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Cycling science: Is it worth drafting uphill?

Laura Greenhaugh
15 Aug 2017

When the road gets steep, do the aerodynamic effects of drafting disappear? Cyclist investigates...

It’s not hard to imagine the scenario: you’re halfway up Alpe d’Huez, trying desperately to stay in touch with the wheel in front. Lungs burning, heart rate climbing, your body is screaming out for respite.

So is it really worth all this effort to hang on when drafting during a climb might achieve only the tiniest of benefits? 

The energy-saving effects of drafting on the flat are well known, but the quantitative benefits of being behind riders on a climb are relatively under-reported.

‘There’s not a lot of objective data on this, because the effects of drafting are hard to study,’ explains Damon Rinard, senior cycling technologist at Cervélo.

‘We have wind-tunnels to study aero drag on individual riders, and velodrome studies to measure drafting in a team pursuit – but applying that to the mountains is hard.’

Back to basics

Time to go back to basics. The bottom line is that aerodynamic benefits are dependent on speed – which rapidly decreases when you go uphill.

‘In cycling, the three forces to be overcome are air resistance, rolling resistance and gravity,’ explains Matt Williams, aerodynamics specialist at McLaren Applied Technologies.

‘On flat ground, all the power goes into overcoming drag and rolling resistance – but as you start to go uphill, the force of weight that is resisting motion goes up quite quickly,’ he says.

‘For a given effort you go slower, because you’re using more of that effort to overcome gravity, and less into actually going forwards.’ 

And as speed decreases, so does air resistance. This can be expressed as: Fd = ½ rv2CdA (where r = air density, v = velocity, Cd = coefficient of drag and A = frontal area), meaning the relationship between speed and drag is exponential.

‘Drag force is proportional to velocity squared, so the force changes a lot with any change in speed,’ explains Rinard.

‘At typical climbing speeds of 15 to 20kmh, wind resistance has already dropped significantly, and at around 12kmh, that’s the point around which wind resistance is roughly equivalent to the rolling resistance of the tyres.’

All this means there is much less scope for saving energy by slipstreaming up a hill, because the force against the rider is so much smaller.

‘Your velocity falls quite quickly, so the aero benefits fall,’ says Williams. 

Totting it up

So what energy savings are available on the hills? ‘On the flat, you might be using 300 watts to overcome aerodynamic forces, for example – and if you save a third of that through drafting, that’s 100 watts less,’ says Williams.

But on a 6% gradient, as much as 80% of energy might be employed to resist gravity, with only 10% against air resistance.

‘If you’re only using 30 watts to overcome aerodynamic drag, even if you still save a third of that you’re only saving 10 watts.' 

In fact, the real savings could be even smaller. ‘Putting numbers on it is the hard part,’ says Rinard.

‘It’s been measured that drafting reduces power required by 30% to 50%, but that’s at normal, flat-ground speeds.

For climbing the power is higher and the speeds are lower, so the savings in terms of drag force are lower as well – but it’s not easy to quantify.’ 

 Nonetheless, David Swain, professor in exercise science at Old Dominion University in Virginia, says there is always likely to be some effect of drafting, however minute.

‘Runners clearly benefit at 15mph, since the four-minute mile was first broken with the help of drafting, and they appear to benefit even at marathon pace,’ he says.

‘There will be a reduced energy cost at climbing speeds in cycling, as long as the hill isn’t so steep as to force a walking pace.’

Putting out power

And the more total power you can put into the system, or the lighter you are, the greater the benefits – explaining why pros stick so close to one another in the mountains.

‘For most of us going up an 8% climb, we’re only going at 8 or 9kmh – and at that speed the advantage is negligible,’ says Tony Purnell, an engineering professor at the University of Cambridge and head of technical development at British Cycling.

‘But if you’re going up like Vicenzo Nibali, it’s a different circumstance. At 20kmh, it’s a tangible technical advantage.’

‘As the gradient gets up beyond 5% or 6%, what they will save is small, but it’s the kind of amount that bike manufacturers would be looking to save in equipment design – so it's by no means insignificant,’ adds Williams.

‘Within the context of a three-week stage race, professional riders are looking for every opportunity to save energy.’ 

Making gains

Pro teams are now looking to better quantify the benefits of drafting by measuring the effective wind speed across the body in real time, which can be hard to predict in the mountains.

‘What’s needed, and has only just become available, are air digital velocity and wind direction sensors on the bike,’ says Rinard.

‘Mavic has a wind sensor that they’re using, and we have an instrument called the Aerostick that measures apparent air speed velocity, wind direction, and rider trajectory, power output and velocity, and records that second by second.

‘But this technology is relatively new in the last three years or so, and most of the data from it is still in private hands.’ 

It certainly seems that drafting is the way forward – and that’s without considering the added tactical and psychological benefits of being able to launch an attack from behind, or letting your teammates do the pace setting. 

‘Having a friendly wheel can make a pivotal difference psychologically,’ says Purnell. ‘And climbs are rarely a steady gradient – so you want to be on somebody’s wheel for the flatter bits, where there really is a significant advantage.’

For us mere mortals, there might be one consideration. ‘You’ve got to know what you’re capable of, because drafting means sticking at the same speed as the rider in front of you,’ says Purnell.

Stick to your own pace

‘People often say, “Climb at your own pace,” and this makes sense. You might get an energy advantage through drafting, but if it’s too fast you’ll go into the red and blow.’ 

To put it in numbers, at 20% gradient an average 70kg rider putting out 300 watts will be going just over 6kmh, by which point air resistance is negligible and drafting is of little concern compared to simply maintaining forward movement.

But Rinard takes the racer’s view: ‘It’s always worth drafting,’ he concludes. ‘And if you’re not drafting, there’d better be a reason why not. If there’s a finish line to sprint to, or an attack to be made, then these are all valid reasons.

‘But drafting helps, unless you have a reason not to. Even if it’s a small amount, it’s free, so why not take it?’

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