Every modification a person makes to their car is something of personal preference. Sometime it's because they want their car to look more like another more expensive car or make it faster, more unique or just because they can. So when it comes to aerodynamics it's no exception. Here are some great tips from Hotrod.com.

Aero Tricks You Can Use

For a Given Aerodynamic Package, It Takes Eight Times the Horsepower toDouble Your Speed. It's Easier to Be Swoopier and Go Faster With theSame Power. We'll Show You How.

By Marlan Davis

Photography by Marlan Davis, The Primedia Archives, Manufacturers

Ever stick your hand out the window at highway speed? Do it palm-first,and the air pushes your whole arm backward. Hold your hand out palm downand you can control arm movement, even against the air pressure. Tiltyour hand upward, and air pressure forces your arm higher. Go theopposite way and tilt downward, and your hand gets pushed downward.You've just experienced the aerodynamic properties of drag, lift, anddownforce. Air flowing over, under, and around a car acts the same way.

Drag and Frontal Area

Palm-first, your hand has lots of resistance. That's why air pressurepushed it back. Engineers call this drag. Measured in pounds, total dragis a combination of the vehicle's frontal area, dynamic pressure (airdensity and velocity), and a shape factor that defines how slippery itis. The shape factor is a dimensionless numerical expression called thecoefficient of drag (Cd). Think of Cd as an efficiency factor, likehorsepower per cubic inch (hp/ci) is for engines: A go-cart engine and asupercharged big-block Chevy might make the same hp/ci (be equallyefficient), but the Rat motor's total power output is vastly greaterbecause it has more displacement. Likewise, the Goodyear blimp and apencil have a similar Cd because they have a similar shape. Thedifference lies in the overall size of the object (also known as frontalarea) that is pushing through the air. In car terms, a Corvette has botha much lower Cd (it's swoopier) and less frontal area (it's smaller)than an 18-wheeler.

Also, aero forces go up with the square of the speed, meaning that ifyou double the vehicle speed, you increase the total drag resistance byfour times. What's worse from a gearhead standpoint, AeroDyn's GaryEaker points out that horsepower requirements to reach a given speed goup with the cube of velocity. At 200 mph it takes eight times as muchpower to push the car through the air as it does at 100 mph. If your carneeds 80 rear-wheel horsepower to run 100 mph, for it to see 200 mphrequires 640 hp. Gives you more respect for those 300-plus-mphBonneville racers, doesn't it? Therefore, if top speed is the goal,anything that reduces frontal area and makes the vehicle's shape morestreamlined is crucial.

Lift and Downforce

Returning to our palm test, your upwardly inclined hand experiencedpositive lift; tilted downward it saw negative lift, or downforce.Nearly all stock cars have natural positive lift. Older cars with bluntfront ends generated lift kind of like your hand did--air getsunderneath and pitches the nose up. Pre-'68 Corvettes, old Cobras, andclassic-era GT-40s were notorious for front-end lift at speed. But evenmodern, fuel-efficient, bubble-cars have lift, paradoxically caused bytheir very efficient wing-like shape.

Positive lift is bad; it makes a car float and hard to control,especially at speeds over 100 mph. It's far better to have downforce topush the car down against the pavement, increasing its traction. "It isimportant to be able to steer the car at the speed you want to run at,"says engineer Chuck Jenckes. "You don't want a buoyant car."Unfortunately, many of the techniques used to increase downforce alsoraise drag. As Jenckes puts it, "You need one to generate the other." Itbecomes a fine juggling act--aero drag reduction from betterstreamlining versus increased drag from more downforce--and which one toemphasize is dependent on the vehicle's inherent design, its intendeduse, and (if engaged in organized racing) the sanctioning body's rulesrestrictions.

More Downforce or Less Drag?

What's more important: going as fast as possible, or being able to turncorners? The answer will dictate whether you want more downforce or lessdrag. For anything that must turn, generating downforce usuallyoutweighs any additional drag that might be generated. "You can make upmore time in the corners than on the straights," says Jenckes.Road-racers always want lots of downforce. With oval-trackers, itdepends on track length, whether the rules permit radical downforcemods, and if the cars are power-limited. Today's Nextel Cup teams payclose attention to streamlining because the rules require apower-limiting restrictor plate and closely limit downforce-generatingmods to those that equalize competition and ensure vehicle safety.Short-track cars benefit from downforce enhancers, especially becausethey are usually traction- limited, not power-limited.

On the other hand, for land speed racing, ultimate straight-line topspeed is the goal, so reducing drag reigns supreme in all cases. Thesecars need only enough downforce to hold traction and to keep the frontend planted. Finally, in drag racing you want to add some downforce fortraction, but the majority of drag-car aero mods are to enhance top-endvehicle stability and improve performance past mid-track and through thetraps. "A drag-car should have no more downforce than needed or you'rewasting your potential," says Racecraft's Mark Wilkenson.

Aero Tricks You Can Use

A multiuse street car must be a jack of all trades: stable at highspeeds, sticky in the corners, versatile enough to survive driveways andgas stations, and tough enough for weekend race duty. Realistically,there are limits to what you can do to an average car without going offthe deep end. Unless you go the route of extremecustomization--chopping, channeling, narrowing, and sectioning--it'shard to significantly improve a production car's drag coefficient. Youcould remove the driprails, molding, chrome trim, and antenna; shave thedoor handles; lose the windshield wipers and side mirrors; racer-tapeall the body seams; meticulously polish the vehicle's surface; close theside windows; lay back the windshield; and block airflow through theradiator grille (to the extent the car doesn't overheat, but quitepractical in a short-duration drag race). These things will lessen draga little bit. However, generating more downforce, even on astreet-driven car, is entirely practical, and some of the things thatadd downforce can also aid aero efficiency.

Spoilers and Air Dams

"The two hotspots where there's the most room for improvement on mostcars are the lower front and upper rear," says AeroDyn's Gary Eaker.Racers realized this as far back as the mid '60s and developed rearspoilers and front air dams to help their cars maintain traction. A rearspoiler is a device attached to the car's upper rear surface (usuallythe trunk lid) with no gap between it and the bodywork. If it has a gap,it's considered a wing, which is far more complex.

As its name implies, a spoiler's purpose is to "spoil" the fast, smooth,low-drag airflow coming off the roof. By sticking up into the airflow, aspoiler causes the airflow to detach and separate, reducing its velocityand creating a pressure rise that decreases the rear lift tendencies.Although a rear spoiler primarily adds downforce, in some situations itcan also decrease drag, depending on the spoiler's height, angle, andlength of extension off the deck. The more vertical the spoiler's angle,the more the downforce at the price of increased drag. Exact resultsvary per vehicle and can only be determined by cut-and-try testing, butsome studies suggest that the most downforce is achieved with a spoilerheight that's about 8 percent of the car's wheelbase. That means about8-9 inches for a 106-inch wheelbase. For any drag decrease, the spoilerheight will usually have to be less than 1 inch, but up to 2 inchesthere is usually no drag penalty.

Source: Hotrod.com