Drag and Draft–How the really fast cars go faster!
There’s a lot more to making a race car go fast than horsepower…it takes an incredible amount of engineering. And one of the most fundamental parts of this engineering is the aerodynamic design of the car. During the course of a race, you’ll hear a fair amount of commentary on the topics of downforce, grip, drafting, and drag…all terms that have more to do with what’s outside the car than the car itself.
It all starts with drag
Creating speed requires an engine plant that can transfer power to the wheels, but once the car is in motion, this power encounters an obstacle: air. Billions and billions of molecules of air that need to be moved out of the way in order to create a clear path for the race car. These air molecules cause a level of resistance that, as the speed of the car increases, becomes a force that impedes the car’s forward motion. How the air is moved also relates to downforce (the weight of the air gluing the car down to the track to grip the racing surface).
To get a feel for the resistance, recall what just about all of us have done at some point in our lives. Remember sticking your hand out of the window of a moving car and holding it up to face the wind? What you immediately feel is the resistance of the air hitting you upraised hand, and the feeling that your hand is bending backwards. If you turned your hand flat, then most of the resistance went away, and that’s because you’ve reduced the number of air molecules your hand was hitting. This is why race cars typical employ a sleek design, with limited appendages and openings in the front. The less blunt surface to face the air, the less resistance.
Drafting Amounts to Managing Drag
If you’ve had the opportunity to witness a NASCAR race at either Talladega or Daytona (the two largest high-banked ovals), you’ve probably marveled at what’s referred to as “pack racing.” Groups of 20 or more cars packed tightly together only inches apart, often moving at speeds faster than the single-car qualifying speeds. It’s an incredible sight, but it’s more than that…it’s a visible definition of using the power of drafting to minimize drag, since only the cars at the front of the pack are encountering the drag effects.
In a pack, the cars behind the leaders are not contending with the resistance. Instead, by virtue of being separated only inches from the car in front, the air flowing over them is creating a vacuum between the two cars that has virtually eliminated the resistance factor, and the same is true in the case of the next cars in line, as long as the separation doesn’t grow large enough to cause the air to hit the front end of the car. As a result, the entire pack of cars is able to move faster than a single car. The cars behind the leader pick up the benefit of a power boost (up to 3- to 5-mph) as a result of their engines not needing to deal with the resistance. Collectively, this power moves the entire pack forward at a faster speed.
In an interview for the National Science Foundation’s “Science of Speed” series, NASCAR veteran Brian Vickers explained the process this way, “In a two-car draft, it’s like one car with two engines.”
And while pack racing has become so fascinating, one of the equally fascinating aspects of the phenomenon is what happens when a car gets separated from the pack. This is where you really can see the affects of drag, because a car that gets maneuvered out of the draft will suddenly appear like it’s dropped anchor as it gets pulled to the back of the line. If it’s a team car, it’s likely that a teammate will make the risky move of opening a spot in the pack so the separated car and get back in line, but if it’s near the finish that’s not likely to happen, either.
Drafting is Great…Until it’s Not
A relatively recent outgrowth of drafting is something called “bump drafting,” where the car behind uses that bonus horsepower resulting from the draft to run into the car in front, pushing both to even faster speeds. This is a tactic that’s often used by teammates to gain positions on the track, but if not done properly can have disastrous affects. It takes an extraordinary amount of skill to successfully transfer momentum from one car to another, and requires careful attention–usually at more than 200 mph–to the center of gravity of the car in front. To effectively bump draft, the car doing the bumping must ensure that the transferred force goes directly through the center of gravity; otherwise, the car being bumped will spin. So, bump drafting is really only effective on straightaways.
The Concept Applies to All Forms of Racing
The effects of drag and drafting are not limited to motorsports. In fact, even long-distance runners in track and field events will use drafting techniques during an event to capitalize on the person in front of them punching a hole in the air for them. Believe it or not, it does allow the runner behind to actually take a break..in this case, a break from fighting resistance. The principle also comes to pay in competitive cycling, where riders will often lay back in the pack during portions of the event to conserve strength.
A Scientific Explanation of the Process
If you want to get a feel for what motorsports personalities think about the whole process of drag and drafting, check out this National Science Foundation video…