Air Dam or Splitter - A Closer Look

Which Device is Better?

Note:  This data was previously posted on another blog by one of our members.  This is the same data as that post; but now under Verus, as he works with us and it is personal data and knowledge that we feel should be shared with the public.  The more people that better understand aero, the more likely people can pick out the good from the bad or design their own.

In this post, we hope to give you a glimpse into how different front-end aerodynamics effect a streetcar.  This is a tough situation to discuss without an example.  The example will be a 1990-1997 Mazda Miata. The Miata was chosen because we have the model, different designs are commonplace, and our friend asked us to help him design an aero package.

Verus Engineering Factory Mazda Miata CFD

Factory NA Mazda Miata CFD

Verus Engineering Factory Mazda Miata CFD Lowered

Factory NA Mazda Miata Lowered to 4in Ride Height CFD

There are 6 total cases we study.

  1. Stock 1990-1997 NA Mazda Miata

  2. Stock 1990-1997 NA Mazda Miata Lowered to a 4 Inch Ride Height

  3. Small Front Air Dam at 4in Ride Height

  4. Small Air Dam with Splitter at 4in 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 analyses was a steady-state incompressible solver with a k-omega SST turbulence model. OpenFOAM was used for pre-processing and solving and all post-processing was done using Paraview and excel.

Verus Engineering Miata Air Dam and Splitter Data

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

Downforce: Negative numbers indicate lift, positive numbers indicate a downward force.

Drag between all setups is all fairly close with the least drag being the setup with the large front splitter with air dam. The two splitter designs also make significantly more downforce than the air dams alone. The two stock Miata setups make lift instead of downforce.  This is expected since most street cars create lift from the factory.  These are numbers and trends for common design choices. Actual designs should be more refined after a design goal is formulated.

The stock Miata simulation (CFD run) had a calculated coefficient of drag of 0.36. The 1990-1997 Mazda Miata had a published coefficient of drag of 0.38.  The CFD case having slightly lower drag is expected from the simplification of the vehicle vs. the real car. These simplifications on primarily the underbody, the wheels, and no internal flow, decreased the drag coefficient from published values by 0.015.  To us, this puts the coefficient of drag between the simulated value and known value within a reasonable error to continue onward.  To reiterate, it passes the “sanity” check to ensure validity in the data.

Verus Engineering Miata Air Dam and Splitter CFD
Verus Engineering Miata Air Dam and Splitter CFD

These photos really emphasize how different setups affect the pressure at the front of the car and midway on the car.

Verus Engineering Miata Air Dam and Splitter CFD

These pictures show how setups influence how fast the air moves around the car.  What is interesting is how much the setups on the front change the airflow behind the car.

Take a close look at all of the photos.  Each one paints a different picture and changes how the car reacts when driven at speed.  So, which one is best?  There really is no “best” design.  There are compromises for each design.  Multiple things should be considered when designing a system.  Things like L/D, downforce, and aero balance should be considered.  These can and likely will change from track to track and from car to car.  It is generally recommended to decide on the rear aero first, and then design the front to balance the car back out.

Please let us know if you have any questions below.  Thank you for your time.