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Drafting has been a highly effective strategy used in bicycle races since the dawn of the sport, with riders not only safely hiding behind other riders, but also getting the occasional tow from various motorcycles and cars in the race caravan. The energy savings are obvious when you’re tucked right behind something that’s blocking the wind for you, but what about when you’re further back, or even when a vehicle is relatively close behind you?
As it turns out, the interactions we’ve all been assuming to be innocuous may actually be anything but, and it’s probably time the UCI rethinks its rulebook and how races are managed.
The unexpected relationship between urban canyons and drafting in races
If you’ve spent any time in a bigger city dotted with skyscrapers and high-rises, you’ve likely noticed that it’s often quite windy at street level. This is due to a phenomenon commonly described as an “urban canyon effect”, where the buildings occupy so much of the airspace that the incident breezes are forced into narrower opening — street-level — thus increasing the windspeed.
These are the sorts of things Dr. Bert Blocken, a research professor at KU Leuven and Eindhoven University of Technology, studies regularly using a combination of physical measurements and testing as well as highly complex computational fluid dynamics (CFD) models. Dr. Blocken’s research team has looked at things like airflow patterns at the cruise terminal in Rotterdam, The Netherlands, and how a “second skin” facade would affect wind comfort on the balconies at a high-rise building in Antwerp, Belgium.
“Strictly speaking, a cyclist, or football, or building, they are all in the category of non-streamlined objects that are exposed to flows below the speed of sound, and in aerodynamics, that is actually quite a well-defined category and you see much of the same phenomena,” Dr. Blocken explained. “So the misunderstandings that we sometimes try to clear up in cycling sometimes are very similar to the misunderstandings that occur in building design, and so there’s quite a strong link between those two fields even though, at first glance, you would think there’s no correlation.”
As such, Dr. Blocken has been looking at all sorts of cycling aerodynamics scenarios for the last few years, and the professional racing world has taken notice. Dr. Blocken’s group currently works directly with two professional teams, one of which is none other than Jumbo-Visma. And remember that CFD animation that went viral on Twitter back in April, showing how respiratory droplets travel through the air when exercising outdoors with others? Yep, that was him.
(1/2) #COVID19 #SocialDistancing when walking/running/#cycling nearby non-family members. Summary in @hbvl & @HLN_BE. Message: keep exercising but stay outside slipstreams. Movie below: for walking.
Research @TUeindhoven @LeuvenU @ansys #CFD #aerodynamics #COVID #CoronaCrisis pic.twitter.com/LFA7EkU5xG
— Bert Blocken (@realBertBlocken) April 7, 2020
The space between us
Most recently, Dr. Blocken has turned his attention toward the effectiveness of drafting in a variety of different race scenarios, and at a variety of distances. Many of the findings aren’t terribly surprising in terms of subjective effects, but Dr. Blocken wanted to finally put some numbers to the various situations.
When a single rider is situated directly behind one other rider, that rider expends 35.6% less energy overcoming aerodynamic drag to maintain the same speed. When it’s a nine-person line in a team time trial, though, the ninth person in the train enjoys a 58% benefit.
In a big group, the savings are even more dramatically amplified.
If you’re toward the front of a large peloton — but still behind fifteen or so other riders that are forming the spearhead of the group — you get a monumental 85% benefit in terms of aerodynamic drag. And if you’re at the rear of that group? You’re quite literally just sitting in, having to only work 5% as hard as you would if you were at the front.
Now, say you’re trying to initiate a breakaway, and you manage to hide behind a camera moto carrying two people and a variety of gear. If you’re directly behind it, you get a 75% energy savings in terms of aerodynamic drag. And if it’s a team car (with a roof full of bikes on top) towing you back up to the field after a mechanical? The only surprise here is that the energy savings is about the same as if you’re behind the moto.
Here’s where things get really interesting, though.
We’ve all seen riders getting a pull from race motorcycles and other caravan vehicles, and while the aerodynamic gains are clearly pretty massive, they’re also usually pretty brief. And extended tows are thankfully rarer still. But according to Dr. Blocken’s analyses, riders still get far more benefit than previously assumed when they’re nowhere near those vehicles.
That same camera moto that provided a 75% aerodynamic benefit when it was right in front of the rider? It turns out that it still provides a 36% savings when there are 5 m of separation. At 10 m, it remains a substantial 23%. And at 20 m? That seems like plenty of daylight, but the rider behind nevertheless gets a 15% helping hand. At 40 m — nearly half the length of a football field! — the wake effect keeps tailing off, but Dr. Blocken’s calculations say it continues to linger at 10%.
Just as before, the numbers aren’t really all that different with a much larger team car, either.
“This slipstream is a very persistent thing, and it extends even further than 50 m beyond a motorcycle or a car,” said Dr. Blocken. “This is actually something that is known from building aerodynamics. If you have a high-rise building in a city, you still find effects of this building downstream for about 15 to 20 times the height of this building. This is an enormous distance, and it’s more or less the same as what we see in cycling.”
Push and pull
Ready to really have your mind blown?
This won’t be a surprise from fans of NASCAR racing, but riders also get aerodynamic benefits when there’s a vehicle behind them. Granted, they’re not nearly as big, but a moto following a single rider at a distance of 1 m saves that rider 3.8% of aerodynamic drag. At 2 m, it’s 1.7%. At 10 m? It’s a tiny — but non-zero — 0.1% at that point. And while the benefits of a team car are remarkably similar to a moto when the rider is behind, the car has a bigger effect when the rider is in front: 0.2% at 10 m, or double that of the camera moto.
Keep in mind, too, that Blocken’s simulations account for a team car, but not the pile of bikes that usually reside up top, which will amplify the numbers further still.
“We based the car model on a typical team car, but we didn’t put anything on the roof,” he said. “As soon as you put stuff on the roof, you’ll see that percentage [savings] will slightly rise, but not that much.”
But how can a vehicle following a rider possibly reduce the amount of aerodynamic drag that rider experiences, anyway? After all, the conventional understanding is that drafting only benefits the person behind, not in front, right?
“We started these drafting studies back in 2012, knowing from the fluid flow equations that if you have an object that moves in calm weather — whether it’s a cyclist or car or whatever — this object is not only disturbing and influencing the flow downstream of it, but also upstream. There’s always an upstream effect below the speed of sound, but this effect is small and that’s why people generally don’t think about this intuitively. But it is true that if you are driving a car behind a cyclist, this car is actually pushing the air in front of the car partly forward, and that creates a kind of overpressure bubble. When the cyclist is suffering drag by suction on the cyclist’s back, and the suction area comes close to this overpressure bubble, they partly merge, and the suction on the rider becomes less. So you are actually literally pushing the cyclist in the back.”
This effect isn’t just limited to riders and vehicles, either; Dr. Blocken says it applies to two riders, too.
“The front cyclist can get 2-2.5% drag reduction,” he said. “[That’s] very small, of course, but it is there. More scientifically interesting was showing what happens when you’re riding in front of a motorcycle or a car. With a motorcycle very close, it can go up to 8 or 9%, and with a car, very close, it can go up to 13%. Of course, a motorcycle is free to move in the peloton, and that can be an issue, but the car needs to stay 10 m behind, for example, a time trial cyclist.”
Stacking the deck
Interestingly, Dr. Blocken says the percentage savings don’t change much with different speeds, and it’s perhaps worth noting that the simulations were all run in calm conditions. However, Dr. Blocken says that crosswinds don’t actually have much influence on these analyses as you might think.
“Certainly, cross-wind is important on some days, but counting all days in a season, in the majority of days, cross-wind is not important or not very important,” he explained. “For cross-wind to be important, the cross-wind should be at least a large fraction of the riding speed, which is already quite high in pro cycling. The importance of cross-wind is often overrated.”
Either way, the sorts of situations Dr. Blocken has outlined here occur regularly and frequently in modern professional racing, which suggests that more than a few races have been altered by the presence of support vehicles mingling amongst the riders.
“What we actually did on purpose is not looking into races and trying to make calculations because it’s very difficult to reproduce what happened on a single day, and we also don’t want to question any rider’s victory because, sooner or later, every rider in the peloton has this benefit, so it’s not one above the other,” he said. “But for sure, these benefits are so large that we didn’t only do simulations; we also tested in the wind tunnel, always to make sure we had a double backup of what we are about to publish. In some cases, we also tested in the field, and you indeed always find similar numbers. So these numbers are there, and I think people inside the professional cycling peloton know very well, and the rider, of course, tries to position him or herself in the wake of a motorcycle — I would do exactly the same thing! But there have also been team managers that have expressed their disappointment about some riders benefiting from slipstreams and motorcycles and cars, and they have a point.”
If you take Dr. Blocken’s findings at face value, then there’s ample evidence to support the notion that race vehicles are — and have been — providing unintended benefits to surrounding riders to the point where the results of these races are potentially being influenced. However, what should be done about it? As it turns out, Dr. Blocken has some thoughts on that as well.
“Because this is allowed by the UCI, the motorcycle can follow very close to the rider,” he explained. “The team car has to stay 10 m behind the rider, for example in a time trial, but that’s not strictly enforced. Even at 10 m behind, there is a substantial aerodynamic benefit that, in some cases, can be decisive. That’s why we, a long time ago, contacted the UCI to suggest that they change this 10-meter rule, or first to enforce strictly the rule they impose, because this 10-meter rule is not strictly enforced. We see plenty of team cars in time trials driving 3 or 5 m behind riders. We also suggested they change this 10 m to 20 or 30 m, so you do not have this aerodynamic effect anymore. But we didn’t get a positive response on that — well, we didn’t get any response on that and that’s been a pity.”
Interestingly, Dr. Blocken doesn’t really blame the riders in all of this. In his view, racers will always instinctively jump on any advantage they can in a race situation, and over time, most contenders in his view will see similar amounts of semi-legal drafting opportunities.
Instead, his view is more aligned with the idea that the sport needs to remove those temptations from the equation altogether, and along those lines, Dr. Blocken is quite pragmatic about his suggestions, too. Getting rid of support vehicles altogether isn’t currently realistic, for example, nor is the notion of using overhead drones instead of on-the-ground camera motos. Certain courses are inherently prone to these sorts of rider-vehicle interactions, too, in particular ones that are run in very tight quarters.
“Sometimes it’s impossible to avoid [riding behind a motorcycle]. If you look at Tour of Flanders or Paris-Roubaix, the roads are so narrow that the motorcycle with the best intentions can not avoid being in a straight line with the rider behind them. So it’s not our intention to blame [the moto drivers]; they also have a job to do. But the effects are there, and there’s no use trying to deny that.
Ultimately, Dr. Blocken feels that the solution is a mix of the UCI merely enforcing the rules it already has on hand, and making slight modifications to other ones.
“Some changes are very simple. If [the UCI] just change the 1 to a 3 [as in, 10 m to 30 m for follow-car distance], the problem is solved. That should not be too difficult. In terms of cyclists drafting behind motorcycles, I think that is an issue of quality of cameras, but given current technology of image stability in cameras, it should be possible to ride more than 30 m in front of a motorcycle. There have also been incidents where you’ve had three to four motorcycles actually — whether consciously or not, I’ll not pass judgment here — helping riders bridge the gap to others in certain stages. And for drafting behind motorcycles, you can also enforce distances there. Now, the rules are that when there’s a break of a given timespan between one group of cyclists and another, the motorcycle can get in between. But the rule doesn’t specify where this motorcycle can go, and can not go, so the motorcycle can get very close in front of the riders, and even when it’s just a few seconds, that can be a major benefit in bridging this gap, for example.”
Nevertheless, even Dr. Blocken admits that the UCI currently perhaps has bigger fish to fry given the rash of severe crashes and injuries that have sullied the sport in recent months. Ensuring race fairness is one thing, but rider safety is still more important.
“I think we first have to take care of safety first and foremost after what we’ve seen this past year, and then move toward this topic.”