Sunday, June 17, 2007 Updated: June 21, 7:30 PM ET
Ins and outs, and arounds and overs, of aerodynamics
By Bill Borden Special to ESPN.com
When a team sets up a race car they do it while it is at rest on a surface plate or scale. They are concerned with balancing the car front to rear to achieve a "neutral" status of the static weight. They manipulate the vertical loads (spring ratings) on the four tires and the geometry of the front and rear ends to achieve what they believe will give them a consistent and reliable tire patch when the race car is at racing speed on the track.
Dale Earnhardt Jr., left, creates "puckering moment of sensitivity" on Carl Edwards' car by pulling out of their drafting.
However, one extremely important element cannot be measured or manipulated at their shop unless they have a rolling wind tunnel tucked away in the back. A rolling wind tunnel is one in which the floor surface moves at the same speed as the air being flowed over the car. The nature of the air flow over the car is drastically different and much more realistic in a rolling wind tunnel. Because wind tunnels cost many millions of dollars to build, maintain and operate, the teams primarily rely upon their respective manufacturers to provide wind tunnel time and engineering support.
Some independent Wind Tunnel operations (such as Aerodyn, LLC in Mooresville, N.C.) can provide expert assistance to the teams. Ron Lutz of Aerotech US -- the premier provider of test equipment such as "balance" machines and rolling wind tunnel equipment in the world -- said balancing the air flow on a Nextel Cup car is like "Trying to balance an ice cream cone on the tip of your finger." Aerotech's "balance" machines measure the multiple forces of physics that can be applied to an object and are so finely calibrated that you could lay a quarter on the corner of a car and the weight change would be noted and recorded.
Teams have a limited availability of balance machines, however. The Nextel Cup teams are still many years behind the technology curve represented by the "balance" machine. They are only trying to resolve the simpler forces of drag created when the nose splits the air around the car plus reducing the friction caused by the air flow over around and under the car at this point in their technology development. "The goal is to gain the most down force with the least amount of drag penalty," said Lutz, who is the U.S. representative and general manager for Aerotech US.
Even the multimillion-dollar wind tunnels operated by the major automobile manufacturers cannot faithfully re-create the winds and forces that the race car will experience when it is operated around other cars on a race track at racing speed.
The air flow through the standard wind tunnel is steady and uninterrupted whereas race cars are constantly moving around and disrupting each other's air flow. And, the cars are static in the wind tunnel so engineers cannot accurately measure air flow around the rotating tires and underneath the car as it travels through the corners unless they have a rolling wind tunnel.
They can only obtain good aerodynamic readings when the car is operating at full speed on the track. So modern aerodynamic research in Nextel Cup has become a combination of wind tunnel time, track time and shop time for the teams and their engineers who spend hundreds of hours analyzing data and shaping the body lines of their race cars while staying within NASCAR's rules and limitations.
Aerodynamics is an area in racing where science and art are joined together to improve the outcome of the effort. Lutz pointed out that "Nextel Cup drivers, are artists who have been forced more and more to become technicians. But, the (Car of Tomorrow) will put much of the 'artistry' back into the driver's hands" by simplifying the science.
Engineers have so many variables to consider that it makes it nearly impossible for them to create the aerodynamically perfect race car. Remember, everything on a race car is a compromise in some form or another. Want absolute stability of the car for driver comfort? Then you will be giving up speed and performance to do it. You have to develop a compromise between the two through testing and tweaking so your car is competitive and your driver is comfortable and confident.
Plus the race car is operating in a constantly changing environment as the driver races his way through the field. You will frequently hear television or radio announcers talk about how a particular car came to life once it got into clean air on the track. Clean air simply is air that is not being disturbed by other race cars moving on the track.
The engineers are addressing two basic actions when they attempt to minimize aerodynamic drag. One is to minimize turbulent air flow over as much of the race car body as possible (friction drag or lift drag) and the second is to recognize that the aerodynamic drag that is created by the nose of the car splitting or pushing the air aside develops a base pressure area of potentially "confused air" behind the car as the air flows around the race car's body and merges back together.
Anyone who has driven on an Interstate behind a big and boxy semi and got buffeted around by the confused air coming from the back of the semi will better understand and appreciate what a race car driver is experiencing almost constantly during a superspeedway race.
When drafting, It is the ability of the trailing car to "steal" the low pressure area of drag formed behind the lead car and transfer it to the rear of the trailing car that allows both of them to go faster. The second car literally pushes the lead car to go faster. The air flows up and over both cars, thus proportionally reducing the aerodynamic drag depending upon the aerodynamic compatibility of their individual designs.
Lutz said this is one of the basic laws of physics that involves the length to diameter ratio or "L to D" ratio in engineering speak. Basically the wind is fooled into thinking it is flowing over a longer car which is more aerodynamic. You are trying to fool Mother Nature, Lutz said.
But, as the old commercial used to say: "It's not nice to fool Mother Nature." Primarily because she can fool you right back. Two cars butted nose to tail will be stable and will go faster. However, if one of the cars moves to either side then a "yawing moment of sensitivity" (he took the air off my spoiler!) is created and causes a "puckering moment of sensitivity" for the lead driver as the direction of airflow and the amount of downforce on the lead car is disrupted and manipulated by the trailing car.
They used to say that, if you were nuts enough to do it, you could walk out on the track to a corner and crash a race car as it passed by you simply by touching the rear fender because they were running that close to the edge. I might point out here that "yaw" is not a Southern term. Ya'll is a Southern term while "yaw" is a physics term. Don't get confused on that.
Although Richard Petty probably never heard of "yawing moment of sensitivity" during his driving days, he had a simple explanation for it when asked how to determine the speed you could comfortably carry when turning into a corner. Petty's short answer to a young driver was: "Cheeks will tell you."
The young driver was a bit confused by Petty's advice, thinking that perhaps he was referring to the smile of satisfaction he would have on his face as he successfully negotiated the corner. It was later that day when he over-drove his car into the corner and experienced a "yawing moment of sensitivity" that it dawned on him -- "Oh! Those cheeks!" Lutz and his fellow engineers call it a "yawing moment of sensitivity" while the older drivers always referred to it as the "pucker factor" or "pucker moment."
The rear spoiler on today's race car does not "spoil" the air, so the name is deceptive. Its purpose is to dam up the air flowing over the roof and rear window to create a cushion of air over the rear deck lid which allows any additional air flowing over the roof to travel across that cushion and the top of the "spoiler" to reduce lift and manipulate the base pressure area at the rear of the car to help reduce drag as described earlier.
The COT has a wing on the rear, which is one reason the teams are struggling to gain a balance of comfort and competitiveness with the COT. All of their acquired spoiler knowledge doesn't transfer to the new wings plus adjustments on the wing are very limited so the teams need to massage other areas on the car to work in concert with the NASCAR specified wing settings. Either way, too much or too little spoiler or wing changes the rear downforce and can impact driver comfort and/or competitiveness.
You will occasionally hear announcers talk about drivers seeking drafting partners. There is more to it than two cars just getting together to gain aerodynamic efficiency. Car "A" and car "B" may draft well together when car "A" is in the lead but then lose their competitive efficiency when car "B" leads car "A" because the aerodynamic flow would be different. Car "B" and car "C" may be more aerodynamically compatible than either configuration car "A" and car "B" had when they drafted together so car "B" would want car "C" as its drafting partner.
For example, the aerodynamics of a particular Ford might be more compatible with the aerodynamics of a particular Chevrolet than with another Ford so you might see some strange alliances forged during a race. Individual driving styles (and trust) may also enter into the equation.
If you have ever noticed the dirt or water swirl patterns on the side of the family car after you drove it through a storm then you were seeing a visual demonstration of aerodynamics at work. Aerodynamic engineers will attach small pieces of yarn to the race car body so they can see how the air flows across it in the wind tunnel. They will also use smoke or long yarn trails to visualize air flow. If all the yarn pieces and/or smoke flow evenly and smoothly in the same direction then they have a good indication that they have an aerodynamically slick design. If the yarn or smoke indicates that air disruptions are along the body then the engineers will manipulate the body panels accordingly to achieve what they believe to be a good aerodynamic air flow. Finding the perfect aerodynamic balance however is like chasing the wind. How do you know when you have caught it?
Bill Borden is a former championship winning crew chief who operated David Pearson's Racing School for many years.