Ground Effects and Aerodynamics

Many people out there think if your car has "ground effects",it is more aerodynamic than cars without them.Except for a few cases,this is far from the truth.Aerodynamics seem tricky to most people,the area is probably the easiest area to understand,but the hardest to improve.I’ll explain what aerodynamics are,how to improve them,and how most ground effects decrease the aerodynamic efficiency of a vehicle.

Aerodynamics is simply how air flows over,under,and sideways a vehicle,whether it be a car,truck,airplane…etc.It is comprised of two main components:

CD (coeffient of drag)
Frontal Area (sq feet or sq meters in europe)

Coefficient of drag is the most widely used number in relating the Aerodynamic efficiency of a car,and is very misleading.It is just a measure of how much out of 100%(like a barn door) a vehicle lets air flow around it.Typically sports cars have coefficients of between .28 and .45(Viper roadster uggh!).The better(lower) the number is,the easier it is for air to pass around a car.It can be misleading because I recently read an ad in MT,and it claimed that the ’97 Infiniti Q45 has the best CD of any production car(I think it was .27).You don’t realize that the car has 10" more height,and a few inches more width than a third generation f-body(length isn’t a factor).This means that it has more sq feet of air to push out of the way.I think that CD is slightly less important than Frontal Area,because like I said it can be misleading.It is hard to decrease CD on an f-body without cutting it up.

Frontal Area discribes how much area in sq feet a car has to punch a hole through the air.The frontal area on a third generation f-body is between 24 sq ft (IROC or TA)and 21 sq ft(base camaro or firebird,no spoilers).These figures are among the best of any car made ever,but improvements are possible.A good example of frontal area is the Mazda Miata.The car has a greater CD than an f-body,but because its frontal area is tiny (I think it is 16.5 sq ft),it is more aerodynamically efficient.I use an equation to compare cars with different CD and Frontal Area figure,the number generated is merely a factor to be compared 1:1.

CD x Frontal Area(sq ft) = factor #

It works well for direct comparisons,because doing it the long way would take 10 minutes to compute(true engineering method).

Another area that should be mentioned is the phenomenom of Downforce.You may hear a lot if you are a Nascar,IMSA,Formula 1,NATC car fans.Downforce is simply how air affects the weight(relative to ground) of a car in motion.It can be negative,neutral,or positive depending on the car and application.Street cars are typically neutral,some are slightly negative(lift).If your car feels good at speed(120 plus),then it is neutral.If your car feels "light" at speed,it is lifting,a characteristic of negative downforce.Very few cars that are driven on the street have any measureable amount of positive downforce. There are two easy ways to increase downforce:wings and/or spoilers. Wings are used by Formula 1 and Indy cars(easy to see).Wings are simply turned over airplane wings,used to push the car to the ground.For street cars,they are so impractical that you should forget about it. Spoilers,both front and rear are typical of NASCAR racecars.Front spoilers that extend lower to the ground produce more downforce on the front tires,shorter ones produce less.Rear spoilers on these cars increase donforce on the rear of the car,the larger the spoiler is(height),the more downforce it produces.NASCAR racing has certain restrictions for spoilers that must be followed,but they are as crucial as suspension tuning for being competitive at 190+ mph.F-bodies,especially camaros have a rear spoiler similar to NASCAR racers,though not as tall,taller spoilers are available and are practical for street cars.The Firebird/TA rear spoilers are as effective,but do not resemble NASCAR rear spoilers.Front spoilers are where IROCs and TAs have an advantage in front downforce,they have lower front spoilers that produce more downforce than base camaros and firebirds.The bad side to tall rear spoilers is that they increase the CD of a vehicle,Lower front spoliers increase the frontal area.You need to balance the spoilers to get the optimum downforce,CD,and frontal area(I told you it was hard to achieve).

The less frontal area you have,the less air there is (sq footage) to push around the car.Every little reduction in frontal area helps.One typical way is to lower your car at least 1".This cuts 1" of tire cutting into the air,on a car with 245 tires,you reduce frontal area 11 sq inches per side,22 total(only 2 front tires count).Double that if the car is 2" lower.There are 144 sq inches in a sq foot.Another way to reduce frontal area is to have a smaller front spoiler,it decrease frontal area,but also decreases downforce on the front tires.Skinnier tires(narrower) also reduce frontal area noticeably.Removing your side mirrors(don’t expect not to get pulled over and pass inspection) also decreases frontal area.Otherwise there aren’t too many mods even worth mentioning that won’t have you cutting parts off your car.

The lower the CD,the easier it is for air to pass around your car.A typical way to decrease the CD of a car is to "smooth" it out,or cut obstructions that air "catches" onto as it passes around your car.Removing the rear spoiler on a car will improve CD,but decrease rear wheel downforce.Removing rain guards(above windows),smoothing doors(removing door handles and locks!)…etc.There really aren’t too many areas to improve the CD of an f-body,but the one area that needs the most help is almost always ignored:the undercarriage.Look under an NSX,911,or rear engine Ferrari and you see a smooth undercarriage-no exhaust pipes,driveshafts…etc.This is the reason why these cars have high top speeds with modest hp.The only way to improve aerodynamics of the undercarriage on a street car is to use "belly pans".Belly pans smooth air under the car,decreasing drag and improving top speed(usually).I ahve been seriously thinking about how to do this,one covering the front suspension up to the front crossmember,a rear one from the rear seat area to rearend,and another from the rearend(other side) to the rear bumper should work very well.If I can manage to keep ground clearance good,it might be commercially sellable.

If you were to look at aerodynamic force,it quadruples for a doubling in vehicle speed. It is hard to explain,but is just a law of physics.Weight factor does stay the same merely doubling with a doubling of vehicle speed,and at higher speeds it becomes a smaller factor than wind(air) resistance.It takes about 100hp for a stock IROC to reach 100 mph(maybe more).It takes about 200hp to get the same car to hit 150mph.If you figure that at 100mph,the car uses 40hp to push the weight,and 60hp to push the air around the car.At 150mph,you need 60hp to push the weight,and 140hp to push the air around the car.At 200mph,you need 80hp to push the weight,and 240hp to push the air around the car,a total of 320hp.These figures are not accounting other factors,but are merely meant to show how aerodynamics are important to top speed.By the way,320ho will not push an IROC to 200mph,otherwise there would be a lot of 200mph cars.By improving the CD and frontal area of a car,the same 200hp may push the car to 155 or 160mph,and the 200mph power needed would be lower by quite a bit,acceleration at higher speeds would be improved also.Lowering vehicle weight has less of an impact on top speed than improving air flow around the car. Had enough?

Most ground effects are meant to please the eye,not improve airflow around a car. Larger(lower) front spoilers increase front downforce,taller rear spoilers increase rear downforce;at a penalty of higher CD and larger frontal area.The "side skirts" of a ground effects kit are totally irreleavant to aerodynamics,neither hurting or helping air flow on a street car;they do look good though.The rear bumper cover can hurt or help air flow.If you have a low rear bumper cover(without a rear belly pan),it acts as a parachute,slowing a car down.

For practicality sake,a regular camaro(or firebird) with the same exact weight as an IROC(or T/A) will have a higher top speed with the same power than the IROC.But,the IROC will(theoretically) handle better at the same speed because the spoilers force the car to the ground more.Its a trade off,but make sure you know what you want before making a decision to get or remove spoilers,otherwise you may:put the car off the road at speed;or may need more power than you think to reach top speed.