becoming a pro, you can crank that competitive strategy up a notch or two. ‘A common tactic among the pros is to “take out” a sprinter,’ says Madison Genesis’s Chris Snook. ‘Basically, when said rider is directly behind you and is the last man, you let the wheel in front go and soft pedal, creating a gap between you and the rest of the break. The idea is that the rider will then have to try to make the jump to close back to the break while you can then sit on and enjoy the tow back, forcing them to use up a lot of energy.’

The components of speed

In the competitive world of cycling and its reputation for marginal gains, you can’t ignore the impact of gear. Drafting, it seems, can have an effect not just on your performance on the road, but in your choice of equipment. As per the size of the rider, following a larger frame with hefty frontal area creates a greater draft – as do the components, albeit a negligible one. ‘The depth of rim influences things, but only really for the lead rider because the second rider’s in his wake,’ says Blocken. ‘There have been a few studies on number of spokes and the rims, but the effect is a few tenths of a percentage.’

But, argues Paul Lew, director of technology and innovation for Reynolds wheels, that’s to ignore the fluctuating forces acting on a peloton, especially in a crosswind. ‘The front wheel in particular reacts differently depending on whether it’s going through clean air or turbulent air,’ Lew says. Clean air is in the slipstream; turbulent air is when you lose it.

‘Let’s say you have a 30mm deep rim and you’re right on the wheel in front of you,’ continues Lew. ‘The air is turbulent but you’re not feeling the effects of the crosswind because you’re in the draft. You don’t have a lot of force on your steering. And then maybe the situation changes – a small gap opens up, the wind changes, the echelon changes – and suddenly that air hits your wheel. If you’re riding a 30mm, you don’t feel it very much. But if you’re riding a 50mm wheel, you go from being easy to handle to having your steering blasted. Riders want to go deep [to have deep rims] so we look at how stable that wheel is in transition.’ It’s interesting to note that wheel manufacturers have to consider not just how a deep rim will cope with side winds, but with the constantly changing air turbulence that comes with riding in close proximity to other cyclists.

On the pro circuit, round the velodrome or with your mates, drafting should be an integral part of your cycling armoury. After all, now that Dr Fuentes is off to prison, where else can you gain extra speed for no more effort?

Paceline pointers

The dos and don’ts of riding in a line…

‘A common novice’s mistake is to spend too long at the front, which causes muscles to fatigue unduly and slows the whole group. For a paceline that’s travelling at 20mph, keep your pulls to a max of 15 seconds before peeling off. Once the speed increases beyond 25mph, pulls should be no longer than eight seconds.’

John Morgan, head of cycling-coach.co.uk

‘Always look at riders ahead for signals to pull out, slow down, do your stint at the front or watch out for a pothole. When you’re on the front you are the eyes of the riders behind so pass on information with the right arm signals and calls.’

Dan Bennett, founder of Progressive Cycling Coaching 

‘Keep slightly to one side of the wheel in front, but always still slightly behind. If things go wrong you can steer to the side. Better still, ride just off the wheel
[ie leave a bigger gap, say 75cm]. You still enjoy the aero benefits but it’s less scary.’

Joe Beer, time-trial and sportive coach

‘Change the side on which you drop back according to the wind direction, so that the riders coming through to do their turn on the front are sheltered as much as possible until the last moment.’

John cobb, aerodynamicist and owner of cobb cycling

Follow that car!

When drafting is taken to its logical conclusion, there are some crazy records to be broken

In 1899, Charlie ‘Mile A Minute’ Murphy drafted a steam locomotive with two miles of pine track placed above the sleepers. A hood was fitted over the back of the caboose, extending around Murphy and his bicycle to shield him on both sides. His first attempt was limited by the slowness of the train. The railway company came back a couple of weeks later with a more powerful engine. Murphy subsequently covered one mile in 57.8 seconds, hence his rather factual nickname.

Dutch cyclist Fred Rompelberg is a man who simply loves chasing heavy engines, amassing 11 world records in his pro career. But Rompelberg’s efforts in 1995 were his pièce de résistance. On the salt flats near Salt Lake City in Utah, and following a dragster with an extra-large fairing protruding from its rear, he pedalled to a record speed that stands to this day – 268.83kmh (167.04mph).

Back in 1928, Leon Vanderstuyft was the talk of the cycling world. Huge crowds flocked to the famous Montlhery circuit just south of Paris for the Belgian’s attempt at regaining his coveted motor-paced world record. They weren’t to be disappointed. Following a three-litre V-twin Anzani-powered pace bike, Vanderstuyft cycled 76 miles and 504 yards (122.77km) in one hour. To put that in perspective, Chris Boardman’s hour record without drafting was a comparatively meagre 56.37km

draf1

Comments