Splitter Length and Side Fences

In this post I’ll examine several aspects of splitter design, starting with length. Some racing organizations regulate the maximum size of a splitter, for example:

  • Supermiata S2, NASA ST6 – airdam, but no splitter
  • Supermiata S1, NASA ST5 – 4″ max
  • Champcar – 12″ max (wow)

In Race Car Aerodynamics, Katz states that the splitter length should be double the chord length, the chord being the distance from the splitter to the ground. So if your car has 4″ of ground clearance, the splitter should be 8″ long. That seems rather long to me, but this rule of thumb may depend on the shape of the nose, and for a vertical airdam, perhaps shorter is ok.

In Competition Car Aerodynamics, Simon McBeath cites a CFD study done on a NASCAR model, using splitters of 2″, 4″, and 6″. In this case the ideal splitter length was 4″, producing the most downforce, and best L/D ratio.

  • The yellow line represents the total downforce, and you can see that the 100mm splitter is just slightly higher than 150mm.
  • The blue line shows downforce increasing fairly linearly up to 100mm, and then leveling out at 150mm. So you don’t want to go longer than 6″ (at least not on this stock car).

In the NASCAR CFD study, the splitter added 10% more downforce than using the airdam alone. A Miata isn’t a stock car, and this is all calculations, so YMMV. This is in contrast to the Hancha Group’s CFD work, who’s theoretical airdam produced 34% more downforce with a splitter than with just an airdam. In my real-world testing, I found a 4″ splitter added .38 to the front coefficient of lift (it increased downforce substantially over an airdam alone) and decreased coefficient of drag by .01. More downforce, less drag, so do it.

The same NASCAR CFD study found that extending the splitter rearwards underneath the car had further benefits, and the longer the better. This isn’t surprising, because it’s effectively creating a flat bottom. The interesting part was that lengthening the splitter rearwards made the splitter less effective, because of a build-up of pressure in the engine bay. Overall downforce did increase, but this was because of body interaction. The CFD model was revised to add vents in the hood, and then the splitter and underbody panel both made more downforce. Hood vents are not just for cooling!

Another unexpected result comes from Competition Car Aerodynamics. In Chapter 9 McBeath explains the wind tunnel work done at MIRA using a championship winning Integra Type R (which always makes me want to go all Dor-Dori and shout “Inte-R!” in a Japanese accent). In the wind tunnel, they experimented by putting different ends on the splitter, with ramps of different sizes, and with and without a vertical fence on the end. Each time they measured the result for drag and lift.

The configurations were as follows, and correspond to the image below:

  1. Shallow ramp.
  2. Baseline configuration with no ramp or fence.
  3. Medium ramp.
  4. High ramp.
  5. High ramp with vertical fence.
  6. Vertical fence alone, no ramp.
  7. Vertical fence, shallow ramp.

In the image above, the total amount of front downforce is the yellow line, and configuration 6, a simple vertical fence, is the winner. Adding a small ramp (configuration 7, which is pictured above), or large ramp (configuration 5) in front of the vertical fence actually reduced downforce and increased drag (the black line). Who’d have thunk it?

McBeath doesn’t reveal the actual numbers (it was a private test, they hold the cards close to their chest), but he did say that the best configuration reduced total drag on the car by 4.8%, and more significantly, total downforce increased by 50%!

I’ll conclude this post with some generalizations about splitter design:

  • You can make your the splitter as long as your rules allow, but the longer it is, the more it will affect the front/rear balance. Also note that it may work just as well at a shorter length. If you want to choose a length and not experiment with it, 4″ seems a safe bet.
  • Extend the splitter rearwards as far as the rules allow. But note that this may increase pressure under the hood, and then hood vents may be necessary.
  • Extend the rearward edge of the undertray as close to the leading edge of the front tires as possible. You can generate downforce from wheel wash.
  • If the undertray curves upwards at the rear, it will accelerate the air in front of it, creating more downforce and drag. McBeath quotes a value of about 4% increase in downforce and 1.5% increase in drag. For me, the effort wouldn’t be worth it, and I’d just use a flat undertray and perhaps rake it slightly.
  • Your splitter may create over 200 lbs of downforce, and so if you can’t stand on it, it isn’t strong enough.
  • Many people use birch plywood for the undertray and splitter, but mahogany marine plywood is better. I suggest Okume or Meranti BS-6566, it’s more weather resistant than birch and 33% lighter. I’d go with 9mm-15mm thick depending on if it’s just an undertray or splitter.
  • The front edge of the splitter should be radiused on the underside to avoid separation of flow. Sharp on top, radiused on the bottom.
  • Add vertical fences on the sides of the airdam to shield the tires and increase downforce.

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