Test your car's aerodynamics for less than $100

J.G.
By J.G. Pasterjak
Jul 13, 2022 | Aerodynamics, C5 Corvette | Posted in Shop Work | From the June 2022 issue | Never miss an article

Photography Credit: Wrecking Force Photography

For nearly its entire run as one of our project cars, our C5 Corvette Z06 has worn some fancy aero bits from AJ Hartman, Shields Windshields and Nine Lives Racing. And while we’ve measured the effectiveness of that gear in lap time improvements, we haven’t determined exactly how the physics come into play. 

These are tests you can do at home, with your own car, to get a clearer picture of how it’s interacting with the air. Yes, you can spend thousands on CFD aero testing using the latest modeling software–the modern way for many top teams–but the aero tests we’ll use on our Corvette can be done in less than an hour using a piece of equipment that costs about $70. 

Splitter Kit

Before we get into the fancy aero at the back of our Corvette, let’s start at the front, where we added a proper splitter courtesy of the attachment and template kit from Nine Lives Racing. Designed to create both a shelf of high-pressure air at the front of the car and a clean path for high-velocity, low-pressure air beneath the car, splitters are an easy* and affordable* way to get an effective aero boost downward.

Why the asterisks? Because those terms are highly variable when it comes to splitter installation. Some jobs are really easy, and others require some–or even lots–of fabrication skill and knowledge to pull off.

C5 Corvettes tend to sit in the latter category. Toward the front of the car, there’s no real exposed structure sturdy enough for a splitter mount, and the structure that does exist is too far back to do much good.

Enter Nine Lives Racing and its Sturdy Boii splitter mounting kit for the C5 Corvette. With this package, Johnny C and his big brain have taken the hard work out of splitter mounting, allowing you concentrate to on the easy bits, like turning wrenches and using basic hand tools.

The Nine Lives Racing splitter mount kit requires some arts and crafts, but it’s a one-day project, even with basic hand tools. We finished off our plywood splitter with truck bed coating while filling the gaps between the splitter and the bumper cover with some rigid foam inserts. Photography Credits: J.G. Pasterjak

The Nine Lives Racing kit consists of a pair of mount brackets, attachment hardware, and templates for locating the bracket holes and cutting out the splitter itself. Nine Lives will also happily make a splitter for you out of materials ranging from the mundane to the exotic, but for our install we chose to cut our own. This step, we figured, would allow us to maximize the limitations of our chosen rule sets.

An exhaustive step-by-step breakdown of our splitter install can be here, and paper has probably increased in price since we started typing this sentence, so we’ll give you the elevator version here: Remove the front bumper cover, install the precut brackets on the frame of the car, use the template–or, in our case, a slightly modified version–to trace the splitter design onto your material of choice, cut out the splitter, bolt it to the attachment brackets, reinstall the bumper and, finally, dust off your trophy shelf.

Outside of the standard install instructions, we gave our splitter a few personalized finishing touches. First, we built it with half-inch birch plywood coated with truck bed liner. While materials like Alumalite, ABS plastic and even carbon fiber are used successfully in DIY splitter construction, the last time we checked, those materials weren’t sold at Lowe’s for $50 a sheet. 

Besides, the wind doesn’t care what it’s hitting–so long as the material is properly supported. After a 150 mph run on Sebring’s back straight, we can report that our carbon-based, layered organic composite material did just fine at speed.

We also had to fill the gap between the bottom of the bumper and the top of the splitter, which would have been a prime location for trapped high-pressure air under part of the car’s structure. This would create lift or at least lead to an area of high turbulence-creating drag. 

Our material of choice was some polyethylene foam we’d been keeping in our “interesting materials” pile. Finally, we found a use for it: fill panels for the splitter. We simply cut the foam to rough shape with a band saw and an electric turkey knife (possibly also useful for ham) and gave it a blast with a heat gun to shape it to the contours of the car. Once we were happy with the shape, we glued it to the deck of the splitter. 

That foam is easy to work with and a great material for prototyping as well as light, non-structural fabrication. The next time you buy furniture or speakers or some other item packed with foam, save it. 

Spoiler Alert

Now that the splitter is handling downforce duties up front–we’ll get back to actually testing it in a few minutes–let’s take a look at the most visible aero accessories on our Corvette. Depending on the venue, we run either an AJ Hartman rear wing or a Shields Windshields spoiler. The flat-panel spoiler is required for SCCA CAM-S competition, while the wing is used for most of our on-track, time trial activities in SCCA and NASA.

To quantify how these devices work the air, we employed a magnehelic gauge. It simply shows the pressure differential between two different sources.  There are two ports on the gauge, one tuned for pressure above ambient and one tuned for pressure below. The unused port can measure ambient pressure wherever the gauge is located, so you’ll want to keep it inside the car and away from any pressure fluctuation–so, if possible, close the windows. 

Now to measure stuff: Simply run a flexible tube from the gauge’s other port to wherever you want to measure a pressure difference. (You can also run two tubes to the gauge to measure the pressure differential across a surface, but we find that ambient versus whatever is plenty useful.)

We bought our Dwyer Instruments analog magnehelic gauge for about $70 several years ago, but today you can buy digital ones for even less. Some of these digital models can also measure a larger spread of differential pressure than our gauge, which is only able to read up to 1 inch of water (about 0.03 psi).

We started our testing by measuring the pressure at the base of our 10-inch-tall, flat-plane spoiler. At race speeds, though, the pressure quickly buried the needle. This showed substantial pressure increase thanks to the spoiler, but we needed a revised test strategy that worked within the limits of our gauge.

Our new plan: Record the speed needed to pull 1 inch of water. For our 10-inch spoiler, this was a mere 50 mph. 

Simple aero: Our flat-plane spoiler started off as a cardboard template that Shields Windshields duplicated in polycarbonate. We then used an inexpensive magnehelic gauge–one tube measures ambient, the other measures air pressure–to test the spoiler’s effectiveness: How much did the spoiler increase air pressure across the decklid? Photography Credits: J.G. Pasterjak

Now, we mentioned earlier that 1 inch of water equals about 0.03 psi–0.036 psi to be exact. This means the air pressure in front of the spoiler increased by 0.036 psi relative to the air around it. In other words, the air pushed down on the bodywork in front of the spoiler, creating downforce. It’s literally just that simple.

Now, 0.036 psi doesn’t sound like all that much, but let’s remember a couple things. First, there are a lot of square inches on a car. Second, aerodynamic forces do not scale linearly, they scale with the square of the speed. So at 60 mph, the forces are a third higher than they are at 50, and by 70 mph they’re nearly double the force at 50. 

Still, the forces at a mere 50 mph weren’t nothing, so to find out just how large an area they were affecting, we started moving the end of the measuring tube toward the front of the car in 3-inch chunks. 

At 3 and 6 inches away from the base of the spoiler, the speed needed for a 1-inch pull changed very little–too little to really be measured accurately on our analog gauge. 

At the 9-inch mark we had to raise the speed by about 10% to get the same pull, and by 12 inches we had lost much of the high pressure that collected at the base of the spoiler.

If you’re getting a weird tingle in the math part of your brain, yes, the predominant area of trapped high-pressure air in front of the spoiler is very close to the vertical height of the spoiler. That’s no accident.

But what’s cool is that our Corvette’s decklid represents nearly 600 square inches. If we can raise the localized pressure by just half a psi, that’s 300 pounds of downforce from just a simple, flat piece of clear plastic.

The Wind Beneath Our Wing

Next, we installed our AJ Hartman wing, which offers a little more than 1000 square inches of area right up in the airflow. Based on suggestions from friends in the aero business, we positioned our sensor tube on the underside of the wing–about a third of the way back. We then headed back out to pull our inch of water. 

Coincidentally, we pulled our first inch at the same speed as the spoiler: 50 mph. This means the wing and the spoiler were nearly as effective as each other in producing downforce at low speeds.

Our Corvette also runs an AJ Hartman Fulcrum Wing. To measure its effectiveness, we took pressure readings about a third of the way back from the wing’s leading edge. To compare the turbulence generated by the wing and the spoiler, we also measured pressure differentials at the rear of the car. Photography Credits: J.G. Pasterjak

But let’s also look at some meaningful notes. First, the wing’s 1008 square inches offer a larger effective area than the 600 or so primary square inches affected by the spoiler. The second factor is the efficiency of the wing in producing this downforce, which we’ll discuss more in a minute.

But first, let’s pause to ponder the humble spoiler–and all those folks who say spoilers are useless at autocross speeds. Based on the simple math and dead-simple testing we’ve done here, spoilers can produce meaningful localized downforce–on the order of 10% of the weight of the car or even more–on faster, national-style autocross courses. If your class allows a spoiler and you’re not using one, you’re leaving performance on the table at the expense of a modification that can be made from one trip to the hardware store.

What a Drag

But let’s get back to our wing and why its downforce was better than the spoiler’s, even though they were shockingly similar at low speeds. In a word, it’s drag. 

To get a rough idea of drag, we stuck the sensor tube on the back of the car–behind the spoiler and in the clear airflow beneath the wing. 

Here, the air around the back of the car was a mess. In the aggregate, the pressure was lower than ambient–probably around half an inch of water at 50 mph–but the needle was also bouncing around rather violently. This means turbulence, and turbulence means drag. 

First off, we could see the drag just from the aggregate pressure drop behind the car. Remember, the body of the car will be pushed by high pressure toward low pressure. If the pressure behind the car is lower than the pressure in front of the car, the car is effectively being dragged backward. Turbulence, the buffeting, chaotic air that flows in multiple directions at once, just makes for a messy wake and even more drag.

In contrast, the pressure behind the car with the wing in place was just slightly higher than ambient and silky smooth. While the pressure at the front of the car was certainly still higher than in the rear–it’s the end plowing into the wind, after all–the wing yielded a smaller pressure differential than the spoiler, as well as less turbulence. And remember, turbulence means drag. 

The wing’s downforce also scales much more predictably as speeds rise. The wing is designed to keep the airflow around it neat and clean, and that air does its work from a crawl to terminal velocity. 

The spoiler, on the other hand, can produce far messier airflow as speeds rise and the air coming over the car starts bouncing off the high-pressure air stacked in front of the spoiler. At some speeds, this phenomenon can actually lower drag, but at other speeds it just creates a mess as all those localized air pockets start to smash into each other and bust up everything.

So far, we’ve really only discussed a single part of the equation in this admittedly rudimentary comparison of a wing and a spoiler. We’ve only compared them by measuring the high pressure a spoiler creates on top of the car, thus pushing it down, and the low pressure a wing creates underneath its surface, pulling down the car. 

But a wing is a two-sided device. While the pressure on the underside is lower than ambient, the shape of the wing creates localized high pressure on its top side. The difference between those pressures is the actual amount of force being exerted on the wing–and, in the end, on the vehicle carrying the wing. When looked at in this light, wings are a better choice than spoilers when the choice is given.

Back to the Front

Now that we have a clearer vision of what’s happening with the air at the back of our car, let’s do a little testing up front.

We started with that Nine Lives splitter, mounting it at the recommended 2-degree downward angle. 

The angle of the splitter can be tweaked using a couple methods–the mounting holes that mate it to the frame are slotted, for example–but we prefer to do small final adjustments at the interface of the mounts and the splitter itself. We simply used body shims as spacers to get our desired angle without tweaking the joint. 

Front aero needs to be considered, too: To make fine adjustments to the splitter angle, we fit some body shims between the mounts and the splitter itself. Photography Credit: J.G. Pasterjak

Again, we used our magnehelic gauge to look for localized pressure readings on the surfaces of the splitter. Not surprisingly, the top of the spoiler was very good at collecting high-pressure air, pulling an inch of water at just 42 mph. Recalling that aerodynamic forces scale with the square of the speed, we could see that by just doubling the speed to 80 mph quadruples that force. 

But a splitter, like a wing, is a two-sided device. There’s also air flowing under the splitter, and that air now gets a nice clean path under the front of the car.

We built our splitter from a simple piece of half-inch plywood, and its first real test came at Sebring. After a weekend of topping nearly 150 mph, the entire rig was still secure. On track, the car was a neutral, grippy delight. Photography Credit: Wrecking Force Photography

As a result, the pressure beneath the car drops a bit versus ambient. The drop isn’t huge, though: At the same 42 mph that the top pulled a full inch of water, the bottom was pulling just 0.3 inch. However, the bigger the difference between pressure on one side of an aerodynamic device than the other, the more aggregate force is exerted on that device. 

Also, remember that the bottom of the splitter has a much larger exposed surface area than the top. The bottom of our splitter covers nearly 16 square feet–2300 square inches. Each square inch isn’t helping as much as the ones on top, but there are a lot more of them to chip in.

Cavity Search

Before calling our aero testing a done deal, we wanted to examine some of the aero forces on other areas of the front of the car–like those big cavities next to the auxiliary lights. 

These big holes mostly open up to nowhere, and their only vent is out the bottom of the car. This can lead to trapped high-pressure air under the car, which is bad. 

Some aftermarket air intakes leverage this area, but our Breathless Performance system grabs the high-pressure air coming off the top of the splitter and into the cooling system cavity, so these areas were just dead space that needed to be examined.

But could we further improve front aero? The magnehelic gauge showed how the front bumper’s gaping holes were collecting high-pressure air at speed. Trackspec offers ready-to-install filler panels. Photography Credits: J.G. Pasterjak

So we stuck one end of our tube into that cavity and, yeah, they’re just traps for high-pressure air. At 50 mph, they were both pressurized to about half an inch of water, which means turbulence in the front and high pressure below from the escaping air. Blocking off the openings–simple cardboard works for testing–dropped the pressure to ambient and reduced the turbulence.

Of course, we weren’t going to just leave the chunks of cardboard taped to the front of the car, regardless of how effective they were. Trackspec Motorsports–we used its aero vents on our S197 Mustang project, and the company has become a one-stop source for NASA Spec Corvette racers–had us covered, literally and figuratively, with a form-fitting set of plates specifically designed to block those pesky openings. They cost $129 and are easily attached with the supplied hardware. Problem solved. 

[These holes are not your friends: filling in the C5 Corvette’s useless “grilles” while adding a tow hook | Project C5 Corvette Z06]

Keep on Testin’

The great thing about having your own pressure-testing gear? You don’t need to go fast to get results. You can get meaningful data while staying under the speed limit. 

And relocating the pressure testing points is as easy as moving a tube taped to your car. To record the gauge’s reading, just mount a video camera–GoPro, iPhone, whatever. This method very quickly yields pressure maps that can show how the air is affecting your car.

We didn’t need high-tech gear for this aero testing. Our setup consisted of our magnehelic gauge, an action cam, and lots and lots of tape. Aero testing is more accessible than you may realize. Photography Credit: J.G. Pasterjak

Aerodynamic forces are an often-overlooked part of life on track, either because we don’t fully understand how to take advantage of them or because we don’t give them enough credit for having an effect at the speeds we typically drive. Yes, the effects are admittedly weaker at autocross and slower track speeds, but they aren’t nothing.

As aero gear becomes more easily attainable–buy it or build it–there are fewer and fewer excuses to not leverage its power.

Join Free Join our community to easily find more Aerodynamics and C5 Corvette articles.
Comments
View comments on the GRM forums
Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
5/10/22 1:29 p.m.

I'm going to set this aside for a proper read, this looks like good stuff. Magnehelic gauges are a fantastic tool to have on hand, every time I start talking about aero I end up talking about relative pressures.

Johnny_at_NineLives
Johnny_at_NineLives New Reader
5/11/22 9:37 a.m.

the really cheap way is to call us. 
We will tell you everything we know for free. :) 

k20a2DC2
k20a2DC2 New Reader
5/11/22 11:12 a.m.

I want to know more about making the aero using composites from mold making to end finishing aero project pieces whether canards to bumpers/ fenders etc Not a lot information on google but have searched in the past anyone knows any good links forums? 

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
5/11/22 11:28 a.m.

That's just "how do I make something out of composites", don't limit your reading to aero parts.

fschlottau
fschlottau
5/12/22 12:06 a.m.

In reply to k20a2DC2 :

Youtube is your friend on this one. AJ Hartman Aero, Bill's Build & Race are some of my favorite channels on that topic

 

fschlottau
fschlottau New Reader
5/12/22 12:14 a.m.

This is a great article on low-budget method - thank you!  What kind of lap-time differences are you seeing with these changes? 

wspohn
wspohn SuperDork
5/12/22 11:14 a.m.

Pretty easy to do aero testing if you have a track with a long straight and a repeatable cornering technique from the entry corner.

Up here in the upper left hand corner we used SIR (near Seattle) with the longest main straight in the area.  Note the rpm as you hit the first braking marker, go change something and try again. Pretty accurate - I picked up 500 rpm between start and finish of testing.

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
5/12/22 11:27 a.m.

In reply to wspohn :

Do you think you're measuring drag or cornering speed there? Because I can see how the two of them would change your results if you're cornering at maximum speed entering the straight. If you're trying to enter the straight at the same speed, you're just measuring drag and that won't tell you anything about your ability to grip the track.

QuikMcshifterson
QuikMcshifterson New Reader
7/1/22 5:25 p.m.

I'd really love to know the lap time difference without the aero pieces and then again with.

clshore
clshore Reader
7/2/22 10:19 a.m.

You can measure the drag created by aero devices using coast down data.

You need a reliable means of capturing speed (speedometer anyone?) , and also elapsed seconds (stopwatch, etc).
A datalogger is great, but using video capture, place a timing device in frame along with the speedo.

Test on a straight level road, ideally without a head/tail wind, and clear of the wake from vehicles ahead or behind you.

Observe the video and note the coast down time vs MPH, then make a graph with Excel or similar software.
Make a baseline run, modify the aero, then a new run.
Add the new data in same sheet as the baseline run to see where and by how much the aero mods changed the results.

This is not high precision work, so expect and ignore minor numeric differences, but anything that's more than 5% should be confirmed with a second (or third) run.

Be safe!
If you can test at higher than legal speeds, on racetrack, or private location, great.
But the effects at lower speeds will be even greater at higher speeds, so lower speed testing can tell you if you are on the right track or not.
 

Our Preferred Partners
vzx9gV4flTgQFCQizcbHb5iYP6nGIhrsoajs2IdM9PUJ9ANvB4QW6IW0aC1UrSPE