BMW Race Car Aerodynamics Part 1: Practical Splitter and Air Dam Design

As the novice racer attempts to make their time attack or race car faster there tends to be a learning curve. The intent of these articles is to help the track day enthusiast or amateur racer avoid some of the common mistakes while giving general guidelines that have been proven to work for our team. The common theme we have found at track day events is that people tend to think of aerodynamic parts like engine parts. However, like engine upgrades, aerodynamic upgrades need to be thought of as a total package. Just as there is little benefit to adding a large header and large ported throttle bodies to your engine without porting your cylinder head, it really doesn’t make much sense to add a large diffuser to the rear of your car without the proper aerodynamic devices leading to it. To give some practical guidelines for designing and applying aero to your BMW we’ll start from the front of the car and work backwards.

Splitter and Air Dam

Although most people don’t think of this component after they have already added a giant rear wing, it really is one of the most important pieces. The splitter is a simple concept with powerful results. Most people understand that the splitter serves to create a clean separation between the air going above the car and the air going below the car. This performs several aerodynamic functions:

  1. Allows the air traveling underneath the car to move across a smooth flat surface, increased speed results in decreased pressure under the splitter.
  2. The air traveling above the splitter will hit a vertical surface (bumper or air dam) and become turbulent. This turbulent air is moving slow, creating a high pressure area.
This seems simple, but its complicated by real world considerations like:
  • How high should the splitter be off the ground?
  • How long should the splitter be?
  • What material should it be made of?
  • How should it be attached to the car?
  • What angle of attack should it have?
To answer some of these questions we will give some general guidelines. The lower your splitter height is, the less air you will have traveling underneath the car. Less air means less lift. So we want to put our splitter as low as possible right? Not exactly. If you’re a curb jumper and use an expensive carbon fiber splitter you probably won’t want to be replacing it every weekend. Conversely if you want to set your suspension up very stiff and stay off the curbs you might benefit from an aggressively low splitter. For endurance racing we have found that 3″ is a good starting point, this allows us to hop over medium sized curbing and go off track every once in a while without risking catastrophic damage to the splitter. However, they will grind down overtime and we recommend the addition of some 1/8″ aluminum skid plates on the corners to take some of the curb wear.
Increasing the length of the front splitter will create more area for the stagnant high pressure air to push down on the front of the car. This of course requires that you have a proper support structure at the leading edge of the splitter. We recommend some adjustable length steel cabling, this allows you to adjust the height of the leading edge of the splitter as well as allow the splitter to move in the upward direction (when it hits the ground) but keeps it stiff when being pulled down. For most BMW applications (unless you have a huge wing) 3″ length from your vertical air dam is a good place to start. If you run a small wing you may want to decrease this, if you have a large wing you may need a longer splitter.
Build material is mostly determined by budget. A popular cheap option is plywood. An upgrade from there would be a material called Alumalite which is a sign board material made from two aluminum sheets sandwiching a plastic which turns out to be pretty strong and durable without breaking the bank. And perhaps the best and most expensive option is carbon fiber. Carbon fiber is obviously ideal for several reasons: it is strong, light, and most importantly stiff. The stiffness of your splitter is very important. However, to accomplish this you don’t necessarily need to use carbon fiber. Plywood with an aluminum skeleton structure bolted to the top of it could provide sufficient stiffness for a splitter. Increasing the number of attachment points to the frame of the car also helps. At speed your splitter will act as a plunger sucking itself to the ground. The less the splitter bows the more effective it will be.

Behind the front bumper we prefer to use rigid fasteners. Typically what we do is build a solid structure to the frame rails or bumper support beam and use DZUS fasteners to attach the splitter to those rigid pieces. Depending on the material you’ve chosen you may want to have more or less attachment points. For our Alumalite splitter we have found that 4 rear attachment points and 4 frontal attachment points is sufficient.
And finally what angle of attack should be used. This is a pretty grey area and some people have pretty strong opinions here. We keep it simple and use an angle as close to zero as we can. Why do we choose zero? Because we don’t have access to a wind tunnel. If you’re reading this, chances are, you don’t either (if you do please give us a call!). This fact means we have to be content with relying on the theory and or application that has been proven by our predecessors and the teams that have developed designs that are most applicable to our vehicles. That means choosing to model our aero after professional race cars that bear the most similarity to ours, (not open wheel F1 cars).
And finally, don’t forget your air dam. The splitter needs to have a vertical wall on the top side for the air to hit and stagnate. Typically this will just be a vertical extension from the top of your splitter to your front bumper. It’s important to make sure that your splitter doesn’t bend down and create a gap between the splitter and air dam for the air to pass through, this will destroy your downforce on the leading edge of the splitter.
Our next article will go over dive planes/canards, stay tuned.