Designing a Race Car Splitter For My NB Miata

Why a Splitter?

The NB Miata isn't known for front end downforce. At speeds, the front end actually makes lift, worsening handling, and decreasing performance. I've already installed coilovers, tires, etc. all of which are aimed at increasing the car's performance. A splitter is a logical next upgrade that will improve tire grip and increase stability.

This blog will describe the primary steps to the splitter build. Specifically, I'll discuss design, materials, and process.

What Does a Splitter Actually Do?

Before jumping into designs, I made sure I actually understood what I wanted to design. I did this by revisiting an older blog post explaining race car aero.

In short, a splitter works by splitting the air at the front of a car. This creates a high pressure zone above the splitter, and low pressure zone below the splitter, creating downforce.

A good splitter must:
• Be flat and stiff
• Extend forward and ideally rearward under the car
• Seal tightly to the bumper to avoid air spill
• Be supported properly

Design Goals and Constraints

Here's what I want from my splitter:
• Functional downforce, not just aesthetic
• Removable, for daily drivability
• Durable, because this car isn't babied
• Affordable I'm a college student; need I say more?

Constraints:
• I don't want to cut or drill anything structural
• I want to preserve ground clearance
• I want to match the heigh of my side skirts
• I need something that holds up at 100+MPH

Materials

While I considered many materials, I've settled on carbon fiber. Although more tedious to design and build, I want to get more practice laying carbon fiber, and carbon fiber will give me an awesome look.

I'm not completely set between forged carbon fiber or twill weave, but I'll most likely settle with twill weave, as it's stronger and mostly simpler to lay.

Just like the in wing project, I've opted to use extruded polystyrene foam(XPS) for the splitter internals. This will be plenty strong, and will work well with carbon fiber. I've opted for 1/4" foam, which will offer a solid core.

I'll use 1 layer of 5.7oz 2x2 twill weave on the top layer of either side, and 2 layers of 11oz 2x2 twill weave in between, which will result in 0.044" of carbon fiber on each side, which is on par with track-level splitters.(Usually 0.03-0.05in)

I'll lay a 3/8" steel bar horizontally into the splitter, where it mounts with the car. This will increase overall strength and provide solid mounting points.

The splitter will mount to the front subframe and will have vertical supports to the frame rails for 4 total attachment points. I'll use factory holes or fasteners for attachment. I will install rivnuts to the bottom of the bumper lip, and I'll use bolts through the splitter to keep the bumper attached.

I want to have small side fins, for which I'll bond round coupler nuts into the sides of the splitter.

Finally, I want to use clevis pins at the mounting points, for quick and easy removal.

Design

I started by modeling the bumper in CAD, and then I added a 6" offset line, following the outer bumper edge. Basically, the radius of the bumper, plus 6". I decided to use this as my splitter outline. Thus, the splitter will extend 6" beyond the bumper. I want the splitter to have a 10 degree chamfer for 1" of the top edge, and a 45 degree chamfer for 1/8" of the bottom edge. This will build a nice side profile.

The supports will bolt to the splitter using 5/16" hardware. The supports have a thru-hole for a clevis pin, allowing for easy splitter removal. There will be 4 clevis-style connectors, two facing forward, and two facing sideways.

I'll inset finish washers atop the holes for fasteners. I'll use flat head hex bolts, with the head under the splitter. This will allow for maximum clearance. A small lip on the inset pieces will keep the bolt heads from contacting the roadway.

I've also designed smaller inset finish washers that I'll set into the underside of the splitter, along the bumper line. These will be used for fastening the bumper to the splitter.

Next Steps

The best next steps would be to copy this shape onto a piece of cardboard, and fit it to the car. Then, make changes based on that fit.

For now, I'm focusing on the wing, but I'll make a detailed project page for the splitter project once it's in full swing.