Nissan Spec-V: Part 22: Improving Aerodynamics Part 4
8/10/2007
It has been a long time since our last update. Progress was delayed due to problems in obtaining some one-off prototype parts. At last things are looking up for project Time Attack. Only a few more details lie between us and the track.
Since our Spec-V is somewhat underpowered (since the industry has only done minimal development for the QR25 motor) we are trying to exploit the racing experience we have to give us every small edge. The biggest example that we have paid special attention to is aerodynamics since it is often ignored or misapplied by most tuners.
Possibly the most critical element of any race car’s aerodynamics is the rear wing. Nearly all wings supplied on the aftermarket are inefficient, non-engineered designs, meaning they have a great deal of drag for the downforce created and are built primarily for aesthetics. Aerodynamic Solutions, the Motorsports Aerodynamics consultants that have helped us with this project designed a custom rear wing for us after studying the cars shape and power level.
The wing is made of strong prepreg dry carbon fiber laid over a foam core to the dimensions that Aerodynamic Solutions specified for us. This type of construction is exceedingly strong, light and stiff. The wing also uses large carbon end plates. We chose large end plates because they give the car more directional stability at high speeds. Since a front-wheel drive racecar is set to oversteer easily at low speeds, we felt the larger endplate would help maintain directional stability on high speed turns to keep our car from getting twitchy even though the chassis is tuned for oversteer. The wing is designed to give 300 pounds of downforce at 100 mph while only having 20 pounds of drag at the same speed. Most aftermarket wings cannot claim such outstanding performance.
Since the wing has so much downforce, if it were simply bolted to the trunk lid, it would probably tear it off. To prevent this, Brian Kono of Afterhours Automotive built a support structure from lightweight thinwall chrome molly tubing and welded that directly to our chassis. Our wing mounts were cut from 6061 aluminum plates and pass through slots in the truck lid where they bolt directly to this structure. Kono made our wing mounts adjustable so we could easily change the wing’s angle of attack to adjust the downforce for different tracks and conditions. Another advantage of our wing is that it will change the flow field around the car and activate more flow through our car’s underbody diffuser, creating a greater pressure differential under our splitter, resulting in increased downforce over the entire car rather than just the back.
Kono also built some carbon fiber canards for either side of our nose. Contrary to popular belief, canards do not create tons of downforce (like dive planes on a submarine), but actually create vortexes low on the side of the car that help prevent air from spilling under the car from the sides, reducing the effectiveness of the splitter and rear diffuser. The lips we added to our sideskirts in part 21 enhance this action. The vortexes actually help seal the bottom of the car off like the ground dragging side skirts seen on racecars during the mid-’70s. This helps make the rear diffuser and the splitter more effective in creating downforce, especially at lower speeds. In our car, the low pressure zone created behind the passenger side canard also helps evacuate air flowing though our engine oil cooler.
Normally it is against SCCA and NASA road racing rules to have side windows; however time attack has no such rules so our car has full Lexan side glass to minimize drag. To prevent from overheating the driver, we have a forced air breathing system that feeds the driver fresh air from a NACA duct in the passenger side rear window. Hopefully this will be enough ventilation for the driver, but if it isn’t, we can add a WRC style roof scoop.
Following some advice from fellow Nitto team members, the Berganholtz brothers, we cut vent holes in our rear carbon trunk lid to minimize pressure buildup and assist ventilation inside.
According to Aerodynamic Solutions 25% of the total amount of drag a Honda CRX has comes from the side view mirrors. The mirrors could create more drag than our rear wing. So, to reduce mirror drag and to save some weight, we replaced our heavy factory mirrors with some lightweight dry carbon aerodynamic mirrors from SPA Techniques. These mirrors weigh about one tenth as much as the stock mirrors. Unfortunately due to our low seating position and the position of several bars in our roll cage we could hardly see from our new mirrors, so Kono fabricated longer stalks to mount our mirrors on. This also lowered aerodynamic drag by reducing skin drag effects between the mirrors and the car’s body.
Hood pins also contribute large portions of drag to a car’s shape. Tuft testing has revealed that due to the location of the pins in the front of the hood, an area of high flow and close proximity to a boundary layer separation, they cause high amounts of turbulence. To avoid this we used Coast Fabrication’s aero latch system to give us positive locking and minimal drag.
Now we are past the hard part of getting the car race-ready. Next we will work on cosmetics and graphics, do our initial chassis set up, and prepare for testing on the track.
Our Aerodynamics Solutions rear wing is made of dry carbon and is very light and strong. It has an engineered profile and creates a lot of downforce with minimal drag.
Our large carbon end plates act like feathers on an arrow and help give the car high speed stability.
Our wing mounts are adjustable so we can tune the downforce in the rear.
The adjustable wing mounting structure places the downforce load right to the chassis preventing the carbon trunk lid from caving in or tearing off.
The canards create a low pressure zone which assists the outflow of hot air from our oil cooler. 
The carnards and splitter work together to increase front downforce. The splitter works by pressure differential over its surface and the canards create vortices to improve underbody flow making the splitter more effective.
The front of the car is full of aero tricks, a splitter, canards and hood vents which manage airflow to improve downforce. The passenger side hole in the bumper feeds the oil cooler while the driver’s side feeds a ram air box for the engine. 
Side skirts with a lip acts to help prevent air from curling under the car, reducing downforce.
Our rear diffuser has vortex generators in it to improve its effectiveness, especially at lower speeds. The vortex generators are the carbon vertical dividers in the diffuser. The wing, canards, splitter, belly pan, hood vents, side skirts and rear diffuser all work together to help hold the car to the road.
The NACA duct feeds fresh air to a blower/filter which sends clean fresh and cool air to the driver’s helmet.
The holes in the carbon decklid help relieve internal pressure and encourage airflow in the driver’s compartment.
SPA’s carbon GT mirrors have much less drag than stock and are a fraction of the weight.
Aerolatches are a big contributor for drag reduction over hood pins.
Our car is looking pretty beautiful; coming...
...and going.