Body - Aerodynamics
Anyone looking for advice on the coolest looking aero kit or GTR style trunk wing has come to the wrong place.
99% of everything falling under the titles of aero kit, ground effects, and big mouth front bumper, are intended
to enhance styling, with little or no effect on performance.
This section covers functional aerodynamic modifications.
Front Air Dam
The original air dam or air deflector, mounted below the radiator, on the front of the Geo Storm, is 2 inches tall.
It prevents air from going beneath the car and improves downforce. If it is missing, there is a noticeable
difference in straight line stability and grip at highway speed.
A taller OEM stule air dam can be made using 1/16 inch thick aluminum angle (one front and one back) for a mounting
bracket or base, with ¼ inch thick flexible rubber for the vertical dam, sandwiched together with a row of ¼ inch or 6 mm
bolts and fender washers to hold the sheet of rubber between the pieces of aluminum. Rubber skirt board is cheap,
but does not weather well and will break if bent sharply. 65-75 Durometer polyurethane sheet is much, much more
expensive, but will last many years. At 4 inch height (width of sheet), it will truly become a driveway scraper,
but the increased grip at highway speed is very evident.
Air dam is also the name for a front chin spoiler which extends from the lower edge of the bumper cover.
This has the added advantage of directing air around the sides of the car without creating a high pressure
area beneath the front bumper, as the OWM air deflector does.
A front chin spoiler style air dam could be constructed either by attaching a wide sheet of flexible material
to the lower edge of the front bumper, or by molding fiberglass to attach to the lower edge of the front bumper.
Canards, also called "dive plates" are triangular winglets attached to the front or rear ends of the bumpers.
They are often triangular in shape, and may have turned up sides or end plates. They are often mounted at an
angle, may be flat, or may have a very obvious curved shape turned up at the rear edge.
These function to produce down force and produce eddies and vortices along the sides of the vehicle. They
are most common to time attack, but have become more and more popular in Street Mod and X Prepared classes
of the SCCA Solo2 autocross program.
A splitter is a flat, horizontal plane of material mounted parallel to the ground, extending under the nose of
the car, and usually incorporating a front under tray or the forward part of a belly pan (see below). The design
of the splitter is usually dictated by the distance that the splitter plane is allowed to extend from the outer
edge of the vehicle, and the minimum height or ground clearance, as stated in the racing class rules. A vertical
fill panel is then made to fit between the splitter and the underside of the vehicle bumper or front valence.
The assembly looks like a front air dam with a flat semi-circular shaped extension from its lower edge.
The horizontal plane interrupts the flow of air in front of the car, reducing the force of the air on the forward
surfaces of the vehicle. The fill panel directs air around the sides of the car. Air passing beneath the
splitter encounters a venture effect which pulls down on the nose of the vehicle.
Splitters are not very practical for street use, where they will encounter bumps and driveways. But they are an
indispensible asset for track use.
Air is going to flow beneath the car as it rolls down the road, and the underside of the vehicle is not
aerodynamic. A belly pan will do a lot to smooth the flow and reduce drag. However, use in competition depends
on the racing class.
Notched Rear Bumper
The lower section of the rear bumper fascia, extending down below the bumper beam or crash structure, covers
the exhaust system and extends low to the ground (for the GSi) to match the side skirts, but it also acts as
a large parachute and creates a lot of drag because it prevents the air flowing beneath the car from exiting
and flowing easily behind the vehicle.
The best solution to this issue is a rear diffuser, described below. But some racing classes outlaw rear
diffusers. The next best solution is to notch out the rear bumper cover, removing all of the material
below the bumper beam and crash structure. This will allow the air to exit from beneath the vehicle, and
provide some additional down force, though not nearly as much as a diffuser.
This is a rather drastic modification meant for racing, and it goes against the accepted rationale that
the lowest rear bumper with the most elaborate sculptural shape is the "best" as seen on the highest trim
level model. But from a performance standpoint, the Base/SOHC rear bumper provides the least aerodynamic
drag, and cutting the bumper skin further will enhance the performance gain even further.
A rear diffuser is an extension of the belley pan to the rear of the vehicle, curving upward as it exits the
rear bumper, and using vertical airfoils to confine air flow to enter from under the vehicle to exit behind
the vehicle. The idea is to pull the air from under the vehicle causing a vacuum that sucks the car down,
increasing downforce more on the rear axle than anywhere else. Functional rear diffusers usually involve
notching out a horizontal line across the rear bumper as high as possible, and enclosing a tunnel that
starts at the lowest point of the rear suspension, and meets at the edge of the notched out bumper. Then
adding vertical airfoils, evenly spaced across the upwardly angled tunnel.
Rear diffusers can provide the same quantity of rear downforce as a large, high mounted rear wing (covered
Those in the muscle car crowd seem to spend a great deal of time making fun of rear wings and spoilers on front
wheel drive cars. There is some deserved criticism to the gaudy, fashion centered GTR wings of the late 90’s
and early 00’s. But any flat assertion that a rear spoiler or wing is useless on a FWD car, is ill informed
At highway speed, more than 60% of the power used by the vehicle is to overcome aerodynamic drag. A significant
portion of that drag is the air flow at the trailing edge, or back of the vehicle, where low air pressure and
turbulence cause drag. A spoiler interrupts the flow of air at the back of the vehicle, moving the flow farther
away from the tail of the vehicle, which reduces drag. Back when car magazines published Coefficient of Drag
stats in their car reviews, the .cd for base models was typically .03 (~10%) higher than the sport model or
high trim model with the rear spoiler. This kind of reduction in drag is just as beneficial to FWD cars as it
is to RWD cars.
A wing is a spoiler that has been lifted from the mounting surface. It allows airflow both over and under, and
provides more down force with less drag. A small, horizontal wing, can perform the same function as a trunk
spoiler, interrupting airflow and moving turbulence away from the rear of the car to reduce drag. But there
are situations that call for increasing rear down force.
A suspension setup for racing a FWD car will likely involve rear toe out and a very stiff rear suspension, in order
to reduce oversteer and improve rotation. These changes will likely cause issues with cornering stability, which
can be easily remedied with increased rear downforce provided by a rear wing.
A FWD car with very little weight on the rear axle may have rear brake lockup issues, which may be resolved with
a increased rear downforce provided by a rear wing.
While effective and functional, a high mounted, pedestal style rear wing will likely draw too much of the wrong
type of attention on the street.
SCCA Super Touring club racing classes and X Prepared Solo/autocross classes regulate wing type and size, with
choice limited to 3d carbon fiber wings. (There is a 48 inch width limitation for wings used in STU Class).
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