F-Body Front Splitter Design And Build

The Purpose

This post will detail my design process for the creation of a splitter for my 1982 Trans Am. I’m building a splitter for two reasons; the first being that I want to. I have spare sheet metal and spare time, so why not make something productive.

The second reason is out of necessity. Thirdgen F-Bodies use a cooling method referred to as “under-breathing.” Where “Front Breathing” cars use a grill on the front fascia to draw in air for the radiator, “under-breathers” don’t. Thirdgens don’t have much in the way of a grill on the front bumper. Instead, the air which travels under the car encounters an air dam, which pushes the air up, through an opening, and into the radiator. As you can see below, my air dam is a bit battered, but not anything major. When cruising at 70mph, my engine temps are reaching a bit higher than I am comfortable with (Probably because I replaced the manifolds with headers), so something must be done. So I figured, why not improve on what I have, rather than replace with something standard.

Battered Air Dam

Design Constraints

This is not a fully fledged race car. In fact, it’s not even a partially fledged race car. Actually, it’s not even a race car. So what ever I design, it has to be fully usable on any street I may encounter, Speed Bumps Included. This means it probably won’t make any difference when it comes to driving, even though it may increase downforce of the car. The main purpose is to increase cooling of my engine for long highway drives in the upcoming summer. That being said, most of the design work will go into maximizing the downforce while minimizing drag, and the added cooling will be a byproduct. Counter intuitive, but roll with me on this.

ALSO! While I have welding experience, I have no welding equipment, so everything will have to either be bolted, riveted, or JB Welded together. Preferable bolted or rivets, as I would like this to last.

Design Factors

Being in my 3rd year of a 5 year Aerospace Engineering program, I have a little experience when it comes to Aerodynamics. However, I won’t be going all out with tons of complex equations and force balances and whatnot. I plan to do most of this design by feel, based on what I learned in class and my job as a lab technician, and hope for the best. That being said, looking at Reynolds numbers and Cl vs. AOA diagrams won’t be out of the question.

That being said, here are the factors I plan to take into account during this design process:

1. Length of Splitter
2. Angle of Splitter
3. Splitter Bracing

The splitter has to be long enough so that it actually grabs more air to shove into the radiator, but not so long that it scrapes the ground on every bump. The angle will determine the coefficient of lift, and the shallower the angle, the longer the splitter can be before it hits the ground. The bracing of the splitter will ensure it maintains it’s shape during all driving conditions. As I’m using a rather flexible sheet metal, bracing will be imperative to ensure it maintains the form I give it.

Diagram of Stock Setup

Above is a diagram of the stock “underbreather setup.” Below is what I expect the airflow to be like. I don’t have any windtunnel data, this is just what my intuition tells me. (I’ve not considered any recirculation zones, but I’m sure there may be a couple)

Airflow Diagram

Pressure Diagram

The pressure Diagram shows where I would expect basic High and Low pressure zones. A high pressure zone will push away, while a low pressure zone will pull towards. Based on these principles, I created another diagram showing force arrows (NOT RELATED TO MAGNITUDE, JUST DIRECTION) showing the general direction I expect the force to point.

Force Diagram

As you can see, there is a high pressure zone at the front of the car, a high pressure zone in front of the air dam, and low pressure zones below the air dam and behind the radiator. This difference in pressure across the radiator is what causes air to move through it. The results in a force pushing up on the nose of the car, actually creating lift.

Adding a Splitter

By adding a splitter, I expect the diagrams to change as follows:

The Airflow Around the Splitter

Pressure Zones Around the Splitter

Force Directions Around the Splitter

So by adding the splitter, I hope not only to increase the airflow being drawn into the radiator, but to also add surface area for the high pressure zone in front of the air dam to push down on, thus increasing down force.

Actual Aerodynamics

Lift and Drag

There are two aerodynamic factors that I must consider during this design: Lift and Drag.
Lift is the force pulling up on a wing (Or down in the case of down force) and Drag is the force opposing velocity

To calculate Lift and Drag, I use the equations:
L = (Cl)(0.5)(rho)(V^2)(A)
D = (Cd)(0.5)(rho)(V^2)(A)
Where:
Cl is coefficient of lift
Cd is coefficient of drag
rho is air density (1.225 kg/m^3 for sea level)
V is velocity
A is area (Length of plate multiplied by the width)

The coefficients of lift and drag are depended on Angle of Attack, which in the case of the splitter is the angle the plate, as well as Reynolds number. For those who don’t know, a Reynolds number is a number used in Aerodynamics to relate flows to each other. The equation for a Reynolds number in this case is as follows:

Re = (V)(c)/(v)
Where
V is velocity
c is chord length, or in this case the length of the splitter
v is the Kinematic Viscosity of the fluid, in this case 1.983E-5 (m^2/s)

Actual Maths

Chord Length

As previously mentioned, the length of the plate is known as the chord, and this is dependent on the height I want to have between the bottom of the splitter and the road surface. The current air dam extends 2 inches, and leaves 7 inches of ground clearance. Insert Trigonometry Essentially, that means if the chord is the hypotenuse of the triangle that makes an angle Alpha with the car, than the total height of the splitter will be sin(alpha)*Chord, or chord = height/sin(Alpha). According to Wikipedia, the average speed bump in America is 3-4 inches in height, meaning the stock air dam leaves at minimum 3 inches of space. So to prevent scraping and allow for any mistakes construction, I’ll say my max height is 3 inches (0.076 meters) which will give me 2 inches of clearance for any speed bumps I may encounter.

Therefore, the length of my splitter is 0.076m/sin(Alpha).

Reynolds Number

So I want data for Highway speeds, so V=70mph, or 31.3 m/s.
Re = (V)(c)/(v)
Re = (31.3)(0.076m/sin(Alpha))/(1.983E-5 (m^2/s)).
Re = 119959(sin(alpha))
An alpha of 40 degrees would result in a Reynolds number of 7.7e4, 30 degrees of 6e4, 20 degrees of 4e4, and 10 degrees of 2e4.

XFoil

XFoil is an aerodynamics application that takes airfoils, and calculates the various coefficients at various reynolds numbers, angles of attack, and whatnot. By looking around some RC forums, I found that at higher angles of attack, the NACA 0012 airfoil behaves similarly to a flat plat. Therefore, i used a NACA 0012 airfoil in XFoil to get the following data
RE————AOA———-CL———CD———-Chord
2e4————10————0.43——0.12———-0.437m
4e4————20————0.82——0.25———-0.222m
6e4————30————-1.13——0.37———-0.152m
7e4————35————-1.25——0.44———-0.133m
7.7e4———40————-1.33——-0.45———-0.118m
9e4———-50————-0.67——-0.51———-0.099m

Cl vs Cd at various Reynolds Numbers

Data Analysis

Now, this is only a simulation on an airfoil that is not exactly the one I’m using, so I have to take these results with a grain of salt. However, it does tell me that going higher than 40 degrees is getting close to the stall range (Where Cl drops off, but Cd still increases). As such, I’m arbitrarily going to choose 40 degrees. It’s the peak of Cl, but also the highest Cd. However, the angle between the Splitter and the stock air dam will only be 50 degrees, so it won’t be sticking out too far.

Lift and Drag

The length of the air dam is about 70inches/ 1.78m (It’s not exactly straight), so that will be my width. My length, calculated with the previous equation will be 0.118m/4.64inches. Therefore, my total area is 0.21 m^2.

With a angle of attack of 40 degrees below parallel, Area of 0.21m^2, and velocity of 31.3 m/s, I can run my calculations for lift and drag:
L = (Cl)(0.5)(rho)(V^2)(A)
L = (1.33)(0.5)(1.225)(31.3^2)(0.21)
=168 Newtons= 38 lbf
D = (Cd)(0.5)(rho)(V^2)(A)
D = (0.45)(0.5)(1.225)(31.3^2)(0.21)
=57 Newtons=13 lbf

So I can estimate that this splitter will have the effect of a bag of salt being placed in the engine bay. Nice.

Are We Done With The Maths?

Yes, we’re done with the maths. Might’ve been a bit excessive for the actual thought being put into this, but it did want to have at least some reasoning as to why I did what I’m about to do, regardless of how off base or full of errors as it may be.

Onto Design Work

The material i’m working with thin stainless sheet metal originally designed for use in ducting. It’s very thin and a bit bendy, but it can be cut with my two sets of tin snips, which is good, because I don’t have a vice to hold it while I cut it with a hack saw.

Cut Out Template

The template for the Splitter consists of four pieces, each with a 4.64 inch section and a 1.25 inch section separated by a 40 degree bend. Each section is the width I measured for either the center section or the side sections.

Coming Together

After all four were cut out, I mocked them up against the stock Air Dam to check widths. It looked pretty good, but I wouldn’t know for sure until I made the necessary bends. Unfortunately, I don’t have a vice or a clamp, so I really can’t make a good angle bend on my own. Luckily, I’m friends with the owner of a machine shop, who helped me make some bends that are about 40 degrees, give or take. Their bender didn’t have any method for telling how far it was bent, so I had to guess, than check it with my protractor.

Bracing

The Bracing I made are designed to stop the Splitter from bending from the force of the air, but also not disturb the original Air Dam. Replacement parts are a hard thing to come by for the early Firebirds, so I tried not to make any cuts into the Air Dam. The Splitter will be mounted “around” the Air Dam, just in case it fails.

Bracing Design

The Braces are triangles cut at a ~50 degree angle (Angle between vertical and the Splitter if the angle between horizontal and the Splitter is 40 degrees) with a cutout for the stock Air Dam. The L-Brackets are made of aluminum and are riveted to the front of the Splitter, and bolted to the metal that sticks out behind the Air Dam. There are four brackets in total. One on each end of the two center pieces.

Finished Mounting

The two side bits don’t exactly fit with the geometry of the mounting points on the car. There is a large bend on either side of the center pieces that I didn’t account for, so for now I’m leaving them off. As a result, I’m losing half of my total surface area for downforce (So only a half a bag of salt in the engine bay), but not any surface area below the cooling duct. Acceptable losses.

Close up view

Far Off View

All Done!

Well, almost all done. As you can tell, the bends weren’t perfect so the tips of each side don’t exactly end at the same height. I haven’t added a piece in the center yet as I wanted to check to make sure this whole setup worked before I put anymore effort into it.

Testing

As for testing, I did go out on a long highway drive, and I did cruise at 70mph for a majority of it. However, it was in the mid sixties, so I wasn’t in any danger of overheating anyway. I’ll have to take it out at another time to check that aspect.

I did find some speed bumps, and my splitter did not hit them, so that’s a plus.

Final Results

After three days of cutting, bending, drilling, riveting, and bolting, I’ve got my self a mildly decent looking splitter which hopefully improves airflow to the radiator. I’ll have to see how it looks after I modify it aesthetically, as the pieces not lining up kind of puts me off. I also want to see what affect it has on hot weather driving before I do anything major. If anything, it will serve as a template for redoing the whole thing in lightweight plastics. For now though, I am satisfied.