Air flows around, under and through your race car, and you can take advantage of it with cheap and simple modifications that have a big performance payoff. The factors with the greatest lap-time influence are grip, power-to-weight ratio, downforce-to-weight ratio, and drag-to-power ratio, so aerodynamic performance really matters. Four of the most effective modifications for closed-wheel race cars are air dams, front splitters, rear spoilers, and rear wings. How effective are they?
“Adding a 4-inch front splitter to my tube-frame Camaro and ducting the radiator air out through the hood required 9 degrees more rear wing to balance it!” said Paul Costas. “Although it reduced my top speed by around 5 mph, it added 3 mph to my lap average speed. I’m consistently pulling over 0.2 more lateral g’s in the fast corners at Texas World Speedway.”
Sure, more downforce and less drag will make your car faster. But more downforce usually means more drag, not less. Finding the optimum setup for each track takes some testing. Also, the balance of front to rear downforce will change your cornering balance with air speed.
Aerodynamic downforce and drag increase with air speed squared. This relationship has a huge consequence. Aero devices don’t do much in slow corners, just when you need them the most. But they produce big forces at high speeds, which are mostly on the straights where you don’t want downforce or drag. Downforce is most useful in fast corners and while braking. Adding the right amount of downforce in a way that produces the least additional drag is the goal.
The optimum size of each aero device changes with the combination of car and track. A track with only slow corners and long straights favors a low drag, low downforce setup. A track with several fast corners and shorter straights favors a high downforce, high drag setup. A high-horsepower car will be faster with larger aero devices than a lower powered car. If you run several tracks, then you will need easily adjustable aero devices.
How can you determine the best sizes for your aero devices? There are three ways: computer-based lap simulation, track testing, and copying the cars at the front of the grid. Using all three of them at the same time is the fastest way to optimize your aero configuration.
FRONT AIR DAM
An air dam can be one of those magic improvements, increasing downforce and reducing drag at the same time. It can also improve engine cooling by increasing the air flow rate through the radiator. An air dam reduces drag by reducing the rate of air flow under the car, which reduces drag caused by all of the protrusions and cavities under the car. Everything that you see under the car is a drag source, so hiding that mess behind an air dam is a simple way to reduce drag.
An air dam can increase downforce by reducing the average air pressure under the car. Maximizing downforce from an air dam requires its lower edge to be very near the pavement, so it has to be flexible to survive in the real world. An air dam is a simple device, so making one that works well is an easy fabrication project.
A splitter is a horizontal shelf mounted under the nose of the car, or to the bottom of an air dam, with the top side of the splitter sealed to the body. A splitter produces downforce from the difference in air pressure on the top and bottom surfaces of the splitter area. Because airflow over the top is blocked by the body or air dam, the local airspeed is low and the air pressure on top is high. Because air can flow under the splitter freely, the local airspeed under it is high and the air pressure on the bottom side is low.
More splitter area produces more front downforce, but only up to a point. The effectiveness of adding splitter area falls rapidly as it extends away from the body. That is because the difference in airspeed between the top and bottom sides decreases with distance away from the body. The useful length is 4 to 5 inches. The downforce that a splitter creates changes with height from the ground. A small increase in downforce can be had by extending the splitter behind the air dam or nose, and gently curving the trailing edge up to create a short but wide ground effect tunnel.
Because a splitter is the lowest point on the car, it is vulnerable to damage from contact. So, it has to be made from a durable material. A splitter that is mounted with cables so that it can move up, but not down, will sustain less damage from contact. Terry Fair, owner of Vorshlag Motorsports, reports that, “The 5-inch ABS plastic splitter on my Mustang added noticeable grip at both track and autocross speeds, but it flexes down at high speed.”
A notchback or flying buttress roof shape produces a giant separated wake behind it, so a spoiler or wing mounted in that wake will not work. On the other hand, a gently tapering roof and rear window shape produces a clean, high energy flow field for a spoiler or wing.
A deck lid spoiler is a simple shape, so making your own is another easy fabrication project. Making adjustments also is easy. The only adjustments are length and angle. It is easy to make the length adjustable by making two panels that overlap partially and fastening them to each other. Changing the amount of overlap changes the effective spoiler length, and the downforce and drag that it creates. There are several ways to make the angle adjustable as well.
A spoiler makes downforce in two ways. It slows the airflow ahead of it, which increases the air pressure on the trunk lid. It also deflects air upward as it flows past the end of the car. The good news is that a spoiler makes consistent downforce. The bad news is that a spoiler also generates a lot of drag along with that downforce, so a big spoiler has a strong effect on top speed. Spoilers have proven to be effective at low speeds as well. Terry Fair’s experiment at an autocross produced a clear lesson.
“Removing my 10-inch tall clear spoiler was very noticeable,” he said. “A low speed push with the spoiler on immediately turned into an oversteering mess without it at cornering speeds above 50 mph.”
A smooth, properly shaped wing with large end plates in a clean airflow field can produce as much as 8 pounds of downforce for every pound of drag that it creates. Raising the rear of the wing increases its angle of attack, and the downforce that it creates. Drag increases with downforce, and it decreases with wing span squared. So, you want your wing to be as wide as you can legally run. Large endplates further increase the effective span of the wing, further increasing its efficiency.
“Running NASA Time Trial events without the rear wing makes the car more skittish in corners, hard to slow into heavy braking zones, and corner exit traction is limited on medium to fast corners,” Fair said. “Now that we have the wing set up, I don’t want to run without it. With stock aero, the car is both slower and harder to drive.”
The downside of a wing is that its performance is more sensitive to its shape than the other aero devices described here. Fabricating a wing yourself is possible, but it requires a far higher level of craftsmanship and more fabrication equipment than a spoiler. Fortunately, there are usually a few options for buying a wing that is good for your application. Wings are sensitive to small changes near the leading edge, so make sure that your wing stays clean and fill any rock chips carefully. The low-speed performance of a wing can be improved by running a smaller angle of attack and adding a Gurney flap on the trailing edge.
AERO BALANCE OPPORTUNITY
Adjustable aerodynamic devices give you a way to deal with the most annoying bit of physics that racers have to deal with. Unfortunately, velocity is one of the factors that influence cornering balance. It is one of the terms in the automotive cornering stability equation. Because of that, the slower you go, the more understeer you will have. Adding aero devices to your car enables you to change the combination of mechanical and aerodynamic balance to produce a cornering balance that is closer to neutral over a larger speed range. The way to do that is to change the mechanical setup toward oversteer and the aerodynamic setup to add understeer.
With a setup combination like that, the mechanical oversteer and physics-induced understeer cancel each other in slow corners, and the mechanical oversteer at higher speeds is canceled by rear-heavy aero downforce. The optimum aerodynamic balance for this setup combination is usually about 40 percent front and 60 percent rear downforce. Rear spoilers and wings are usually more effective than front air dams and splitters, so it takes more development and testing to generate downforce at the front than at the rear.
Air dams, splitters, spoilers and wings are proven ways to modify airflow and make a race car faster. They can be cheap and easy to make and adjust, and they offer a big performance gain per dollar. You don’t need expensive wind-tunnel testing or computational fluid dynamics expertise to improve your car’s aero performance. The experiences that Fair and Costas had are good indications of that. Track testing is part of what you do anyway, so you can test these proven aero devices and benefit from the performance gains that they offer.
MORE DOWN FORCE
Rules for your class may limit aero modifications that you can make, but if the rules allow extensive modifications, there is much more aero performance potential available. Here are some more proven modifications that you can consider developing on your car.
A ground effect diffuser under the rear of the car can produce a lot of downforce very efficiently, but only if it blends into a smooth, continuous cover panel under the car that extends all the way to the nose. It takes smooth, high-speed airflow under the car for a diffuser to perform well.
If your car has an effective air dam or diffuser, side skirts will help reduce the leakage of air into that low-pressure region under the car. Side skirts can increase downforce and decrease drag at the same time.
Radiator airflow is a huge air leak that reduces the effectiveness of your air dam and/or splitter. Ducting most of the radiator exit air up and out through the hood can produce a large downforce increase, and perhaps even a drag reduction to boot.
Sealing the nose air intakes to the radiator face so that no air can leak around the radiator is another way to improve aero performance and cooling. The same goes for every other heat exchanger airflow path.
Lowering the whole car is a particularly effective way to improve aerodynamic performance. Maximum downforce occurs when the front is lower than the rear. Minimum drag usually occurs with the car low and level.
Any surface on the hood that has outward curvature is a lift producer. Adding louvers to the location with the tightest curvature will reduce that lift considerably, and it will provide an additional outlet path for under-hood airflow.
Another way to increase front downforce is to add Gurney flaps to the leading edges of the front wheel openings. That will deflect air out, producing an easier path for air to exit the under-hood or wheel well area.
A cold air intake for the engine is an effective aero modification, with most of the benefit due to lower air temperature rather than higher pressure. Likewise, locating the exhaust pipe outlet in a strong low pressure area provides an aero gain. Exiting the exhaust out the side of the body produces a strong jet of exhaust airflow, which adds drag and exhaust back pressure.