In a previous article that was published about four years ago, we offered an explanation of the way the aerodynamics of downforce works for a stock car, something that had never before been adequately defined. In today's racing circles, aero effects are discussed on a regular basis.
Body shapes and methods have evolved over the years that improved stock cars' aero efficiency related to drag and downforce, but until recently, no one was told why certain methods are effective. Let's readdress aero effects and downforce now that our knowledge has progressed.
The addition of a high wing to the rear of this Daytona prototype helps provide a lot of d
Just as I was about to write this piece, I read the morning Daytona newspaper and my friend Godwin Kelly's story about four crew chiefs being barred from the Daytona 500 and fined for illegally helping out their cars' aero balance situation by attempting to improve the rear downforce of the cars.
Those Nextel Cup cars are suffering due to having to run hard tires, and the stability factor is at an all-time low. Any help with rear downforce is a huge help. That sounds strange, but with all of the technology concentrating on the front downforce, the cars have become unstable because the rear loads are not enough to keep up with the front loads. We'll explain this in greater detail later.
Our interest as short track racers is not so much with drag efficiency, but in how to make more downforce, especially in the front of the car. Going back 30 years or so, we could see that the automotive manufacturers and many racers began to redefine the body shapes to make the cars cut through the air more efficiently with less drag. The research done for the space program as well as in the design of modern aircraft provided much of the early impetus for the factory teams and local racers to improve their cars.
The wing on an airplane provides lift by causing the air passing over the top of the struc
The squareback sedans became fastback cars. Square corners became more rounded. Spoilers and wings were added to production model cars to make these configurations legal for competition use.
Issues such as drag coefficient, frontal area, and downforce were looked at more closely. Teams that were at the cutting edge of this new area of racing technology were the ones who ran out front.
One of the proving grounds for short track racing is the annual Daytona Speedweeks races at New Smyrna Speedway (NSS). Every year, teams from all over the country show up with "stock"-bodied cars that adhere to the strict rules of their respective sanctioning bodies only to discover that there are no rules at NSS.
Years ago, we noticed that after about the third night of racing in the nine-night series, most of the cars were transformed into sleek, wedge-shaped oddities with huge fantail rear spoilers attached.
This is how we evolve in stock car racing-by going to the extreme. When we are reeled back in by the rule makers, we have to get more creative. In this modern day, we now have a better understanding of how aero works to provide additional downforce.
Because the air under the hood is at a lower atmospheric pressure than the air above it, a
Downforce, or its close cousin lift, is a byproduct of differences in air pressure on two sides of an object. Similar to how an airplane wing produces lift, a car can produce antilift, or downforce, so that more load is placed on the four tires to provide more overall grip. The more grip we have, the faster we can drive through the turns. The bonus is that downforce provides added grip (traction) without the negatives associated with added weight or mass.
The wing of our stock car consists of the hood and rear deck areas. We need to design our bodies so that we can direct the air around the car in order to vacuum air out of these two cavities.
If we can use the swift flow of air that is flowing past the sides of the car to help vacuum air out of the engine compartment, we can lower the pressure along the underside of the hood, as with the airplane wing. High pressure on one side of an object will push that object in the direction of the lower pressure, or high toward low.
To accomplish this, teams use wider, angled front noses that direct the oncoming air around the sides of the car to the wheelwells in such a way that a low-pressure area is created just outside the wheels. Air rushes out of the engine compartment to fill this void, and the pressure under the hood is reduced.
As the car travels through the air, some of the airflow is directed around the nose and to
The average atmospheric pressure at sea level is 14.7 psi on all sides of an object, even our bodies. If we reduce the pressure under the hood to 14.5 psi over an area of just a square yard, we would generate about 260 pounds of downforce (0.20 [psi difference in pressure] x 362 [number of square inches in a square yard] = 259.2 pounds [total amount of downforce generated by the pressure differential]).
A counter effect to the air flowing out of the wheelwells relates to the air that flows under the nose of the car. This air will replace some of the air that is being suctioned out of the wheelwells. It also reduces the low pressure and the effect of downforce. That is exactly why we need the front spoiler, or valance, on the nose to be as low as possible. The popular big bar and soft spring (BBSS) setups tend to help this situation by allowing the overall chassis to compress into the track to help eliminate the flow of air under the nose.
Once we have modified the outside of our cars to provide the airflow that will create the low-pressure areas we need, the next step is to try to provide the greatest amount of surface area under the hood and rear deck that would be exposed to those low-pressure areas, thereby increasing the downforce effect. We also want to try to limit the volume of air that could flow under the nose and make what does flow less able to diminish our goal of low pressure by routing it away from our low-pressure areas.
One way to increase front downforce is to open the area under the hood that is available for pressure reduction. One area that we can utilize is the top of the radiator shroud or airbox. Most designs offer a flat top that is built very close to the hood.
We can redesign this structure so that the top of the airbox is curved from a sideview starting at the very top of the radiator. As it comes forward, it is at an angle of 70 degrees off horizontal and then curves forward to intersect the top of the openings in the nose. This shape creates a large cavity under the front part of the hood and nose that is now available for reduced pressure and added downforce.
A low-pressure area in the rear of the car, similar to the front, can be created with a li
We can modify the shape of the window posts to either promote the flow of air to the rear
...or to route the air away from the spoiler to decrease drag for larger and faster racetr
If we build the bottom of the airbox wider at the front, less air will flow under the nose and upward to the low-pressure area. With less air being replaced, more low pressure is retained and more downforce is available.
This team built the airbox in front of the radiator with the top angled down as it runs fr
At the rear of the car, we can manipulate the shape of the spoiler, the rear window posts, and the body just in front of the rear wheelwells. By routing the air that is flowing past the sides of the car out and to the sides of the rear wheelwells, a similar suction effect takes place to create a low-pressure area under the rear deck. There are obvious limits as to body shape in this area, but a little reshaping can help.
Racetracks that require less downforce and tracks where we can benefit from reduced drag will cause us to rethink how the air flows past the roof (greenhouse area) and onto the rear spoiler. If we reshape the post that connects the rear window with the side-window openings, we can direct air away from the spoiler and greatly reduce aero drag.
The rear spoiler produces two effects at the same time. It provides aero downforce generated from low pressure on the underside as well as aero drag, which creates a cantilever effect that redistributes some of the weight of the car from the front to the rear. Many racers mistake this effect for downforce because it tightens a loose car by adding more weight to the rear of the car.
If we put the car on a set of scales and have four guys pull straight back on the rear spoiler to simulate drag, we would see a change in the front-to-rear percentage of total weight distribution. This is what drag from the spoiler does. If we need the car to turn well (and who doesn't?), then the creation of a high drag effect cannot be in our best interest.
It has been found that the optimum angle for the rear spoiler is between 55 and 60 degrees from horizontal, depending on the type of racetrack. The longer and faster the track, the less spoiler angle we need to run.
The ideal nose for creating downforce in a stock car would be narrow at the front with fla
The shape of the front nosepiece can help determine how much air is directed under the car and how much moves up and over the hood. Obviously, the more air that passes over the top, the less air that will invade the low-pressure area under the hood.
Vertically flat noses tend to build a pressure front that pushes the air in all directions and disturbs the orderly flow of air. This will force more air under the nose as well as disturb the free flow of air around the side of the car and past the wheelwell. The suction effect that produces the low pressure is less effective, and the car will have less frontal downforce.
If the nose were angled with the lower edge farther forward, the oncoming air would be cut more cleanly and would not build up into a pressure front. Less air would flow under the nose, and the flow of air around the sides of the nose would be cleaner, with less turbulence.
The overall balance of the setup in the car has an effect on downforce. A car that is set up so that the rear suspension is rolling more than the front will cause the car's left-front area to rise. This allows more air to flow under the car and replace the low-pressure air, which results in less downforce effect.
In a NASCAR Cup test I was involved in at Daytona in early 1996, a team rearranged their spring rates so that as the car negotiated the turns, the left front traveled more. The left-front shock travel increased over an inch, and the car picked up an honest 3 mph.
Short-track teams are now using larger sway bars and softer springs so that body roll is reduced and the front end is more compressed into the racetrack to eliminate much of the airflow under the nose. These cars turn better due to the increased downforce this creates.
A flat nose will create a pressure front much like a bubble. This impedes the air trying t
A slightly angled nose tends to cut the air more cleanly, and more is directed up and arou
Rear spoiler angle will produce both downforce and drag. The greater the angle from horizo
Major-league teams use wind tunnels to perfect the aero efficiencies for both downforce and drag. Each session may be split to include a short/medium-track car and a superspeedway car because the goals for each are different.
In the wind tunnel, the attitude of the car is adjusted to simulate the way the car dives and rolls as it circles the racetrack. There is also a feature at some tunnels that will yaw the car, or rotate it as if it were on a round table. The latest technology utilizes a moving platform under the car to better simulate a real race car driving into the wind.
A very low angle on the spoiler offers less drag, but also very little downforce. The opti
The key goals with stock car aero design and testing is to create a body shape and inner construction that will provide more downforce to enhance turn speeds, produce less drag, and promote a more balanced race car.
If we work hard to develop 600 pounds of downforce on the front end and the rear is not able to keep up with that high amount of grip, then the car will be loose-nobody can drive a loose car fast. So there are limits to how far we can go. It is also possible to create so much front downforce that the car is undriveable.
Work toward a good balance of front-to-rear aero downforce to help produce more overall grip. Do not overdo your efforts to help the car aerodynamically at the expense of handling efficiency. Make sure the basic chassis setup is balanced, and then the combination of both aero downforce and handling will enhance your on-track performance.
Many short track teams gain more aero downforce by utilizing the big bar and soft spring (BBSS) setups. This produces a very low and square nose attitude in the turns, helping to reduce atmospheric pressure under the body.