DIY Downforce – How to Design Front Splitters, Diffusers, Flat Bottoms, Wings and more!

A guide to improving your car’s aerodynamic downforce performance efficiently, without wind tunnel testing or CFD analysis.

Improving downforce can seem like a big challenge when you don’t have the tools to measure increases in performance. How do we know if the wing we put on our car or the air dam we spent a weekend making help at all? The tools of the aerodynamic trade can be out of reach for mere mortals. Renting a wind tunnel is cost prohibitive for many large scale race teams, let alone a few friends looking to have fun at the track. Computational Fluid Dynamics (CFD for short) is a great tool that brings aerodynamic testing right to your desktop computer, but takes a decent budget to purchase the software and a lot of training and experience to be able to get useful information out of it. Well, I’m here to tell you that dipping your toe into dramatically improving the downforce and aerodynamic efficiency of your race vehicle is much more accessible than you think. Certainly, a wind tunnel and CFD can take you to the next level, but right now, a lot speed is being left on the table due to misinformation, misunderstanding and a fear of making your vehicle perform worse.

First things first, this guide is applicable to production based cars. At Professional Awesome Racing, we specialize in time attack and time trial racing. This guide can be helpful for touring car racing, endurance racing, club racing and more, but is not intended for open-wheel formula cars. This information is also valuable regardless of the horsepower or speeds your car can reach. We’ve worked with 90 hp Mazda Miatas and 1000 hp Mitsubishi Evolutions and the information can work with both. Commonly, we see people saying that trying to improve downforce for lower power cars is wasted because the cars don’t have the horsepower to make up for the added drag. There might be a few instances where this may be true, but for the most part, this isn’t the case. You can make downforce without adding much drag, and in some cases, even reducing drag! We’re also assuming you have at least a little knowledge of car aero performance, but we’re going to try and make it welcoming for everyone. Enough with the intro, let’s get down to business.

Front Aero

We’re going to start at the front of the car and work our way to the back. I know when I first started getting into upgrading aero on cars, getting the front to perform was always my biggest fear. At the back of the car, you could slap on a wing and make it as big as your wallet allowed, but how do you get more performance out of the front? Well, after years of reading, testing and research, my mindset has flipped. Downforce at the front of a car is relatively easy! Why? Well, the air hitting the front of the car hasn’t been jumbled and made turbulent yet. The rear of the car is dealing with all sorts of vortices, boundary layer effects and issues that you really need CFD or a wind tunnel to study. Designing for the front is made much easier without having those issues to deal with.

Splitters and Air Dams

So where to start first? Since we’re dealing with production based cars, let’s talk Splitters and Air Dams. The splitter is a common, effective and very efficient way to make a healthy amount of downforce. It literally splits air hitting the front of the car, separating a higher pressure zone on top from a lower pressure zone beneath. What do you get when you combine those two pressure zones? Downforce!

The thing about splitters is, most every application is vehicle specific AND race series specific, meaning it’s really hard to find an off the shelf solution. So since you’ll likely have to construct your own, we should cover some important info on splitter material first. The material should be stiff, relatively lightweight, easy to work with, impact resistant and not too hard on the wallet as splitters can take a lot of abuse being mounted so closely to the ground. Common material choices are plywoods, plastics, sign board materials like Alumalite, carbon fiber honeycomb composites, kevlar composites and aluminum. Each material has its pros and cons, we use and sell Alumalite for many of out splitter designs as it’s a good balance of strength and weight to cost. For more detail info on splitter material choices check out our video (coming soon!) on what the positives and negatives are for each.

Once you have made your choice on what material to use, a critical aspect of your splitter system is mounting it to the vehicle. It’s easy for your splitter, if big enough and properly designed, to see 500+ pounds of force trying to pull it off of the vehicle. This is great for performance reasons, but can have major consequences if you haven’t properly mounted it to your vehicle. We’ve seen a lot of people mounting their splitter directly to a factory plastic bumper and then seeing failures when the plastic isn’t strong enough to hold it in place. Ideally, your splitter should have an aluminum or steel frame that distributes the downforce loads over a wide area and then attaches directly to the chassis of the vehicle via rods, tubes, cables or other mounts that have a lot of tensile strength. I cannot stress this point enough. We’ve see a lot of splitter failures recently as designs get more aggressive and sooner or later, someone is going to get hurt when a splitter rips off during a high speed turn and all of the front end downforce on the car disappears. Be warned! Be careful!

So now that you have your splitter material and know how to attach it to the vehicle, what next? The splitter should mount to the vehicle as closely to the ground as possible, without hitting the ground under braking or in corners. This is easier said than done, but critically important. Splitters’ downforce performance improves dramatically with reduced ground clearances. The closer you get it to the ground, the more downforce it will produce. The problem is, if you fully seal it to the ground, not only do you wear down your splitter or damage it quickly, you also can greatly reduce downforce due to the airflow beneath it being cut off, which can be very dangerous. This leads me into the importance of having a well designed suspension setup that can keep the aerodynamic pieces under control. The more stable you can keep your splitter in relation to the ground, the more consistent your splitter’s aerodynamic performance will be. With that said, if you stiffen the suspension a lot to provide a stable aero platform, you can reach a point where you start sacrificing mechanical grip which can hurt cornering performance. Finding a balance is key and will require testing for your specific vehicle. Running a softer sprung suspension, with lots of low speed damper control, a digressive damper curve and progressive bump stops is a common way to keep compliance in the suspension, while also having a stable aero platform. But again, testing is key to finding the right setup.

With the splitter mounted low, but not too low, another aspect that is commonly overlooked is splitter angle. Now this can require testing to find the right balance as well, but in general, if the rear of the splitter is angled upwards slightly, you can see a gain in downforce with minimal increase in drag. Angles should be in the 1-4 degree range, but testing here can yield better results. Using string tufts under the splitter,then driving it on a safe piece of road or track and then seeing what angles and speeds show attachment, can allow you to be more or less aggressive, depending on the average speed you’re competing at. Higher angles will create more performance, but may require higher speeds to get proper air attachment. If you’re not regularly cornering at those speeds, it may be in your best interest to reduce the angle of your splitter.

The area in white is your air dam. This vertical, flat piece helps create a high pressure zone, which then pushes downward on the splitter.

Still on the subject of splitters is your splitter air dam. This is an oft overlooked and critical piece of your splitter system. The air dam is the vertical component of your splitter that attaches perpendicularly to your main splitter plane and parallel to your bumper. We see many splitters that do not feature this component and if you’re one of the offending people that have installed a splitter without one, you’re really hurting your performance. The air dam’s primary goal is to efficiently create a high pressure zone on the portion of your splitter that’s exposed in front of your bumper. This piece should be well sealed to your splitter, so as to not let air pass to the backside of your splitter. What we see happening are gaps left in this area, that allow air to bleed above the splitter and below the bumper, killing the potential downforce creating high pressure zone, increasing drag by not efficiently moving the air around and over the vehicle, and finally, hurting cooling performance by not forcing air towards intercoolers, air filters, oil coolers and radiators (and potentially allowing the air to get to the backsides of these cooling systems, hurting performance even more!). Your air dam, again, should seal tightly to your splitter and tightly to the bumper to control the air as much as possible.

Splitter extension, forward of the front bumper, is another important aspect you should take into consideration and the rule here is, the longer the better… as long as you aren’t smashing it into the ground under braking and cornering or compromising the height at which you can run your splitter. Realistically, this works out to be about 4”-8” of extension in most cases. The longer you make it, there is a diminishing reward as the high pressure area created by the air dam and front bumper can only be so large, but try and make it as long as feasibly possible or within what your racing class’ rules allow.

The outboard splitter end fence has a lot of knock-on effects downstream that are vehicle specific. Without a lot of testing, it is wise to keep conservative in these designs.

Splitter end fences are one area of splitter design that are really car dependent and with that in mind, CFD and wind tunnel testing are important to maximize their designs. Because of this, I suggest going very conservative with their design, with somewhere on the order of a 4” tall rectangle being used that’s parallel to the centerline of the vehicle.

Splitter diffusers are perhaps the most underlooked area for performance increase and has one of the best upsides for untapped aero potential. The simple goal of the splitter diffuser is to increase the velocity and mass of the airflow underneath the splitter. The more air flowing at faster speeds means an increased low pressure zone beneath the vehicle. The lower the pressure you can get beneath the splitter, the more downforce to be had. The additional side benefit is the possibility of moving air away from potential high pressure zones, such as where the tire meets the road, which has a side benefit of reducing drag and lift. Win-win! We sell splitter diffusers of a few different shapes and sizes that are have been optimized in CFD and will work in a bunch of applications. It’s possible to design your own as well, but care must be taken to ensure proper attachment of airflow to the diffuser curve. It’s easy to get overly aggressive which can cause airflow separation, causing a reduction of downforce and increasing drag.

It’s wise to also protect your splitter from damage if/when it comes in contact with the ground. It’s extremely important to use a material that has high hardness, so it doesn’t wear away quickly, but also maintains its strength under high temperatures that are caused by the friction of asphalt at high speeds. We’ve found titanium to work very well in those situations.

A few final splitter tips, tricks and ideas. If you can design a mounting solution that allows for adjustment of height and pitch separate from adjusting your suspension height, this allows for dialing in the aero and suspension to the track separately. Again, care must be taken to ensure the splitter is securely attached though. You may also want to consider using cables for the front mounting positions of the splitter. The reason being is that the splitter is most likely to hit the ground in this area and using solid rods can damage the rods or the chassis of the vehicle. The cables will just flex out of the way, saving your car from damage. The downside of cables are that they cause more drag than you’d expect considering their size. Sometimes making something work in practice is more wise than trying to make something as aero efficient as possible.

Front Bumper

We spent quite a lot of time on the splitter, because of how important they are to an efficient aero package, let’s work our way back just a touch. The front bumper is the next area where we see lots of room for improvement on many cars we see at the track. First, get rid of any unnecessary openings in the bumper. For styling purposes, we see a lot of ducts and vents that go nowhere on front bumpers and with a little sheet metal and a few rivets, these can be sealed up, which almost always reduces drag and increases downforce.

Speaking of unneeded openings in the bumper, it’s almost a guarantee that every track event will have the turbo car with the gigantic intercooler fully exposed to oncoming air. It may look cool, but not only is it causing more drag and a reduction in downforce, it’s likely hurting the cooling performance of the car. This is where ducting comes in handy. With a properly designed inlet duct, the opening for your intercooler, radiator, oil cooler, etc… should be roughly a 1/3rd the surface area of the heat exchanger. So if your intercooler is 20” x 30”, (this would be 600 square inches) the opening to you duct should be about 200 square inches and then smoothly transition to the full area of the intercooler/radiator/oil cooler… etc.

We take our aero ideas from many sources, plane engineering being a great source and a wealth of knowledge.

This technique was adopted from cooling systems of WW2 fighter planes that looked for efficient ways to cool their massive engines. In a perfect world, you’d then create an outlet duct that would then shrink down to the 1/3rd size again and dump into a low pressure zone. Figuring out well defined low pressure zones is tough though, so testing should be done very carefully to ensure proper airflow out of the duct. This is a prime example of why doing the front aero is easier than the rear. It’s easy to find the high pressure area of the front bumper, but the low pressure zone following the car is only easy to find directly behind the rear of the vehicle.

Cooling in general is another topic I want to touch on briefly. Any cooling inlets on your car will increase drag and likely reduce downforce, but, obviously you can’t get rid of them. What do you do? Size your systems accordingly. For time attack/time trial vehicles, measure oil, air inlet, coolant and brake temps to learn trends. We found that we were over cooling the oil in our vehicle and did not need a full size oil cooler for the 2 or 3 laps we’d run at a time. This allowed us to seal up the ducting for the oil cooler, improving aero performance and then we removed the cooler completely, which reduced the chance for leaks from the extra lines, as well as reduced weight. Brake rotor temperatures can be monitored via brake paint as well as infrared (IR) temperature sensors. It might turn out you don’t need the brake cooling ducts in your bumper and you can also better set up brake balance and pad choices by optimizing temps, but that’s a whole other article! Endurance racers, the same applies for you, but perhaps in reverse. Long story short, monitor your temps and adjust cooling capacity accordingly!

Front Fenders and Hood

A big difference between how a car works on the street and how it works on the track is heat management. Race cars produce quite a lot of heat, and airflow is an absolute requirement to keep temperatures in check and keep things from breaking. This is where the hood and front fender area are critically important. Generally, you’ll have air coming into the engine bay  through a radiator, oil cooler, potentially an intercooler and this air needs a place to go. Hood venting is how you allow the air to escape efficiently. The problem is that your hood sees a variety of pressure zones. As oncoming air moves up from the bumper and curves over the hood, you can see a velocity increase, reducing air pressure and causing lift. Then just a little ways away, the airflow hits the windshield, slowing the air down and increasing pressure, causing drag. Finding a smooth flow of laminar air and adding venting, much like we sell here, will allow the underhood air to vent. Aside from improving cooling efficiency, as the high pressure air under the hood is released, you’ll see a reduction in lift, improving the downforce of the vehicle.

The same principles apply to the fenders as well. The rotating tire in the fender can cause a large increase in air pressure, which pushes upwards on the fender. Adding fender venting, which is available at the Pro Awe shop, can reduce this pressure, helping improve downforce numbers. This is a common practice in prototype racing and large increases in downforce can be achieved with properly designed vents. Another side benefit can be an increase amount of airflow around your brake system, reducing the need for added brake cooling. Additionally, all this venting can work hand-in-hand with your splitter diffuser system, allowing more air to pump through your diffusers, decreasing the pressure under the splitter even more and creating more downforce!

Front Tire Deflectors

On many track cars, wider tires are shoved into the relatively small factory fenders and can be visible extending out beyond the sides of the car when you’re looking at the car from the front. Exposing tires in this manner will create drag and lift. Simple side spats can keep air from hitting the rotational air mess caused by the tire and improve your aero efficiency. If you do it right, this can also be turned into a splitter support and/or a small air dam. A great way to kill 2 birds with 1 stone!


I will just touch on this topic briefly. It’s very difficult to know how canards will affect the aero performance of any specific car without testing. In generally, they are used for balance changes to the vehicle in race series that have very limited aero rules when larger changes can’t be made. That being said, I’ve seen some vehicles where they are extremely efficient and critical for aero performance and other vehicles where they are very inefficient, but a necessary evil for other aero problems on the car. I’d look into these only when adequate testing can take place, be it at the track, CFD or a wind tunnel is available.

Rear Wing

We were just on the front bumpers and all of a sudden we’re jumping back to the rear wing. What gives? Underbody aero, such as flat bottoms and diffusers are extremely complex and vehicle specific, we’ll touch on them later, but testing is so critical to these parts, we will skip them for now.

A rear wing is a very efficient way to balance out the massive front aero you’ve added to the car via a splitter, but there are some considerations to keep in mind. First, work with companies that provide data for their wings. If you have followed our advice and made a nice, simple front splitter, you might be making 300 pounds of downforce on the front of your vehicle at 80 mph. Let’s say you went to town and have a huge splitter with monster diffusers, all working properly, you might be up to 600 pounds of front downforce at 80 mph. You need a rear wing to match accordingly and having data is the only way to figure out the best wing for your needs.

Without data, if you purchase a wing, you’re just guessing. We use APR Performance because we’re provided with data to make the best decision possible.

Once you have that data, you need to know what percentage of downforce you want on the rear of your car. Generally, you work with the front and rear weight distribution to find a good match. If for example you have a weight balance of 50% Front / 50% Rear on Miata, you’ll shoot for roughly a 50/50 aero balance. What about a 60F/40R Evo? Here you’d go for a 60/40 aero balance. That being said, I’d generally recommend to shift that balance slightly towards the rear to have a car that understeers with aero balance. It’s just a little safer, easier to drive and generally more confidence inspiring.

Once you know how much rear downforce you want to make, take a look at the wing’s downforce vs. speed data. Most wing data is provided via free stream testing, meaning, they aren’t the exact numbers you’ll see on your car and this is very important. Putting the same wing on the back of a slippery Toyota Prius or on the back of a convertible Miata will see different performance. The Prius will send cleaner airflow to the wing and the wing will produce more downforce than it would in the airflow mess a convertible Miata produces behind it. With this in mind, you’ll need to oversize your wing based on how the air flows towards the back of the car. You can move you wing mounts up to get cleaner airflow above the car and/or move the wing back to create move leverage acting on the chassis as well. You will have to balance these options when choosing and mounting the rear wing. Just like the front splitter, rear wing mounts are critical for safety. The large, higher and/or more rearward the wing is mounted, the more stress is placed on the mounts. Go conservative here and make sure the wing doesn’t fail or you could find yourself going backwards into a concrete wall in quite a hurry.

Rear Spoiler
Spoilers can be used with rear wings to improve performance of the wing and rear diffuser. They can also be used for balance adjustments or on their own to make a car without much front aero a little easier to drive at high speeds.

I’m a fan of rear spoilers for multiple reasons, but it should be kept in mind that they aren’t massive downforce generators. Spoilers can be used on cars to balance out high speed oversteer, instilling more confidence while driving. They can also be used in conjunction with a rear wing to improve rear downforce performance and add stability. With a properly designed rear diffuser, a spoiler can also be used to alter underbody downforce, but this would require careful planning and testing to execute.

Flat Bottom and Rear Diffuser

Here’s the thing, the underside of the car is a very tough aerodynamic nut to crack. You’ll have tire wake to deal with, which is way more intense than you can imagine, the cool new front splitter you’ve designed will have major cascading effects behind it and inflow from the sides of the car is a major challenge. A flat bottom needs to be properly sealed to the chassis above for the diffuser to work properly, which can be a big challenge on a production based car. Without the right flat bottom, the rear diffuser will not work properly, if at all.

Rule of thumb for flat bottoms and rear diffusers, without proper testing, be conservative. Even one of the highest downforce cars in history, the Allard J2X, has a relatively modest rear diffuser angle. It is possible to be more aggressive, but without testing, you might just be wasting your time.

That being said, I know some of you will insist on going this route without testing. Starting the diffuser farther forward can yield positive results when trying to enhance the overall downforce of the vehicle, just keep your aero balance in mind as the point the diffuser starts will be the point of lowest air pressure, so the car will be “pulled down” from that area. Keep your angles conservative to reduce the chance of flow separation under the car, which will increase drag. I would say no more than 10 degrees of difference compared with being parallel to the ground. Do string testing to ensure you have attachment of airflow to the roof of the diffuser. If you see attachment, you’re doing pretty well and perhaps you can even push further. The center of the diffuser, when looking from the back of the car, is generally the area that works best, the outsides will likely struggle no matter what you do. Use vertical strakes to enhance attachment and keep area of healthy flow separate from areas of turbulence.

How to Test
We might have gone a bit overboard here, but you get the idea. Wool tuft testing can be a great way to visualize what’s happening on the surface of your car. Use it to test for separation on diffusers and wings to ensure they are working properly!

String or wool tuft testing can be a great way to visualize airflow on the surface of your vehicle and ensure your components are working as tested. I think some of the best ways to use strings are to check for flow on the surface of wings and diffusers to ensure they are working as expected. Grab yourself a GoPro and find a safe place to drive your vehicle. In the best case scenario you can drive at specific speeds and document what is happening to your strings at each breakpoint, let’s say every 10-20 mph difference. The faster you go, your airflow will more likely stay attached to the surface. Test at speeds you’re likely to be cornering at. I’ll see huge downforce numbers regularly tossed about because vehicles are tested at 160+ mph, which may not make sense if you have a car that spends the majority of its time at 60 mph in the turns. If the strings are steadily flowing in the same direction, you likely have good surface attachment. If you see strings going all different directions, even pointing towards the front of the car, that is likely flow separation. You want to avoid situations like these on wing and diffuser surfaces. Be careful to make sure the strings have enough room to move freely and not touch one another or any sharp edges as they can get stuck and give you false information.

If I haven’t stressed this enough, I’ll stress it again. Gathering data and testing are critically important to the performance of your vehicle. Using a program like AEM Data can make it relatively simple to learn what makes your car faster and what doesn’t.

If you really want good data, find yourself a good data acquisition system and datalog shock movement. This information can be used to do rough calculations for downforce, aero balance, shock settings and much more. Going into how to use the data is outside the scope of this article, but check out A Practical Guide to Race Car Data Analysis by Bob Knox if you have more interest in learning.


Thanks for reading this far and hopefully we’ve given you some advice to push you in the right direction for your car’s aero performance.

Have more questions? Feel free to comment below.

Want us to help you on your specific car? Reach out to us about our consultation services at

Looking for parts you read about in the article? Head to Professional Awesome Aerodynamic Parts.

Tell us what you’d like to learn about next and we’ll try and help you learn from our experience. Thanks again!

9 thoughts on “DIY Downforce – How to Design Front Splitters, Diffusers, Flat Bottoms, Wings and more!

  1. How are you measuring/evaluating the changes? Increasing downforce will (normally) increase drag, without adding more HP to overcome the drag the decision needs to be made where best to use the available HP (straight line speed or downforce).

    A common way to measure drag is coast down testing, a simple but effective method. However this also assumes that the car is able to be driven on the road. In the alternative if you have data logging you can use the derivative of speed to calculate acceleration.

    A ‘crude’ way is to multiply the acceleration G force by 9.80665 to give acceleration in m/s². However this requires there to be almost no lateral G forces and the car to be at wide open throttle to remove as many variables as possible. However if you use the same part of the track then at least it is repeatable.

    No matter what method, as speed increases acceleration will decrease due to the impact of drag. If the data shows that acceleration has improved then baring no other changes drag has decreased.

    Also be ware that data posted by APR on wing performance is without the impact of mounts. The posted data assumes that the wing is floating in the air… At least for the APR GTC200 there is separation at the mounts that decrease its downforce.

  2. For a street car, the easiest way to measure changes in drag is fuel economy and driving a lot of miles! I found on the Miata that a noticeable increase, 2-3 miles per gallon, in fuel economy that was repeatable. Obviously, there are other methods to test with, but we’re looking for big changes, although this method means we’re measuring without units. IE, I don’t know how much the CD on the car dropped, but it most certainly did, meaning I was headed in the right direction. Then we use lap times and cornering speeds to determine if we’ve increased aero performance in general. Is it super accurate and precise? No, but that’s not what I’m looking to achieve.

    On our race car, we use shock pot data that is damped and we use data from the same track and measured at the same speeds in the same sections of track. With this method, we can have high precision on whether or not an improvement has been made in downforce, although, 500# of downforce at once track might be completely different than 500# at another track. Accurate? No. Precise? Yes.

    Regarding the wing data, you’re absolutely correct and I reference that to some degree within the article. The data you see on the charts will not be exactly what you get behind the car, but it’s better than nothing!

  3. Wow! This is great! Do you think adding a front lip spoiler (think Ezlip) would have any effect or is it just not big enough?

  4. The front lips you see on eBay can make a little difference in reducing drag and maybeeeee reducing lift, but the effects will be minimal. If you can stick it on with double-sided sticky tape and it doesn’t fall off, then you know the forces it’s seeing aren’t that great, but anything to keep airflow from going under the car will help in some way (unless you have tunnels are a rear diffuser, but that’s a whole other story).

  5. This is a great article, thank you for sharing. Is it possible to utilize the venturi effect and install ground effect to generate negative pressure(similar to old school F1 cars) on a street car?

  6. It’s totally possible, but there a few considerations to take in mind.
    1)Body control- The car’s body has to be very stable for safely and consistently creating underbody downforce.
    2)Flow disruption- The car has to have a consistent flow of air to make this work, but items like wheel wake and inflow from the side skirt area can both help and hurt flow.
    3)Testing- With the first two points in mind, you really have to test effectively to make a system work. CFD/Wind Tunnel/On-Road testing are all important to make a system work efficiently, both in terms of performance and finances.

  7. Very well written and encompasses a large part of race car aerodynamics in very clear and visiual way! I have always wondered about the negative pressure area behind the car (in the wake right at the body) and i have seen some time attack cars use a tunnel from under the car routed inside the body and exits in the negative pressure zone at the back of the car. Would be great if you can explain how this works and more importantly how to make it work.

  8. Diffusers/tunnels at the rear of the car make perfect sense to utilize the low pressure behind the car, to create a suction effect under the car and it’s even better with a rear wing that can help create even more suction. The issue is, it’s beyond the scope of this article to help design a good system, because the further downstream you are on the car, the more the effects of wheel wash, boundary layer and dispersed flow creates problems with proper attachment. Testing, CFD and/or wind tunnel all have to work together to design a really solid rear diffuser system. Hopefully, we’ll get to dive into that in a further article, but for now, we just don’t have a great explanation to provide.

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