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by Matthew de Vroet
October 3, 2017
Photography by Kristof Ramon
We’ve all done it. You’re riding along when suddenly a faster rider overtakes you and you immediately jump across to sit on their wheel. You’re swiftly up to speed and find yourself riding five kilometres an hour faster, all while feeling more comfortable and putting out less power.
It’s no secret that drafting works, but how much energy do we really save from it? Do the gains from drafting stop when you’re riding uphill? Is is just those in the middle of the bunch that save energy, or can those on the front benefit as well? These are some of the questions that have been asked at mid-ride coffee stops for decades now.
There are three forces we need to overcome when riding a bike: the effect of gravity, rolling resistance, and wind resistance. When we’re riding at 30km/h on a flat road, wind resistance accounts for approximately 80% of the total resistive force. When we increase our speed to 50km/h, wind resistance increases to a massive 94%. Quite simply, the greater the speed, the greater the drag1. And the greater the drag, the greater the benefit to riding in someone’s slipstream.
So how much energy can you save from drafting? Interestingly, there seems to be little consensus among researchers that have investigated this topic. Studies have shown drag reductions of between 27% and 50% for riders that are drafting, with the exact reduction depending on a number of variables — the size and on-the-bike position of the rider in front, likewise with the rider drafting, the distance from the wheel in front, the direction and strength of the wind, and more.
In short, drafting provides a considerable effect, but its magnitude depends on many factors. But by taking a closer look at some of the studies in this space, we can understand how much we benefit from sitting in the slipstream in different situations and where drafting is most effective.
According to a 1979 study by Chester R. Kyle, you should be able to reduce the drag you experience by up to 44% when riding in a group, assuming you’ve positioned yourself correctly in the bunch.
So where’s the correct position? Depending on the size of the group, it’s usually best to be right in the middle of the bunch with riders in front, behind and to either side of you. This means you are protected from the wind from each angle. You don’t want to be right at the back — while it may seem like you gain the most draft there, Chester found that with gaps opening up between the riders ahead, you’re not sheltering from the wind as best you could.
It’s worth noting that Kyle’s findings were based on perfect alignment. He discovered that, depending upon the amount of overlap and side spacing between riders, the reduction of wind resistance could be anywhere from 0 to 30%.
How close to the wheel in front should you ride in order to get the best draft? Again, there’s doesn’t seem to be one clear answer. The graph below shows the different results various researchers have found when trying to determine the optimal distance from the wheel in front:
Graph from Belloli 2016.
Kyle’s 1979 study suggests that it’s best to be as close to the wheel in front as possible, and that the benefits of drafting decrease as you move further away. Other, more recent studies came to different conclusions.
Nathan Barry’s 2014 paper suggests that it’s better to be 10cm from the wheel in front than it is to be even closer. Marco Belloli’s 2016 study (labelled as “present study” in the graph above) used wind tunnel testing to show that the drafting effect is strongest up close, but that it’s not a linear relationship between distance and drag reduction. Rather, Belloli found it’s better to be roughly 35cm from the wheel in front than it is to be 20cm. Note, though, that drag is still being reduced by 38% when the trailing rider is a metre away from the rider in front — a significant reduction.
From these studies, it’s difficult to conclude what the optimal distance is to keep between your front wheel and the back wheel of the rider ahead. That said, the table above suggests that as long as you are within 40cm of the leading rider you should be getting a large benefit.
It’s obvious to anyone who’s ridden in a group just how much assistance you get by sitting behind those at the front. But it turns out that riders leading a bunch — the ones pushing into the wind — also get an advantage by being part of a group.
In their 2007 paper Andy G. Edwards and William C. Byrnes found a reduction in drag of 1.63% for lead riders, due to a “pushing” effect from the riders behind. This seems to be because the drafting rider fills the space behind the rider ahead, ensuring the air flows past both riders, rather than allowing turbulent airflow behind the leading rider.
Obviously the rider in front still needs to expand much more energy than the cyclist who is following. But it is worth noting that there are gains to be had while sitting on the front of a group, compared with riding alone.
Bert Blocken came to a similar conclusion in his 2013 study on the aerodynamic drag of two drafting cyclists. Blocken wanted to find the optimal strategy for team time trials and how all riders could be used to full effect. He found that in a four-rider paceline, the lead rider’s drag is reduced by about 2 to 3% compared to if they were riding solo. The second rider in line experiences a reduction of about 27% while the third and fourth riders see drag reductions of approximately 35%.
So wheel-suckers might not be such a horrible thing after all. Whether you are riding a team time trial or simply on your commute home from work, a rider sitting behind you could actually help you ride faster, even if it doesn’t feel much easier.
Tune in to a mountain stage of a Grand Tour and you’ll see the top teams arranging themselves in a line as they ride uphill. But is there even a benefit to drafting when the road tilts up?
As mentioned earlier, the faster you ride, the greater the drag you’ll experience. When you ride uphill, your speed drops and so does the wind resistance you face. Meanwhile, the impact of gravity is what you’ll spend the bulk of your energy overcoming, not wind resistance.
Indeed, as Matt Williams, an aerodynamics specialist at McLaren Applied Technologies, told Cyclist, up to 80% of a rider’s energy might be used to fight gravity on a 6% climb, compared to roughly 10% to fight wind resistance. Compare that with the 80% wind resistance contributes to the total resistive force acting against a rider on a flat road at 30km/h.
Because wind resistance isn’t as much of a factor uphill as it is on flat roads, drafting provides less of a benefit. This is why you don’t see riders climbing in the drops very often or using specific aero frames for large mountain stages. They need to be as light as possible to overcome the effect of gravity, rather than trying to get aerodynamic.
It’s clear to anyone that’s ridden in a group that there’s less advantage to be had while drafting uphill than on the flat, but it’s difficult to quantify that difference.
However, in a 1998 paper entitled “Cycling Uphill and Downhill” David P. Swain wrote: “At very slow speeds (on the order of 16 km/h or less) air resistance is negligible, and drafting becomes nearly meaningless.”
So why do the pros still draft each other uphill? For a start, the pros will climb at well in excess of 16km/h on all but the very steepest grades. Even if they don’t save as much energy as they would by drafting on the flat, they still save some energy by sitting in the wheels. And in bike racing, saving energy is the name of the game, particularly in the case of three-week Grand Tours.
And then there are the psychological benefits. Having a wheel to follow up a climb can help you pace yourself, or simply provide extra motivation to hold a faster pace than you might otherwise. This is something we can all benefit from, not just those who can climb mountains at well in excess of 20km/h.
As we’ve seen, the research is far from conclusive when it comes to quantifying the benefits of drafting. This is due to a number of factors, not least the fact that drag reduction changes with the size and on-the-bike position of the rider ahead and the rider drafting, plus the environmental conditions. Regardless, we all know that drafting works and one thing the studies can agree on is that you can save enormous amounts of energy by drafting correctly.
1. The power required to overcome wind resistance is actually proportional to the rider’s speed cubed (assuming there is no wind).