Splitter or Air Dam – Which Design is Best?

How do you choose which front end aerodynamic route you need to go? This is a tough question to answer so lets work through an example.  The example will be a 1990-1997 Mazda Miata. The Miata was chosen simply because 1. I have the model and 2. because different designs are commonly used.

CFD Models

1. Stock 1990-1997 Mazda Miata
2. Stock 1990-1997 Mazda Miata at a 4in Ride Height
3. Small Front Air Dam at 4in Ride Height
4. Small Air Dam with Splitter at 4 in Ride Height
5. Large Air Dam at 4in Ride Height
6. Large Air Dam with Splitter at 4in Ride Height

Note: The air dam and/or splitter is 2 inches off the ground in study 3-6

The solver used for these analysis is a steady state incompressible solver with a k-omega SST turbulence model. Again OpenFOAM (as always) was used for pre-processing and solving and all post-processing was done using Paraview.

Cd = coefficient of drag
Cl = coefficient of lift
L/D = lift divided by drag / aerodynamic efficiency

Drag between all setups are all fairly close with the least drag being the setup with the large front splitter. The two splitter designs also makes significantly more downforce than the other setups. The two stock Miata setups makes lift instead of downforce which is expected since most road vehicles create lift from the factory.

Note: These are numbers and trends for common designs choices. Actual designs should be more refined after design goal is formulated.

The stock Miata simulation had a calculated coefficient of drag of 0.36. The 1990-1997 Mazda Miata had an indicated coefficient of drag of 0.38. The simulated Miata has a lower coefficient of drag; which was expected from the simplification of the vehicle. The simplification of the underside has an estimated coefficient of drag decrease of 0.015. That along with the simplified wheels and no internal flow, puts the coefficient of drag between the simulation and the indicated coefficient of drag within a reasonable error. This means the simulation passes the “sanity” check to ensure validity in the data.

XZ-Plane Pressure Cut Plot

XZ-Plane Pressure Cut Plot

Ground Pressure Cut Plot

Ground Pressure Cut Plot

XZ-Plane Velocity Cut Plot

Okay, now what does all this mean???

The main question is actually a trick question since there is not one “BEST” design.  Different designs are all about compromising.  First L/D and downforce goals should be known for best track times ***This is different with different tracks and cars*** The rear aero should be decided first and then the front design should balance the car back out.



12 thoughts on “Splitter or Air Dam – Which Design is Best?

  1. Rob

    Paul, very interesting CFD study, thanks for posting it. What ride height did you use for the stock Miata (setup 1)? I’d like to know how much it was lowered when you dropped it to 4 inches for the other setups.
    I was surprised to see drag increase when the car was lowered (setup 2). I have always assumed that a lowered car presents a smaller frontal area so there would be less drag. Note also the lift (negative downforce) was greatly reduced from -106lbs to -31lbs by simply lowering the car. I believe these two results, greatly reduced lift and increase in drag, may simply products of the model’s simplified (flat?) underside.
    If you compare the stock and lowered pressure plots (setup 1 & 2) you see that the pressure above the car is almost identical so there must be downforce generated under the car to explain the majority of the 75lb decrease in lift. I believe the underside of a real Miata is simply too messy to generate almost 75lbs of downforce by simply being lowered.
    I would like to get your take on my theory and could you describe the bottom of your Miata model?
    Thanks again,

    1. paulwlucas Post author

      I see your comment, and will answer you question tomorrow or Saturday. I am doing some cfd work on our frs/brz splitter design right at the moment. I want to give you a thorough answer, and that will have to be in a few days.


  2. cord

    Hi Paul,
    Would you solve the rear aero first as a rule, and then balance the front? Wouldn’t the front in some instances create new/different issues at the back of the car?

    1. paulwlucas Post author

      Yes it would. You would still need to know how the changes effect the other parts down the car. It is hard to explain, but it is used to rebalance Cl. I will email you a picture of a graph that will explain it better than I can type.

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  4. Nick

    Would it be possible to get the front/rear split on the downforce numbers? I think it’d help to get a ballpark to balance aero when adding a rear wing.

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  6. I

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  7. Johnb284

    cheers for the actual article i’ve recently been on the lookout with regard to this kind of advice on the net for sum time proper now so numerous thanks dcefdfaekcbb

  8. Cpaulv

    I am trying to start design work on my 1998 Mercury Mystique GS I would like to know how to start to do a front splitter and air dam flat underbody and rear diffuser. Not sure what approach to take also I put on a spoiler from the sport edition of my car. Thanks.

  9. Josh

    Hi Paul,
    I was wondering if you had any resources to recommend to a completely green openfoam user. What’s the best way to learn?



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