As speeds and G-loads increase, the aerodynamic properties of a car begin to matter more and more. As we know, it is the natural tendency of a car to generate lift at speed. The air flowing over, under, and around the paneling of the vehicle can have as profound an effect on the handling characteristics as suspension or tire selection.
The un-nerving, albeit natural speed governor of a car going light at speed is the nemesis of grip racers. Ever in search of traction at high speeds, car designers have turned to aerodynamics to settle their machines.
Problems And Solutions
Performance vehicles generate undesirable lift that ultimately reduces traction, steering authority, cornering speed, and overall safety in a few major areas. While very few of us are lucky enough to have access to a wind tunnel for testing, not all homespun or aftermarket aero remedies are aesthetics follies.
Among the issues faced by racers is rear end lift. Several factors may influence this tendency — the first and foremost of which is the profile of the car body over which the air flows. Given different leading and trailing contours, a body shape may create different characteristics. As the rear of many cars naturally slopes earthward so does the airflow as it clings to the surface of the car before delaminating. Acting as a very crude lifting wing, the profile of the body may induce rear end lift.
The addition of rear wings or spoilers aspire to negate this effect and lend favorable down force — thereby synthetically increasing the weight over the rear tires and improving traction under power and cornering loads. Given that the classic retro design and layout of the Cobra Jet is relatively neutral and simultaneously aerodynamically dirty — with a gaping mouth for cooling, and open cockpit — we opted to take an all around approach to bringing this classic into the jet-age.
Snakes Don’t Fly But Some Get Wings
We Turned to G-Stream and its line of competition performance products for our rear wing needs. Dave Martis of G-Stream gave us the story on the history of the company’s offerings and school of thought.
“We wanted to develop a rear wing for our cars to plant them at the track and make them more competitive. We went through a dozen or so iterations and did a lot of on-track testing, and some wind tunnel testing over the years,” Martis said.
“The basic wing is modeled off a heavy-lift, low-speed aircraft profile, and we modified the NACA profile to incorporate a wicker (the little kick up at the rear). The construction is a hand-laid, epoxy-resin, carbon fiber and we have a structural foam core that we bond inside of the wing,” he continued.
“The unique thing about our wing is the mounting — because of that structural core-support that runs the length of the wing it allows you to put those mounts anywhere along the width of the wing so you can use it in a lot of different applications.”
“Everybody wants numbers, but most of the data out there is of a wing profile in free air — well that does no good. Once you bolt the wing to a car — it doesn’t matter if it’s a Mustang or a Porsche, the wing is going to act differently on every car because of the body profile. The wing works, what more can I say,” concluded Martis.
With built-in adjustability we can tailor the wing’s angle of attack into the incoming air mass. By tuning the angle and ultimate rear downforce we can be sure to best balance the front to rear ratio — and idealize the total aerodynamic package.
Charming The Business End Of Our Cobra Jet
As important as maintaining traction and down force over the rear (drive) wheels so is properly tuning the front for steering and turn-in response. As air strikes the façade of a car it deflects in a multitude of directions. Air that escapes the radiator inlet and is too low to pass over the hood and windshield is diverted underneath the floor of the car.
What Is Ground-Effect?
Ground effect is a concept rooted in aviation. As an airplane flys close to the ground (within a wingspan) the lifting ability is exaggerated. This phenomenon is a result of the down wash from the wings having a surface to “push off” from — giving the airplane a little cushion.
In order to redirect some of the air making it’s way under the FFR Cobra Jet we turned to the composite wizardry of TruFiber for a custom crafted fascia, and APR Performance for a well-sorted splitter.
The open mouth stance of the Cobra Jet does not include any kind of conventional front bumper or plane to mirror the ground. To fill this aesthetic no-man’s land, and lend a custom one-off aerodynamic touch, we reached to Phil Truong of TruFiber. Our carbon fiber front fascia is one-piece that sets our FFR Cobra Jet apart from the rest.
“TruFiber manufactures carbon fiber and fiberglass parts like hoods and accessories for American muscle cars. We have the ability to do custom work but this project was an exception — however having never done anything for such a vehicle as the Cobra Jet. So when it was brought to our attention we decided to give it a try,” explained Truong.
We designed our fascia not only to improve the aesthetics of the FFR Cobra jet business end, but to serve as a functional aerodynamic, and cooling aide. The nature of the stock body is such that the receding leading edge of the fender arch exposes the front of the tire to oncoming air. By filling this gap with the fascia we will better direct air down the flanks of the car.
In the center of the fascia is a recessed ramp resembling a NACA duct, this small trough serves to direct more air into the oil cooler keeping operating temperatures down, and pressure up.
Starting with a digital rendering we took the rough design to TruFiber who with a little foam and a deft sculpting hand began to work their magic under the chin of this particular breed of viper. With a foam core sculpted and fiberglassed over the smoothing began to refine the rough plug.
With a plug completed the surface could be smoothed and mold release agent applied. First laying down a gelcoat for a glass-smooth surface a female mold was generated over the plug. With this set of one-off tooling the actual production of our carbon goodness could commence.
“Our process is a vacuum infusion system. We lay the carbon fiber in the mold and put a bag over it. The vacuum infusion sucks resin from one end and draws it to the other. The good thing about vacuum infusion is that it gets rid of all the excess resin,” said Truong. Because the majority of the weight of a composite part is in the resin, removing any excess that doesn’t add to the strength is an important weight-saving procedure.
“Like any other part we make — when it’s completely done and cured — we trim all the edges, wet sand, and apply an automotive clear coat to protect against UV damage,” Truong concluded. Sunlight exposure will cause resin bound parts to yellow over time so the addition of a protective clear coat is a conscientious addition the extend the lifespan of composite parts.
APR Performance Splitter
A splitter is a deceptively simple piece of aerodynamic kit that pays dividends when properly used to correct airflow underneath a car. Generally a flat plate made of metal, composite materials, or wood in budget situations — a splitter protrudes off the front bumper and parallel to the ground. APR Performance has put a lot of thought into the design and function of these aero aides and we spoke to K.C. for some background.
“What we want to have is lower pressure, faster-moving air below the car and higher pressure slower-moving air above the car. The splitter itself does not actually create downforce. What the splitter does is that it increases the area over which high pressure can build up. The more high pressure there is above the splitter, the greater the net downforce,” according to K.C.
For a unique application like our FFR Cobra Jet — a mass-produced splitter solution does not exist. The addition of our one-off fascia additionally complicates sourcing a known quantity in terms of fitment.
In order to ensure fitment we generated one of the fabricators best friends — a cardboard template. With this corrugated representation of the underside of our vehicle we could project a desirable profile for our aero accessory to follow. With this guide sent off to APR a correctly shaped splitter could be cut out of carbon fiber laminate.
Installing the splitter involved some trick hardware, and a pair of links. With adjustability built into the links the angle of attack this front blade presents can be adjusted to suit handling needs.
While aerodynamic modifications are aesthetically pleasing and functional when properly applied, they still require careful consideration. Concessions between drag and downforce may need to be weighed, and the development of an aero package considered from all angles.
According to G-Stream, “The aero downforce created by the wing at the rear of the car must be balanced by the downforce at the front of the car. Excessive rear downforce will cause high-speed understeer, so you can only use as much wing pitch as your front downforce will permit.”
So, when you consider aerodynamic improvements for your performance application remember to approach the task with a unified goal, and seek to develop a cohesive system that play well together. If you have any questions about the route we took here, or the products used drop us a line or reach the vendors below.