Drafting: Regular and Bump

I looked through the blog and realized that I haven’t really blogged about drafting. I’ve blogged about all the rules NASCAR’s enacted in the last couple of years to try to control drafting, but not drafting itself. So, with Talladega coming up, here goes

Daniel Bernoulli: A Founding Father of Racing

Sure, NASCAR itself started with Bill France. But racing wouldn’t be possible without the scientific achievements that led to the technologies used in racing.  Many of those developments happened way, way before the automobile was even invented.

Daniel Bernoulli was a mathematician who, in 1738, published the principle that explains two of the most important technologies in the history of NASCAR: the carburetor and the shapes of race cars.

Bernoulli came from a very talented Swiss family, including his father, Johann (an early contributor to the new field of calculus) and uncle Jacob (who worked in probability theory). Daniel’s brothers were pretty accomplished as well, but Daniel was recognized as “the smart one” — which caused him problems. History doesn’t tell us if he was beat up at school, but we do know that Daniel and his father competed in a scientific contest at the University of Paris.

Daniel won and his father kicked him out of the house. The two men had a very tenuous relationship from there.

So what did Daniel do? This:

Crystal clear, right? No matter: Here’s all you need to know for the purposes of Talladega:

So take a look at how air flows over a race car. The key here is that longer, greener arrows mean faster moving air (and thus lower pressure). Shorter, redder arrows represent slower moving air (and thus higher-pressure air).

Some air goes under the car, but we’re interested in the air that goes over the car. Starting from the front:

  • The air slows down at it hits the nose/splitter
  • It starts to speed up as it goes over the hood
  • Then it slams into the windshield, which redirects the air — and slows it down
  • Again, it speeds up as it goes up the windshield
  • And reaches maximum speed on the roof
  • As the air travels past the roof, it starts to become turbulent. It’s the same effect as you see if you stir cream into your coffee with the back of a spoon. The cream swirls and so does the air.
  • The air slows down as it hits the spoiler
  • The air goes up and over the spoiler, creating even more turbulence as it flows over the rear of the car.

When we look at the forces on the car:

For forces up and down: Green areas mean less pressure (lift) and red areas mean more pressure (downforce).

  • Notice the green areas on the hood and the roof? It’s no coincidence that these are the areas where flaps are designed into the car. These areas can crease significant lift. When the flaps deploy, they slow down the air, which increases the downforce in those areas
  • There are three areas with significant downforce
    • The front of the car, which creates downforce on the front tires
    • The rear of the car, which creates downforce on the rear tires
    • The base of the windshield — which is where air goes into the engine

What we’re interested at plate tracks, though are the forces acting on the front and rear of the car.

  • The air slamming into the front of the car creates front drag
  • The turbulent air in the rear creates a vacuum, which means it pulls backward on the car, slowing it down


FoxSports and NBCSN have much better graphics than I do, but this should give you an idea. The whole point of drafting is to get two cars close enough so that the air thinks they’re a single entity. Compare the diagram below where each car has front and rear drag when they run separately.

When they get close enough, the air doesn’t get turbulent behind the first (orange) car. It flows right onto the trailing car.

  • Note that the trailing car gets a little less front downforce and the rear car gets a little less rear downforce
  • But they gain the advantage of eliminating drag.

So instead of having front drag and rear drag on each car, there is less overall drag on two cars running together than there is when the cars are running apart. You can see a better animation of this in the ‘Drag and Drafting‘ video we did for the National Science Foundation.

Terminal Velocity

Remember that drag increases like the speed squared, which means the faster you go, the more drag. At most tracks, the force the engine can produce can overcome the drag, regardless of the speed.

At plate tracks, the plates limit the engine power, so you reach a speed where the force of the engine can’t exceed the drag force. The cars achieve a terminal velocity. It’s called terminal because you can’t go any faster than that. (The same thing happens to falling objects. They speed up the longer they fall, but at some point, they reach terminal velocity and don’t fall any faster than that.)

This is why teams go to extremes to minimize their cars’ drag. Even a little less drag means a little higher terminal velocity.

Bump Drafting

Here’s why bump drafting came about. When you’re drafting, the leading car breaks the hole in the air. It’s got a larger drag burden than the second. That means the trailing car doesn’t have any front drag. It get’s pulled forward in the leading car’s wake.

This means the trailing car can actually go faster than the car it’s drafting behind. 

Except that only works while drafting. You can try to use the momentum gained from drafting to pass, but it’s a difficult move.

So if you’re behind a car and you’re traveling faster than that car, you may actually have to put on the brake to keep from running into your drafting partners.

Or… You could use the extra speed you have to make you both go faster.

Momentum Transfer

Bump drafting is nothing more than a very well-controlled collision — at least, when it’s done right and you don’t crash the guy you’re trying to help.

The device in the video below is called Newton’s Cradle. You pull back one (or two) balls and let it (them) go. The nice thing about this video is that they then show the collisions in slow motion.

This is called momentum transfer. Momentum is the mass of an object multiplied by how fast it’s going. So here’s how this works.

  • The rightmost ball (which I’ll call Ball 1) gains momentum when it’s let go.
  • Ball 1 transfers its momentum to Ball 2
  • Normally, Ball 2 would move, but Ball 2 is right up against Ball 3
  • Instead of moving, Ball 2 transfers the momentum to Ball 3
  • Ball 3 transfers momentum to Ball 4
  • Finally, when Ball 5 gets the momentum, there’s nothing stopping it from moving, so it does.
  • It then returns and transfers the momentum back.

In a perfect world, this would go on forever and forever. This is not a perfect world. Every time the balls hit, they lose a tiny bit of energy, which slows them down a little. So after ten or twenty smacks, there’s no energy left in the system.

Momentum Transfer Applied to Bump Drafting

All racecars are approximately the same mass so we can talk about it terms of just the velocity. It’s the same thing in the Newton’s Cradle example because the balls are the same mass.

  • The trailing car gets pulled ahead in the wake of the leading car
  • The trailing car goes faster than the leading car (meaning the trailing car has more momentum than the leading car)
  • This only works for a short time because the trailing car closes the gap
  • When the trailing car hits the leading car, it transfers some of its momentum to the leading car
  • The leading car speeds up
  • The trailing car is pulled along in the leading car’s wake
  • Rinse. Repeat.

If you’d like to learn about inelastic collisions, take a look at this blog I wrote for Cocktail Party Physics about Brad Coleman running into a coyote during testing. But not if you’ve just eaten. There are pictures.


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