Drafting

[jonathanhayes]

Watching on TV, I hear the commentators often refer to one car's "drafting" a car directly ahead of it.

That leads to several questions:
Assuming the cars are traveling approx 300 KPH, how far behind can a F1 car be and still get a drafting tow?

Second, if drafting is done correctly, both cars should get an increase in speed; yet that doesn't seem to be the case. Question: is there really any drafting happening?

Third, if the following car is not within drafting distance, then it's plowing through dirty air; does it take more or less energy to penetrate dirty versus clean air?

Thanks

Jonathan hayes

[Sawtooth-spike]

It only helps the car behind as effectively there is a big hole in the air behind the car in front, thus less drag, less drag means more speed, however u dont wanna be in that pocket behind a car in a highdown force corner, cus the less air you have going over ur wings the less downforce you have.

becasue modern f1 cars are so Aero heavy, all the little winglets and thing need nice smooth (for the lack of a better word) air to work really well, so when you follow another car the air is all messed up so the little winglets and stuff dont work as well, so u dont go as around corners.

In other racing series like nascar, the cars are much simpler in terms of aero so get in the draft, tow, or slipsteam and the you can go faster for less effort.

[Tom]

I don't know the answer to part 1, but it will depend alot on individual car setup, namely downforce. Here drafting is known as slipstreaming.

2. The lead car will not get an increase in speed, the following car should. We don't see enough drafting in F1 (hence the CDG wing) but down the Indy straight you'll see a fair amount. F1 cars are now being designed deliberatly to stop people drafting so as there's less chance of them being overtaken, the FIA are keen to change this.

3. It takes much less energy to go in the clean air behind a car then in the dirty (turbulent) stuff.
The way this works is, in laymans terms, the lead vehicle will force the air around it and will therefore leave a hole in the air behind it (a vacum) for a few meters. Because its a vacum aerodynamic parts have a far reduced effect creating less drag and allowing a higher top speed, as well as the vacum actually 'sucking' the following car into the back of the leading car. If two cars are drafting one another there will be an even bigger hole in the air and the drag behind them will be reduced further...
If you are in the turbulent air you're meeting more air resistance than usual so your speed will decrease, also turbulent air will create unwanted vortex in the wrong places so the air won't flow over the car as well as it should and as a result downforce might be affected either way.
Drafting uses much less fuel aswell although it creates less downforce when cornering so its harder to keep the car on the road.

[C/\D]

If my mind was'nt playing a trick on me, BAR had had an research on this topic in 2004.
They were testing the slipsream effects on the F1 cars.
The test has been done in 320 km/h by BAR.
firstly the gap between two f1 cars was equaled to a half of a f1 car's lenght.
Then they made the gap longer periodicaly. In the end they reached 8 F1 cars' lenght.
The CFD datas were examined by BAR engineers.

When the gap decreases, drag and downforce decreases too.
In 8 f1 cars' lenght distance. The following car has 13 percent of drag decreased also it has 18 percent of downforce decreased.
When the distance was only a half f1 car's lenght. The following car has 28 percent of drag decreased, and 45 percent of downforce decreased.
The interesting part is here. BAR expressed that when the distance was only a half of a f1 car's length, the leading car has 5 percent of downforce decreased, also it has 4 percent of drag decreased. Very interesting, also in some single seater series when the teammates are in double formation, they use each other to increase velocity.

[mini696]

The interesting part is here. BAR expressed that when the distance was only a half of a f1 car's length, the leading car has 5 percent of downforce decreased, also it has 4 percent of drag decreased.

The leading car loses downforce and drag because the two cars 'share' the drag between them. When the air comes off the rear wing, it creates a massive turbulent area, if there is a nother car behind it (especially in Nascar) this turbulent area is decreased, therefore there is less drag and less downforce for both cars.

In Nascar terms when drafting with two cars, the leading car loses rear downforce, and the trailing car loses it in the front. If there are three cars drafting, the leading one loses rear, the rear one loses front, but the middle car loses downforce at both ends. This is why bump drafting into certain corners is severly frowned apon.

Very interesting, also in some single seater series when the teammates are in double formation, they use each other to increase velocity.

They do this in Indy quite often to get another couple of tenths in qualy. Not so much this year because the harder tyres made it much more difficult to follow in the final turn.

[Tp]

The car in front produces an upwash of air flow (i.e. from the diffuser), which reduces the downforce of the car behind (especially the front wing, as its close to stalling, any airflow not parallel to the ground, causes a drop in downforce - up to 30%!!)

[checkered]

Hi Jonathan, welcome

to the forum. The basic principle of drafting is fairly straightforward and one doesn't need a racecar - or a car at all, for that matter - to personally experience what a reduction in wind resistance means. It is not advisable, though, to go around tailgating people on public roads in the name of gathering empirical data! For your first question, the answer is manyfold but if you're willing to put an hour or two into reading a couple of technologically minded articles (I'll suggest a couple later), you'll propably already know more about the subject than 95% F1 fans in general care about.

In short, what's certain is that there's a clearly measurable and significant effect some 40m (roughly the same in yd) behind the leading car at 200mph (320kph) and drivers have claimed that they begin to feel the difference at around a 100 meters distance.

As to your second question, if the following car is close enough, yes, there is a slight advantage (0.4% at half a car's distance), but if the chasing car is four car lengths down, the leading car's L/D (lift to drag ratio, even if it's "negative lift") is adversely affected and it's a disadvantage. If the gap is eight car lengths or more, there's no meaningful effect on the leading car. So, drafting "done correctly" as far as the leading car is concerned, happens at very short distances (albeit I'm sure the driver would be happier if he hadn't a fellow competitor so close by ...) whereas the chasing driver will be happy to draft in a straight line and considerably less so in a high DF (downforce) curve where he would sorely need the front end grip that is missing.

I think you have slightly misunderstood the concept of "dirty air". When slipstreaming/drafting, you're always in "dirty" air i.e. in a flow affected by another competitor. It is called dirty since it's an unpredictable environment for the driver to be in - the aerodynamical balance and thus the grip changes. The flow and the downwash from the leading car isn't vertically or horizontally uniform and there are abrupt changes in the energy of the airflow that are evident only by the vehicle's interaction with vortices and such. So "dirty air", I think, isn't merely a question of aerodynamical force, but its nature.



Now, where I'm getting all this from? Here goes:

An ever so slightly outdated article about overtaking in F1, from grandprix.com, 1995:
Overtaking in Formula 1

Two articles on the (aforementioned) Advantage CFD study from 2004, first one from a Fluent paper:
Motor Sport Drives CFD Technology to a New Level
It's a PDF, 2.44 megs, you want to start reading from page 8, "Transient Manoeuvres"

The second one an article from Racecar Engineering (an excellent publication), again a PDF, 0.46 megs:
Draft dodging

Obviously there are also very talented contributors on F1Technical and as the issue of slipstreams, dirty air and overtaking is constantly discussed there are some excellent examples of lateral thinking to be found in the discussions, like from reca here:
F1 aerodynamics, drafting and dirty air, page 2

Not to mention how much effort Miqdad Ali (a.k.a. miqi23) put into investigating the merits of the CDG (Centerline Downwash Generating) wing, put forward by the FIA some time ago - and this goes very much into slipstreaming:
Aerodynamic study of the CDG concept
The FIA's new CDG wing

I think you'll find yourself entertained for a while just by going through those, for starters. I hope I got all the links correctly.

Edit: Obviously, the velocities must not be very great for drafting to work - just think about cycling and how the teams protect and aid their main contender with drafting, or how cyclists in general can co-operate by forming different "competitive units" - or how quickly a single cyclist that loses touch to his group drops behind. Here's a link (a bit off topic):

Science of cycling: Aerodynamics: page 2

The priciple is exactly the same, though. I came to think of this only because of Ciro's excellent link (below), the article about NASCAR "social sciences" is -definitely- worth reading and perhaps could also offer clues about how F1 could encourage a more "social" approach to racing in general as opposed to encouraging the very selfish one that is so prevalent today. But anyway, going back to cycling, apparently I needed a couple of dormant synapses reconnected ... after having a rather sudden and violent encounter with a car some years ago (a bike is no match, as if my direct experience would make that more obvious), I've found that certain negative connotations can cause an unconscious but eventually noticeable aversion to making certain associations.

[Ciro Pabón]

Drafting in NASCAR: Social Science at 190 mph on NASCAR's biggest Superspeedways

Drafting, which looks like tailgating or slipstreaming, occurs when a second car tucks closely behind the first, filling part of the vacuum. The car in front loses some of the drag at its rear. The second car still has a vacuum at its rear, but now has less air resistance in front. As a result, both cars quicken a bit — the two combined speed a few miles per hour faster than either can alone. This push-pull effect is stronger the closer the second car gets to the first. Indeed, the second may even touch and push the first in a tricky maneuver called "bump drafting." But slipstreaming so closely hinders airflow into the trailing car's radiator and can cause its engine to overheat. Most drafting calls for a half to a full car length between the two cars. (In contrast, open-wheel, motorcycle, and bicycle racers do not gain extra speed by drafting, because a trailing racer does not generate the forward aero push that stock cars do.)


This has two marvelous effects for Daytona-type racing. First, there is an unusual incentive to cooperate. So long as two racers stay together in a partnership, they can catch up to or pull away from equivalent cars that are not drafting, and keep pace with rivals who are already in line. But there is also an incentive to defect at some point — eventually the time comes for this opportunistic partnership to dissolve, as the second car aims to get around the first. And when this occurs the advantage usually goes to the second, because of the "slingshot" effect. If he can drop back a little, by a car length or two, and then gain speed back into the draft zone, perhaps as the cars descend off a high-banked turn onto a straight, he may use his tiny extra momentum of a few miles per hour to swing out and pass the car in front, which loses momentum as soon as the vacuum reappears behind it.

Famous races have been won this way. By now, it is a simple calculation. But it was "discovered" in 1960, at the second-ever Daytona 500. Junior Johnson reluctantly agreed to drive a Chevrolet, knowing it lacked speed and power compared to Plymouth entries. But he found that as long as he left the pits behind a Plymouth and drafted Plymouths on the track, he could stay with the front-runners. He won the race after the lead Plymouth spun out temporarily when it lost its rear window to the suction of the backdraft. In another famous case, near the end of the Firecracker 400 at Daytona in 1974, David Pearson knew for sure he had the more powerful engine as he led a two-car draft, ahead of Richard ("The King") Petty. But Pearson worried he would still fall prey to a slingshot. So entering the last lap he suddenly slowed and let Petty shoot past him by nearly two hundred yards, then used his car's extra power to regain speed back into the draft and slingshot by on the final turn, leaving Petty and everyone else awestruck and dumbfounded.

The "forward aero push" is the bow wave of the car following in the slipstream.

[pRo]

If you're really into this, you can have a play with this stuff yourself. All you need are wool strings and tape. (and preferably a (video)camera) Cut the wool strings to length of 2" or so and tape them all over the car. You need roughly 100 strings per car, more is better.

Then just go for a drive and see how the airflow affects the strings. Obviously they should all be "pointing" backwards on the hood, roof and sides etc. But the interesting areas are generally the front and rear, especially the rear. When you find some turbulence spots, you gain speed by managing to get rid of them. It's not easy and it won't be fast, but it'll be great fun and hopefully have a useful result too. Go on, try it!
(obnote! don't break local laws on public roads! )


About drafting, like someone already mentioned, the first car actually does get faster, if the other car is behind it. Also easy to see with this, just pay attention to the wools on the leading car when another car drives right behind it and you'll see why. The rear turbulances are reduced, which means the car in front needs less power to keep that speed, or the other way around, it can use the same power to go faster.

I was really surprised none of the teams used this in Indy qualifying, or did I just miss it?

[Ogami musashi]

Turbulent Air is named dirty because of the instabilities named by Checkered, but mostly is associated with drag in the case of vortices because when thoses vortices lose their energy they catch boundary layer and take away with them a part of it thus leading to lost in lift/downforce wich is associated with lower lift/drag ratio.

But Turbulent air can be stable enough and not produce, or i should say induce, drag in the case of vortices as long as they are stable and don't mix with boudary layers and or freestream.

Applied to F1, slipstreaming/drafting will always poses the problem of the downforce reduction wich represents so much time lost in turns so CDG/moveable aerodynamics will aim for the gold compromise, that is not having a L/D ratio decrease but an increase (same downforce, less drag, or more downforce for the same drag, thus having more power to apply) wich , if it works, will lead again to another supra complex variation of an effect (already complex in nature) that is used simply for years in many other sports, but that's the way the F1 is don't you think?


By the way as pointed out by Vasselon in the so called Magny cours technical preview (wich i call more of a Vasselon interview), the aeros are not thought by this person as being the responsible for difficulties of overtaking but rather the tyres!

In anyway i'm sceptical about the "all thoses aero appendices makes the air dirty", i think this is actually quite the contrary for all thoses littles things, they make it more efficient then stable, wich is not the case for some parts however (rear wings for example) but we'll need to see CFD results for that.

[ss_collins]

Can you draft in F1 today - yes there have been quite a few draft passes or blue bayous attempted this season. But I suspect a lot of drivers don't fully understand how to get the best out of drafting, as its rare to see F3/GP2 drivers understanding how it can be employed - F1 driver on occassion can be seen failing to make what should be an easy pass. I suggest you either watch a Formula Vee or Ford race on the silverstone national circuit or visit an oval.

[jonathanhayes]

Being not sure of what the appropriate protocol is for a Forum, I’ll just charge ahead.

First, thanks for all your responses.

While I’ve not yet had the opportunity to go through the various references some of you provided – I will within next day or so and thanks for them as well - , here are some reactions.

It seems that I was wrong in assuming that whatever draft occurs would so quickly diminish as the distance between the cars increases that there would be essentially no effect three or fours car lengths back. At the same time, I find it so surprising as to be hard to believe that the maximum effect is merely something like .4%. (Maybe I misunderstood the point here, and what was being suggested is that a decent draft can only be accomplished if the gap is less than half a car length.) That would mean an increase in speed from optimized drafting at 320 KPH of something like 1.4 KPH.

I also find it surprising that if, in fact, it takes more energy to penetrate dirty air, that cars follow directly in a car’s path when they are pretty far back. I’d think they’d want to steer clear of that dirty air effect until they were pretty close behind.

Finally, I’ll share with you an article I read perhaps a decade ago about looking at drafting in NASCAR as an exercise in game theory. As a non-NASCAR fan, I don’t know if the facts are still true. Drafting was really on a significant factor in certain (restrictor plate) races, but in those races, it was significant indeed. What happened was that each of the cars had a spotter positioned on the top of the stadium. Since the best way for one car to pass another was for the passer to link up (create a drafting duo) with a car directly following, and then the two conspirators could pull out and pass the leader, who bereft of a following draft partner would immediately lose some speed. The spotter’s job was to negotiate that pairing. What could happen, however, was that the second car would make a deal with the third car, and then as they were supposed to pull out as a two car drafting unit, the second car (the leader of the two car team) would feint and leave the third car out by itself, forcing it to fall back. The game theory question was how should drivers behave if and when they’ve been lied to. What I found most interesting and ironic is that this most American of sports, built on a foundation of “traditional American values” was, in fact, based on lying and exacting the appropriate level of retribution.

Thanks again

Jonathan Hayes

[m3_lover]

You have to taken into account that there is not a lot of long straights in F1 that cars can take fully advantage of a tow.

[Ray]

Finally, I’ll share with you an article I read perhaps a decade ago about looking at drafting in NASCAR as an exercise in game theory. As a non-NASCAR fan, I don’t know if the facts are still true. Drafting was really on a significant factor in certain (restrictor plate) races, but in those races, it was significant indeed. What happened was that each of the cars had a spotter positioned on the top of the stadium. Since the best way for one car to pass another was for the passer to link up (create a drafting duo) with a car directly following, and then the two conspirators could pull out and pass the leader, who bereft of a following draft partner would immediately lose some speed. The spotter’s job was to negotiate that pairing. What could happen, however, was that the second car would make a deal with the third car, and then as they were supposed to pull out as a two car drafting unit, the second car (the leader of the two car team) would feint and leave the third car out by itself, forcing it to fall back. The game theory question was how should drivers behave if and when they’ve been lied to. What I found most interesting and ironic is that this most American of sports, built on a foundation of “traditional American values” was, in fact, based on lying and exacting the appropriate level of retribution.

Thanks again

Jonathan Hayes

It's not that you are 'lying' to them. It's strategy. You always look for the advantage. Alot of times they will draft with a partner until close to the end of the race, and then it's every man for himself. Also they may find, from the drivers point of view, that his partner is not always the best car to draft with and change who they draft with. You can sometimes tell who really drafts well and who doesn't. It's best to find a guy you draft well with and stick with him, and sometimes you must leave a guy hanging to find the best one.

[checkered]

At the same time, I find it so surprising as to be hard to believe that the maximum effect is merely something like .4%. (Maybe I misunderstood the point here, and what was being suggested is that a decent draft can only be accomplished if the gap is less than half a car length.) That would mean an increase in speed from optimized drafting at 320 KPH of something like 1.4 KPH.

Perhaps I should've been clearer about the terms. The 0.4% "advantage" is for the leading car in a two F1 car formation 0.5 car lengths apart from each other. It actually indicates the amount of improvement in the aerodynamical efficiency i.e. L/D (lift to drag ratio). That is special since most of the time both cars lose L/D efficiency while in a meaningful drafting or slipstreaming proximity from each other, even if the trailing car can gain speed in relation to the leading car in terms of reduced drag. The leading car's overall DF (downforce, negative lift) at 0.5 car lengths apart is reduced more than what is gained in efficiency - 3.4% - but obviously drag is reduced even more since the trailing car separates the leading car from a portion of its wake.

(The figures, albeit they sound very "accurate", are also just indicative ... the study made the comparisons in 2004, but modelled the effect based on BAR's 1998 designs.)

At a half a car's length distance the trailing car loses a whopping 44.7% of its body downforce. Little surprise then, that even if drag is also reduced considerably, the L/D efficiency is also reduced by a significant 23.2% - thus making the car much harder to control.

I also find it surprising that if, in fact, it takes more energy to penetrate dirty air, that cars follow directly in a car’s path when they are pretty far back. I’d think they’d want to steer clear of that dirty air effect until they were pretty close behind.

It doesn't take more energy to penetrate "dirty air". Not by definition anyway. Generally speaking, there's quite naturally less pressure in the wake, that's why the trailing car loses drag, but also downforce. More downforce than drag is lost because of the "dirtiness" i.e. the aerodynamical nature (see Ogami Musashi's post) of the flow, energy preserved in the vortices left behind by the first car separates the airflow from the aerodynamical surfaces of the second car earlier than intended, or the flow in general doesn't meet those surfaces in an optimal attitude to begin with. Still, a skillful driver of the trailing car can seize an advantage of the potential gain in relative speed to the leading car.

But do read the articles, I'm certain that is much more efficient than me trying to paraphrase the content of those in my own imperfect way. Also, I hope that the true aero experts weighed in here, their insight could "cut to the chase" in much clearer and efficient terms.