Why Two and Not Three Cars in a Draft?

The most talked-about feature of the racing at the ‘new’ Daytona is the two-car hookup.  Just in time for Valentine’s Day, drivers are finding that the term ‘drafting partner’ is more accurate than ever before.  Why two and not three-, four- and larger packs that used to be characteristic of Daytona?

Drafting 101

Anytime you move forward, you are working against something.  To walk through a swimming pool, you have to push  water molecules out of your way.  To drive through air, you have to push the air molecules out of the way.  The faster you go, the more air molecules you have to push out of the way in a given time.

Aerodynamic Forces on Cars

I’m going to focus on just the forces acting along the length of the car, ignoring sideforces.  The key to my drawings is that the length of the arrows and their color indicates speed.  Long green arrows indicate fast moving air, while red short arrows indicate slow moving, denser air.   Some air gets under the car, while most of it goes up and around.

We are interested in two primary features:  The front of the car acts like a wedge, pushing air out of the car’s way.  The air molecules resist this motion, creating a force that pushes in the direction opposite the car’s motion.  As the air passes over the car, it becomes turbulent at the back end, creating a partial vacuum at the rear of the car.  The physical phenomena at the front and the rear of the car are different, but they have the same effect:  they slow the car down.  We can get rid of the little arrows and just represent the force of the air as arrows pushing against the front of the car and pulling backward on the rear of the car.

If the two cars are far apart, each car experiences forces on the front and the rear of the car.  When they get close enough to each other, they appear as essentially a single object.  The trailing car is traveling in the aerodynamic shadow of the first car, so it doesn’t get the huge blast of air on its hood.  The trailing car prevents the air from getting as turbulent at the rear of the first car, so the force sucking back the first car is reduced.  (To learn more, or at least see much better drawings than mine, see the Science of SPEED video.)

Drafting 102

Every television program explains the very basic aspects of drafting, but we need to go a little deeper to understand what’s different now.

The most important change in the car has been the improved match up between the rear and front bumpers with the new car.  To get the maximum benefit from drafting, you really want the two objects to look like one, which means that they need to be as closely matched as possible.   Compare the  diagrams to the right.  In the top diagram, two different shaped objects will have turbulence between them because of the height difference.  (If the heights were reversed, there would be extra front drag.)

The second diagram shows two shapes that are the same size, but not very close to each other.  They are so far apart that both experience the front drag and the rear turbulence.

The lowest diagram shows two objects of the same size fit right up against each other.  The air travels over the two objects as if they were one.  The better the back end of the first car and the front end of the rear car fit, the more of an advantage you will get from drafting.  I apologize for my terribly drawing.  Graphics has never been one of my strong points.

With the old car, cars could usually add 5-10 mph by drafting.  We’re seeing much larger increases now – qualifying speeds are running 185 mph, while we’re seeing 205+ mph in the draft.  This tells me that the cars fit together aerodynamically significantly better than the old cars did.

Why the Two-Car Draft Works Better than Before

The new car was introduced in 2007 and although the splitter has changed, that’s likely not the big effect.  Drivers report that the repaving has really changed the character of the track.  It’s got more grip, but the biggest effect is probably the smoothness.  The key to good drafting is maintaining the relative positions of the two cars:  they have to be close.   Now the third dimension becomes important.  The figure at left is meant to be a top view of two cars driving to the right, so we’re looking at the path of air around the sides of the cars.

On the top figure, the two cars are in perfect alignment, so the air can flow past them smoothly.  If the trailing car stays the same distance behind the leading car, but slips slightly to the right, you’ve introduced edges.  The top part of the trailing car (in red) now is having to push air aside.  On the right sides of the cars, the misalignment of the trailing car means that there is a rear edge, and that means turbulence.

The old Daytona was bumpy.  Those bumps made the cars move up and down relative to each other (which would decrease drafting effects).  The bumpiness also made it harder for the drivers to control the cars, which made it more difficult to keep the cars aligned and close to each other.  The new, smoother track seems to allow the drivers to keep the cars tucked up.  The pull of the draft is so significant that we’re hearing drivers say that they have to ride the brakes.  This is mostly unheard of -usually, crew chiefs have to remind the drivers to pump the brakes before hitting pit road because the brakes get very cold since the only place they were used was on pit road.  Jaime McMurray had a brake rotor fail during practice and trashed his primary car when he blew a tire running over a piece of the broken rotor.  That’s a surprising thing to happen at Daytona.

Hot Engines

One piece of evidence supporting the hypothesis that the cars are staying together better is rising engine temperatures.  Air hitting the front of the car does produce drag; however, it also provides the air that goes into the car and cools the water in the radiator.  If you draft too long, the trailing car’s engine starts to overheat.  If you move to the side to try to expose the intake vents, you increase the drag and decrease the effectiveness of drafting.

One rumor is that NASCAR is going to require pop-off valves that would decrease the maximum temperatures the engines could reach.  (How that works is a separate article.)  This would decrease how long two cars could draft before they would have to separate or switch positions so that their engines didn’t overheat.

Why Two and Not Three

Go get three oranges from the kitchen.  Try to juggle two of them.  Not super easy, but not impossible.  Now juggle three.

The reason we’re seeing two-car drafts and not three is that it is very hard just to keep two cars in position with each other without hitting each other or overheating.  You’re asking the drivers to keep their minds focused on a lot of things, all while driving 200+ mph.

When two cars hook up, they take off.  A third car would have to be right there in position, ready to latch on. All three drivers would have to focus on keeping the pack together.  That’s far different than the old version of drafting, where becoming and staying part of the pack was easy… and fast.  It’s additionally complicated because the old version drafting didn’t require the trailing driver to use his brakes.  We’re seeing a lot more sudden drafting breakups as drivers realize they are overheating.  Do you want to try to get precisely positioned behind someone at 200 mph who is dragging his brakes?  The probability of getting two things to function together precisely is low.  Add a third and it becomes very, very difficult to do.

The Fix?

Speeds reached 206 mph during the Shootout, which makes aerodynamicists nervous because a sideways racecar going 200+ mph has a strong proclivity to unexpectedly start doing an airplane impersonation.  The usual head-first, tail-last position is just fine at high speeds – there is no reason that a stockcar can’t race at 230 mph or more; however, if the car gets turned sideways at that speed, it can become airborne, even when its roof flaps deploy.  There’s not a magic “take-off” speed below which it is safe because it’s a combination of the speed and the angle the car makes with the direction it is traveling.  We would be fine racing at 210 mph, provided that no one gets turned sideways.  The consequences are uncomfortably large if a car does get airborne. Everyone remembers what happened when Carl Edwards got airborne at Talladega and no one wants to take a chance on that happening next Sunday.  Most prognosticators are predicting that NASCAR will make a change after qualifying.

One quick fix (which has been used before) is to decrease the restrictor-plate size.  This probably isn’t practical because the change in size to compensate for the higher speeds would have to be larger than NASCAR would prefer to make.  The engines are tuned to work with a specific plate size, and changing the plate significantly could disproportionately affect one engine shop relative to others.  This change would address the speeds, but it wouldn’t do anything about having only two-car drafts, which seems to be a problem if you believe twitter to be a representative sample.

The fairly simple fix is to limit the time two cars can be hooked up by making it easier for the radiator to overheat.  If you force the radiator to start leaking steam at a lower temperature, the drivers can’t draft in pairs for as long as they can now.  This is pretty simple to implement and the primary consequence will be sleepless nights for the engine tuners.

Mother Nature will help as well:  the race will be during the day and temperatures will be higher, so there won’t be as much grip on the track.  That should slow down the speeds as well, but it won’t change the preference for two-car drafting.

Etc.

Did you catch what Craig Ferguson said about NASCAR drivers and their understanding of science on the Late, Late Show?  It’s in the first third of this clip.


Also published on Medium.

11 Comments

  1. Fantastic post. I need to use some of your posts the next time I teach mechanics.(As in Legrangian mechanics, not auto mechanics.)

  2. I think nascar made a mistake when they repaved the track. They should have taken out some of the banking. The car builders will always stay ahead of the track and nascar will always have to make rule changes to slow them down with the steep banking. This nothing more than artificial racing with cars running around at full throttle. As long as this condition continues I would rather see them use the road course.

  3. Welcome back. Thought that maybe NA$CAR didn’t like you anymore since you told the truth about the use of corn as fuel.

  4. Very good article. I wish many other “journalists” would learn to write this informative instead their “stir the pot” creative style.

    thanks

    Bob

  5. I think if Nascar gets the cars to run faster on the striaghts so they have to back off in the turns it would solve a lot of their problems! Atlanta and Charlotte are examples of what I am talking about! The speeds they run at Daytona and Talledaga are to slow!!!! They need to fit the track! or Nascar needs to take the banking angle down!

    Also letting teams instead of Nascar decide the rear ratio gear, here they give them a lower gear where resistor plate engines used to turn 6,700 max at Daytona and Talledaga, now they rev 9,700 Nascar I think did this to promote passing in this helps promote the two car draft as well!

    As far as the cars going airborne! I am all for safety!!! there is past history of racecars going airborne its part of the sport!

    Just look at a replay of Bobby Allison wrecking the Penske Cam 2 Mercury at Rockingham in mid seventies, going aprox 150 miles per hour!

    Enjoy your columns! Just some thoughts!

  6. i personaly belive they need a different set of rules for retrictor plate races since the teams build different cars for them likw you can run 4 cylinders with a 2 barrel carb/efi so they can be green like they are supposedly trying to do, save the teams some money and get rid of the restrictor plate

  7. It’s time that we went to 305 cid engines for the superspeedways. Run them wide open. 290 cid even.

    Race attendance is down as well, and that should be the impetus to take out the first ten rows of the grandstands. Speeds are going to continue going up, a car (or parts) WILL go into the stands. Are we going to wait until it happens or be proactive? Two drivers were killed the year before earnhardt died, but NASCAR was not proactive and waited until something happened that they thought would affect their bottom line.

  8. Great post! I’ve been watching NASCAR for 20 years and no one has ever been able to explain drafting to me in a way that makes sense. I’ve even got an engineering degree (although it’s electrical, not fluid dynamics), but I have never gotten drafting until now. Thank you very much for your easy to understand explanation of how it works.

    Regarding the two car draft this week, I’ve watched both the shootout and the duals, and frankly It’s not very entertaining to me so far. It’s just not Daytona without the huge pack of cars. I know it’s not safe for the drivers, but I’ve not heard much commentary on how much more safe they feel with the smoother track and faster speeds. I’ll be interested in how this new draft works with the entire field and the full length race.

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