How to build a splitter for only $100 | Against the wind: Part 3

Tom
By Tom Suddard
Sep 20, 2022 | splitter, Shop Work, Aerodynamics, Against The Wind | Posted in Shop Work , Features | From the Nov. 2022 issue | Never miss an article

Photography by Tom Suddard unless otherwise credited; Lead by David S. Wallens

Our LS-swapped 350Z has sat quietly while we’ve worked on its digital twin, one built in CAD. Without getting our hands dirty, we’ve virtually tested all manner of aero add-ons: wings and splitters, vents and dive planes. 

One big takeaway from our time in Morlind Engineering’s virtual wind tunnel? We needed to build a splitter. 

And while we arrived at this conclusion with supercomputers and cutting-edge software, “I need a splitter” is a nearly universal truth in racing. 

So here are three ways to build yours: the $100,000 path, the $1000 path and the $100 path. We’ll cover all three and end the installment with a shiny new splitter on our 350Z. 

Laying Ground Rules

Time for our usual disclaimer: Before you build anything for your car, read your rule book. Most classes limit what you can do, especially when it comes to aerodynamics, and your splitter will be useless if you’re not allowed on track with it. 

Our open rule set–NASA’s TT2 class allows splitters that protrude as far as 12 inches from the front of the car–meant we didn’t have much to worry about in this department. 

Materials and Shape

Thanks to Morlind Engineering’s aerodynamic analysis of our 350Z, we already knew which size and shape of splitter would theoretically make our car fastest. But what if you don’t have that ace up your sleeve? Start with generalizations, build a few iterations, and experiment from there. 

First, think about the underside of your car: How far back should your splitter go, and how wide should it be? The answer to both questions is almost always, “As much as possible.” 

Flat, smooth bottoms make more downforce and less drag, and that’s awesome for a race car. Most rule books limit how far back your splitter can extend, but you should aim to get it at least to the centerline of your front axle. The farther back, the better. 

We decided our splitter should extend to our front axle centerline, since that was a convenient mounting point and would save us materials. We’ll add a flat bottom to our car down the road.

Morlind Engineering’s virtual wind tunnel work didn’t just produce these pretty, colorful graphics. It showed us what to place where. Now we had a blueprint for our wing, splitter and other aero add-ons.

Next, how far out should the splitter extend? This is where computational fluid dynamics, aka CFD, really shows its strengths. It told us exactly how big our splitter needed to be to balance our chosen Nine Lives Racing rear wing. 

Splitters can be made of relatively inexpensive plywood, so there’s no shame in building multiple sizes and experimenting during a test day. And remember, most rule books limit how far splitters can extend.

Finally, there’s the shape of the splitter. Should it be a flat sheet? Should there be ramps? Walls? Should the corners be rounded or square?

As we learned in our last installment, these features can have a big impact on how much downforce and drag your car makes. We decided to build our splitter with ramps, even though our CFD study concluded they wouldn’t be effective unless paired with dive planes. We’re planning to add those parts next time.Plus, they’re beautiful pieces of handmade Morlind Engineering carbon fiber. When in Rome, right?

So, what sort of exotic material would we use to make our splitter? Carbon fiber? Fancy space-age composite? Foam? 

Nope, plywood. Why? Well, because we asked the experts. Nine Lives Racing owner Johnny Cichowski put his own V8-swapped Miata through the wringer, testing all manner of splitter materials over the past few years. In the end, he settled on two preferred options, both of which are in his catalog. 

“Right off the bat,” Johnny C. explains, “prepreg carbon fiber is the best splitter material. Period. It’s incredibly light, stiff and durable.”

But, he adds, it’s also expensive. “Even after bringing the entire process in-house,” he explains, “complete with the giant autoclave and everything, a carbon-fiber Miata splitter costs the consumer $900 to $2700. The performance might not be worth hanging a rent check 2 inches off the ground.”

What’s the lower-buck option? High-quality, half-inch birch plywood. 

“Alternatives like foam, Alumalite and similar all have been tested,” he explains, “but the required bracing to make them rigid enough for aerodynamic duty made them heavier than the birch options. Maybe that’s why the European Space Agency uses birch in its testing. Then there’s the price difference: A birch splitter for a Miata costs less than a few hundred dollars, and that includes a tough proprietary rocket nose coating to seal it from the weather and prevent skids.” 

Mounting

We’d picked a shape and a material, so now it was time to think about the next piece of the puzzle: mounting the splitter. 


Splitter mounts need to be light, stiff and easily replicable in case of damage. If a splitter flaps around, it will unpredictably change the amount of downforce it makes, which means slower lap times or even dangerous behavior. 

And if the front of the splitter deflects enough to touch the ground, it will block all airflow under the splitter. No airflow means no downforce, which is why a flapping splitter can suddenly remove all front downforce mid-corner. That’s bad.

The $100 Method: How to Build a Splitter in Your Garage

Enough with theory, let’s build a splitter. Step one? Measure the car, which means you need to know two key things: what the outline of the splitter will look like and how it will mount to the car. 

Measuring the outline is easy. Grab a big piece of paper or cardboard, park your car on top, and put the car on jack stands (or in our case, on the lift). Then, using a plumb bob on a string, trace the outline of your front bumper onto the paper with a Sharpie. 

The goal isn’t to draw your splitter’s final shape, but rather your bumper’s shape. This will give you a relatively precise baseline to measure out from. Need a splitter that protrudes 4 inches? Using a ruler, draw a line 4 inches away from your first. Just like that, you’ve drawn your splitter profile. 

To fill in the rest–cutouts for the wheels and the rear of the splitter–pair this same plumb bob technique with a tape measure and a ruler. It doesn’t have to be perfect, and you can always trim the edges after a test fit. 

Time for the next challenge: mounting points. These are different for every chassis, so next you should pull off the front bumper and look around. Find a few sturdy places that can support both the weight of the splitter and the downforce it creates–about 300 pounds in our case. 

Up front, we’d install a splitter. The low-buck method involves pen, paper, a plumb bob and some time. 

Mounting the splitter level with the underside of the car is a good starting position, though you may want to play with splitter rake at some point. Bonus points for adding adjustability to your mounts. 

Don’t forget, your splitter won’t make downforce just at the front edge–the force should be fairly evenly distributed across the entire underside–so design your mounts accordingly. 

After a few minutes of looking around with Johnny C., we had a plan: We’d use our 350Z’s bumper beam bolts to support the front of our splitter, while the rear would attach to factory threaded holes in the front subframe with four bolts. Perfect. 

But how do those bolt locations correlate with the outline you drew? Tie a loop into the string on your plumb bob, and you’ll be able to bolt it directly to the mounting points you plan to use. Mark these locations on the paper, then measure the length of the plumb bob’s string to determine the bolt holes’ relative height to the car’s other features. 

Just like that, you’ve figured out how to record a fairly accurate location in three-dimensional space of each relevant part of your splitter mount. Pair this information with CAD, meaning either cardboard- or computer-aided design, and you can design a splitter and mounts that fit perfectly the first time. 

With our design finished, it was time for a trip to the splitter superstore, meaning whatever your local flavor of home improvement warehouse is called. Buy a sheet of high-quality, half-inch plywood, a jigsaw, some 1/8x1-inch 5052 aluminum bars, and a few spray cans of truck bed coating. (The extra-tall cans seem to work best.)

Mix those ingredients with your cardboard template, and after a few hours you’ll have a surprisingly strong and light splitter for less than $100. 

The $100,000 Method: How to Build a Splitter in a Multimillion-Dollar Professional Racing Complex

We’d learned how to build a splitter the old-fashioned way, but how do the pros do it? To find out, we brought our 350Z to the home of Morlind Engineering and Nine Lives Racing, a sprawling race complex full of incredible tools. Case in point: We built this splitter a few feet away from an extra Nissan DeltaWing somebody left lying around. 

Morlind’s owner, Rob Lindsey, agreed to walk us through the design process, which happens entirely in the computer. It starts with 3D-scanning the car’s front bumper. Since we’d already scanned our 350Z for last installment’s CFD work, that part was easy. 

Next, it was time to design the mounts. Rather than measure with a string and a plumb bob, Rob walked to the equipment closet and grabbed something much better: a FaroArm.

What’s a FaroArm? It’s a measuring tool like your tape measure, but that’s sort of like comparing a sextant to a GPS unit. A FaroArm is technically a brand of coordinate measuring machine, but it’s become the Kleenex of the genre. 

The arm knows the precise angle of every joint, and it’s moved around by hand to touch the tip to different parts of the car. FaroArms are a five-figure financial investment, but they’re invaluable for tasks like these. The arm knows exactly where the tip is within a few thousandths of an inch, even across distances as long as the entire front of our 350Z. 

Or you can go the high-tech route to that splitter, employing CNC machines, a FaroArm and composite add-on ramps. 

Using this tool, Rob measured every mounting bolt we’d planned to use in about 5 minutes and instantly put those measurements into the computer to pair with his splitter profile. After some CAD time planning brackets, the splitter was complete. Rob had designed bolt-on mounts that were height adjustable, strong and easy to assemble. 

Once Morlind was finished designing our digital splitter, the shop beamed it over to its manufacturing partner, Nine Lives Racing, to be put into production. Not surprisingly, there’s no jigsaw involved in this process. Instead, we watched as owner Johnny C. created toolpaths, then fired up a huge industrial CNC router to cut our splitter from half-inch birch plywood. 

The splitter headed out to get its rocket nose coating, and then we watched the router change bits and cut the mounts out of aluminum. After a few bends, a few rivets and some powder coating, our $100,000 splitter was ready to install. At least, it would have cost $100,000 if we’d bought all the tools to replicate this process ourselves. 

After we finished prototyping the splitter on our 350Z, Nine Lives Racing added every part involved to its catalog: the splitter ($376.92), mounts ($275) and ramps ($445), bringing the total to $1134. And if you’d rather just skip the measuring part of building your own, Nine Lives Racing will sell you a paper cutting template for $10.99.

Bolting It Up

Splitter built, we installed it on our 350Z and stepped back to admire our handiwork. Sure, we mostly watched as Rob and Johnny C. did all the hard work, but either way, our race car had a splitter. 

To increase its effectiveness and lower drag, we screwed garden edging (also purchased from our local splitter superstore) along the gap between the splitter and front bumper. Our 350Z was finally ready to return to the track. 

Once we had a splitter, we needed to solidly mount it to the car. If this sounds a bit daunting, you can crib from our notes: After our R&D, Nine Lives Racing added this very setup to its catalog. Dive deeper on our YouTube channel

Join Free Join our community to easily find more splitter, Shop Work, Aerodynamics and Against The Wind articles.
More like this
Comments
View comments on the GRM forums
Shawn_D
Shawn_D New Reader
9/20/22 8:43 p.m.

So, what's the source for this proprietary "rocket nose coating"?

Tom Suddard
Tom Suddard GRM+ Memberand Director of Marketing & Digital Assets
9/20/22 9:41 p.m.

In reply to Shawn_D :

NLR said it's a trade secret, but it's a pretty cool process. 

VolvoHeretic
VolvoHeretic GRM+ Memberand HalfDork
9/20/22 11:36 p.m.

"And if the front of the splitter deflects enough to touch the ground, it will block all airflow under the splitter. No airflow means no downforce, which is why a flapping splitter can suddenly remove all front downforce mid-corner. That’s bad."

I'm having a difficult time visualizing why this is true.

Also, is a flat bottom side to the splitter the best shape? Wouldn't a curved wing like airflowish like shape be preferred?

VolvoHeretic
VolvoHeretic GRM+ Memberand HalfDork
9/20/22 11:36 p.m.

Deleted duplicate. 

jfryjfry
jfryjfry SuperDork
9/21/22 12:08 a.m.

This may be common knowledge but my buddy ran a nascar truck team for a minute and they set up their suspension so the splitters just touch the ground.   Of course they're not deflecting but the truck compresses in the corners, lowering it so it scrapes.  And they're thick - they are not folding down.  
 

 

stafford1500
stafford1500 GRM+ Memberand Dork
9/21/22 7:47 a.m.

To VolvoHeretic: it is not a case of the splitter touching the ground removes ALL downforce, just that it reduces the downforce by a very large portion (~30% is typical). The downforce is generated by the air reacting on the bottom surface at increased local speed. On the flat vs curved surface question, there is a case to be made for non-flat surfaces, but the first 90% of performance can be had with a simple flat shape. After that testing and development are required to get the rest.

To jfryjfry: On the trucks, the splitter is just under 1/2" thick so it does have some stiffness benefit over thin materials. However, the material spec'ed for the Truck series is not the stiffest stuff out there. It does deflect. Setting the trucks up to just touch the splitter to the ground is typical, but generally only happens on the first few laps with low tire pressures. The peak downforce occurs with the splitter not touching the ground (by a small amount, less than 1").

I post this while I am waiting for the wind tunnel to give me the green light to start my test this morning...

Johnny_at_NineLives
Johnny_at_NineLives GRM+ Memberand Reader
9/21/22 6:21 p.m.

In reply to VolvoHeretic :

we need air to make aerodynamic parts work. we can reduce the amount of airflow to manipulate the speed of said air. But if we seal off the airflow (by touching the ground), we are working in a vacuum and have no air to make the parts work. 

Johnny_at_NineLives
Johnny_at_NineLives GRM+ Memberand Reader
9/21/22 6:26 p.m.

In reply to jfryjfry :
also, NASCAR is another level. they generally want as little drag as possible. Because 200mph. also their splitters are quite short (depending if we are talking about cup or Xfinity). They will do things slightly differently than club racers should. One thing is for sure,  no matter the class the height of the splitter is very important to downforce. The lower the better, but once it touches the ground you lose all downforce. get it low but not too low.    

 

Our Preferred Partners
LkanCl3nBT9pf7bw4RgAalqTfI66rQ4qnkHmxP72b2hGRTCAvRRu9wUjLsXYAoNm