Designing Aerodynamics for Track Performance

[Ron Sutton]

Skip this part if you know me or simply don’t care.

A little bit about me:
I’ve been racing for 35 years in 80+ classes of racing, most professionally, including drag, road course, karts & oval track. From low powered Formula Fords & F2000 cars to Mountain Motor drag racing … Midgets, Sprint Cars, Super Modifieds … to GT road race cars, Modifieds, Late Models & NASCAR Stock Cars. I’ve had the good fortune to work with & learn from winning teams & smart people in SCCA, USAC, SRL, NHRA, IHRA, IMSA, Indy Lights, Indycar, Grand Am & NASCAR.

Most of my modern experience is in road racing & oval track, as I haven’t been to a drag race since 1987. My operation has had 3 names, but settled on Ron Sutton’s Winner’s Circle about 20+ years ago. It was a statement to our commitment to winning. But in 2006, we joked about changing it to Ron Sutton’s Third Place Circle when we had nine 3rds & only one win … then we designed & built new cars in the off season & got back to winning. Today, my business that offers parts, complete cars, technical services & car/track consulting is named Ron Sutton Race Technology.

Many of you know what “married up” means. I did. My sweet wife Kim is very supportive of my crazy career & involved a lot, but doesn’t work in it on a day-to-day basis. When I look back, the numbers are a little overwhelming. We’ve built 64 tube chassis pro style cars & 140+ sportsman cars for clients. We’ve had a chassis shop, parts store, track stores & engine R&D over the years. I’ve owned 79 race cars myself … including 9 full time race teams until 2012. Most years we were at the track 3-4 days a week, 40+ weeks out of the year. How I stay married is completely a mystery, and I know I’m fortunate.

After I broke my back racing aero karts at Sears Point in 1991 I realized it’s the friends & relationships I have in the motorsports that I value most. That didn’t change how hard we worked … it just changed my viewpoint about relationships & teamwork. Frankly we started winning more races as I learned how to build better teams & developed more friendships with competitors & resources within the sport.

I live & breathe competition, and I believe winning comes from doing your homework, testing, working hard, testing more, learning from books, classes & mentors … and testing more. I’m not the smartest guy at the track, but when he goes home I’m still there testing & learning. I figure I have 2500+ days of track testing under my belt. As a driver, owner, crew chief or driver coach I have 498 personal wins & 22 championships. My clients' wins total in the thousands.

We closed my 9 team operation & parts business at the end of 2012. The economy being so bad for so long dried up too much sponsorship for us to continue racing as we did. When Dale Earnhardt Jr. had 13 races unsponsored in 2013, you know it’s tough. I sold all the parts, equipment, race cars & rigs and took some time off to relax ... whatever that is. I enjoyed a little slower pace, writing a series of tuning books and having fun building my new ’57 Chevy PT/Street Fighter, which is the prototype for my new Warrior cars. It's on hold now as I am busy helping so many clients build new cars or improve their current car. I’m really enjoying meeting a lot of good people in this part of the car sport. I already knew Mike Maier for several years, as he raced Midgets in USAC too. For those of you who know him only from AutoX, Mike won races in USAC Midgets too. He’s a hardcore racer & a winner.

I’m not an engineer. I’ve worked for them & with them … and had several work for me … but we’re from different planets. One of my friends said, “Ron is a race car designer that did not go to engineering school, so he speaks car guy.” I’m a veteran car guy committed to staying on the leading edge of performance & racing technology. I like to think I’m pretty knowledgeable in my areas of expertise … but I learn every week, if not every day ... and there is a ton I don’t know about street cars, style, paint & body, upholstery, car accessories, etc, etc, etc.

I know I’m tired of talking about me … so let’s outline the purpose of this thread.

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[Ron Sutton]

Larry & I discussed a game plan.

I'm going to launch a few Forum Threads dedicated to Track Performance under the Advanced Tech & Performance Discussion category.

This Forum Thread is for discussing & learning about “Designing Aerodynamics for Track Performance”.

This thread has a narrow focus, just as the title says.
For a thread focused on: Overall Handling & Tuning for Track Performance ... click HERE.
For a thread focused on: Front Suspension & Steering Geometry for Track Performance ... click HERE.
For a thread focused on: Rear Suspension & Geometry for Track Performance ... click HERE.
For a thread focused on: Measuring & Modifying Your Front Suspension Geometry ... click HERE.
For a thread focused on: Safety for Pro-Touring Track Cars ... click HERE.
For a thread focused on: Brake Selection ... click HERE.

I promise to post advice only when I have significant knowledge & experience on the topic. I don’t like to guess, wing it or BS on things I don’t know. I figure you can wing it without my input, so no reason for me to wing it for you.

A few guidelines I’m asking for this thread:
1. I don't enjoy debating the merits of tuning strategies with anyone that thinks it should be set-up or tuned another way. It's not fun or valuable for me, so I simply don’t do it. Please don’t get mad if I won’t debate with you.

2. If we see it different … let’s just agree to disagree & go run ’em on the track. Arguing on an internet forum just makes us all look stupid. Besides, that’s why they make race tracks, have competitions & then declare winners & losers.

3. To my engineering friends … I promise to use the wrong terms … or the right terms the wrong way. Please don’t have a cow.

4. To my car guy friends … I promise to communicate as clear as I can in “car guy” terms. Some stuff is just complex or very involved. If I’m not clear … call me on it. I’m writing some books and want car guys to understand them. When you’re really not clear on something I said … please bring it up & help me improve.

5. I type so much, so fast, I often misspell or leave out words. Ignore the mistakes if it makes sense. But please bring it up if it doesn’t.

6. I want people to ask questions. That’s why I’m starting this thread ... so we can discuss & learn. There are no stupid questions, so please don’t be embarrassed to ask about anything within the scope of the thread.

7. If I think your questions … and the answers to them will be valuable to others … I want to leave it on this thread for all of us to learn from. If your questions get too specific to your car & I think it won’t be of value to others … I may ask you to start a separate thread where you & I can discuss your car more in-depth.

8. Some people ask me things like “what should I do?” … and I can’t answer that. It’s your hot rod. I can tell you what doing “X’ or “Y” will do and you can decide what makes sense for you.

9. It’s fun for me to share my knowledge & help people improve their cars. It’s fun for me to learn stuff. Let’s keep this thread fun.

10. As we go along, I may re-read what I wrote ... fix typos ... and occasionally, fix or improve how I stated something. When I do this, I will color that statement red, so it stands out if you re-skim this thread at some time too.

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[Ron Sutton]

Aerodynamics is a complex subject, and while we may not all become experts at it, that doesn’t mean we can’t benefit significantly from the parts we do understand. So don’t feel like you have to be an engineer or aerodynamicist to learn & use aerodynamic concepts, aids & features. I have aerodynamic knowledge & experience … but I am not an engineer or aerodynamicist. I design winning race cars utilizing aerodynamic features on an ongoing basis.

Mechanical Grip:
Most of us understand how to achieve “mechanical grip” through our choice of springs, sway bars, shocks, tires and tuning settings of the front & rear suspension geometries. What we do with these mechanical tuning tools can increase or decrease grip on the tires.

Aerodynamic Grip:
A little more advanced is understanding how changing airflow, decreasing lift & increasing downforce can add grip to the car. There are a lot of tools we’ll discuss. They will be new to some of you & old hat for others. My goal with this thread is to share what I know, as limited as it is, and open discussion on how we can improve our PT & Track cars’ aerodynamic grip.

Both:
Because when you combine Mechanical Grip & Aerodynamic Grip … now you’ve got a fast, drivable track car. That is our goal here … to add aero grip to our mechanical grip. We should not try to use aerodynamics as a band-aid for a poor handling car. We should add them together for an optimum handling, class winning, kick in the pants to drive, safe, track car.

Where allowed by rules professional race teams run wings or spoilers in the rear, splitters, air dams or spoilers up front, flat bottom belly pans & diffusers underneath, ground effects or side splitters on the rockers, spill plates wherever possible & smooth, undisturbed body design … all to go faster. There is no need to debate if aerodynamics works in this day & age. Aerodynamics starts to have a measurable effect at speeds of 50 mph & above.

Properly utilizing aerodynamic knowledge, methods & aids can make your car faster on track, regardless of the car’s weight or power. Professional race teams, and the top sportsman race teams, embrace aerodynamics & maximize the use of aerodynamic methods & aids. But for most sportsman racers & Pro-touring track cars, aerodynamics is an afterthought & underutilized. I find aero aids that increase downforce or decrease lift to be cheap way to add grip & speed to the car … with more car stability & safety.

For aerodynamics, the 4 major ingredients are:
1. Force
2. Drag
3. Turbulence
4. Structure Design

Reducing drag is important to top speeds. Achieving downforce is important to handling.
If we’re running Daytona or Bonneville … we’ll put reducing drag as a priority over downforce. But in pro touring cars, short track race cars & road course track cars … downforce is king. F1 cars & Indy Cars in road course trim have horrible Coefficient of Drag numbers. But they have awesome downforce, and that is the key to more tire grip, higher cornering speeds & quicker lap times. There are some things you can do that will both decrease drag & increase downforce … and many other things where you must choose one or the other. Our priority needs to be downforce.



We care about three variations of “aero force” in track cars: downforce, lift & side force. Every object travelling through air creates either lift, side force or downforce. In the wind tunnel, all of these are measured in pounds of force. Downforce is the most common discussed term. It is referring to “force” pushing down on the body of the car, created by the air traveling over the body … and wings if they are utilized. If we can transfer this force effectively to the chassis, we will be adding force on the tires, therefore increasing the tires’ real world grip.

Lift is the opposite of down force. It is referring to “force” pushing up on the underside of the car, created by air traveling underneath the car. Left alone, this force is lifting the car and reducing force on the tires, therefore decreasing the tires’ real world grip.

On the track, at speed, downforce is our friend & lift is our enemy. They counteract … or fight … each other, but not necessarily evenly. It’s not uncommon for a typical passenger car to have 200-300# of downforce from air traveling over the body shape … and 600-800# of lifting force from air traveling underneath the car … at high speeds. When you add “gross lift” & “gross downforce” numbers, you end up with the “net lift or downforce” number.

We care about the total, but knowing how they look front & rear is critical too. If we have a car with:
• 150# of downforce on the front
• 75# of downforce on the rear
• 400# of lift in the front
• 200# of lift in the rear
• We end up with a net lift of 250# in the front & a net lift of 125# in the rear.

This is a “floaty” car overall … but the front being light is the biggest danger. This may seem like a small amount to some people but, at high speeds on track, I assure you the effect is not small. In this example, we’ve lost 375# worth of tire grip. On a 3200# car, that’s 8.5% less grip … most off the front tires.

It should be obvious that just adding a rear spoiler is NOT the end all solution to this problem outlined above. Yes, adding downforce front & rear can be part of the solution. But the smart place to start is reducing the lift … then look at adding downforce … as a package solution. A big key to good aero is to reduce the amount of air getting under the car in the first place … and getting what does go under the car out as quickly & smoothly as possible.

What you really care about is the net downforce number.

If we worked on this same car & did nothing to increase downforce … but did reduce the amount of air getting under the car … and helped the air that did to exit well. We could easily achieve:
• 150# of downforce on the front axle
• 75# of downforce on the rear axle
• 100# of lift in the front
• 50# of lift in the rear
• We end up with a net downforce of 50# in the front & a net downforce of 25# in the rear.
• This is a gain of 450# of downforce.
• That is a LOT of additional tire grip.

Of course, adding some downforce to go with that reduction in lift, would be even better. What does this added downforce do for your track car?
• Add grip to the tires
• Increase car control & safety
• Increase the speed capabilities the car can safely run at
• And if balanced, helps increase cornering speeds

A key component of aero downforce is “aero balance”. Before the COT & Gen 6 cars in NASCAR, teams had a pretty wide area to work in with body design & placement. Of course they did a ton of work to reduce undercar air & a lot of shaping to create downforce on the body. When you look at a car body, you may say “where is the downforce at on the body?” If it’s done right … it’s all over. There is downforce on the hood & front fenders, windshield, roof back glass & deck lid. After testing, the wind tunnel operators may say we have 1260# of net down force with this body shape.

But the downforce is not exactly even. At that time, NASCAR allowed the teams up to 10” of fore & aft range in mounting the bodies. So for short tracks that were challenging to get & keep the car turning in the corners … the teams would mount the bodies farther forward on the chassis to place more downforce on the front tires. At 1.5-2 mile super speedways where turning is easier, and rear grip is important, the teams would mount the bodies farther back on the chassis to place more downforce on the rear tires.

A tidbit of trivia for you NASCAR fans: When NASCAR would step in & change the rear spoiler rule … say from 6.5” to 5” … this messed up the aero balance so much, that teams had to cut off the bodies, throw them away & start over. Today, this is not the case, as the body rules have been tightened up a zillion miles compared to just 10 years ago.

Side force is not talked about much outside of professional oval track teams. But it is, or can be, a big deal. Side force is referring to “force” pushing on the side of the car … or wing in some cases … when the car is in a state of yaw. When a car enters the corner … and continues through the corner … the body on the outside of the corner is pushing through the air … similar to how the front end pushes through the air in a straight line. The opportunity for side force is greatest at initial corner turn in and reduces as the car gets to the middle of the corner.

This “side force” can help hold the car on the line at higher speeds. It acts like a tire grip increase. The air pushing on the outside of the car is countering … to a degree … the centrifugal forces that want to make the car slide off track. Side force can also be utilized more on one end of the car to help balance the handling.

In bodied cars, the shape of the front fenders can be designed to “catch” more air … to help the car turn into the corner better. In the NASCAR Modifieds we run on the West Coast, with 52-54% rear weight bias, being loose on corner entry is a challenge. We designed our cars’ bodies & the structure behind them on to achieve 65# of side force at the rear quarter fenders. This allowed our cars to be driven in 6-8’ deeper without getting loose on entry. Besides the lap times being quicker, that 6-8’ often meant the difference between making a pass or not.

Side force is most obvious on winged sprint cars. The veteran sprint car racers will tell you the “side boards” on the wing are more important than the wing surface. They want both side force & downforce, but the side boards aren’t just along for the ride to “help” the wing surface. The side boards play their own critical role in cornering performance. The aero side force is so strong in winged sprint cars, they can carry much higher cornering speeds around a quarter mile dirt track than any other car can on pavement … unless the pavement car also has large side boards.

The side force is so strong on winged dirt sprint cars … the car rolls the opposite direction when cornering. Yes … the car rolls to the inside of the corner, not the outside. It’s not a little bit either. All the modern sprint car chassis have the inside frame rail raised 1” … so it doesn’t bottom out on the ground in the corners.

Aerodynamic drag that we’re concerned about in track cars comes from three areas:
1. Frontal area
2. Surface turbulence
3. Flow detachment in the rear

Before we dive into each one, remember, anything you do that messes up the airflow in the front, will affect the air flow in the middle & rear of the car. For that matter, things that affect airflow in the middle, or rear, of the car … can & have affected the air flow at the front. Just not always. One end doesn’t operate by itself. Airflow is a “whole car” deal.

Frontal area drag:
Wind tunnels actually work backwards to how cars operate in the real world. In a wind tunnel the car is stationary & we blow wind past it at 120-160mph. In the real world the air is more or less stationary … and this high speed object comes along at 120-160 mph & punches a hole in it. For this reason, most experienced aerodynamicists tell me they’re “not looking for absolutes … just clues & trends” in the wind tunnel.

The parts of the car that are punching a hole in the air … are the nose & grill area of the front end … and the greenhouse. The nose & grill area includes everything at the front of the car either flat, raked or slightly curved. This includes the grill, headlights, bumper, spoiler, leading edge of the hood, etc. Most speed calculations that include aerodynamic drag numbers have you calculate the square inches of this area.

The next major part punching a hole through the air is the greenhouse. This may be a new term for some of you, but it is simply the part of the cockpit above the hood, doors & deck lid. The total greenhouse includes the windshield, side glass, A, B & C pillars, rear glass & roof. But the parts punching a hole in the air include the windshield, A-pillars & leading edge of the roof.

Obviously, the highest pressure area in a car is the nose & grill area. The second highest pressure area is at the base of the windshield where the airflow over the hood surfaces meets the steep angle of the windshield. This is where the air is pulled from for cowl induction with no negative effects.

Both the nose/front end & front of the greenhouse need to taken into account for true surface area pushing through the air stream. Both of these create less “air stall” and drag if they are raked back at an angle. The more the better … up to a point far past what we’re capable of achieving in our PT track cars.

Bow Wave:
As the car punches a hole in the air, it is creating a “wave” of air bowed around the front of the car. Think of a boat pushing through water … if it had a shape of your car’s front end … instead of the pointed v hull/bow it has. This is called the “bow wave” and can be as close as 3’ or as far out as 30’ depending on the car’s speed and the shape & rake of the nose & grill area. Rake angle plays the biggest role here. Raked noses are going to push the bow wave out the least & 90° vertical noses are going to push the bow wave out the most.

Why do you care?
• The farther the bow wave is pushed out by the nose shape & rake … the more drag the car has … with no gain in downforce.
• If the bow wave is bigger (height & width) … and too far out front … the air will not attach itself to the car. This is bad. More later.
• In lesser cases, it affects the front of the car … so no front downforce.
• Of course any airflow surface detachment at the front, will negatively affect the rear too.
• In worst case scenarios, the bow wave directs airflow over the whole car creating horrible lift & drag.

Surface turbulence drag:
The air flowing over the body surface is called the boundary layer. This layer of air tends to adhere or “attach” to the body because air has viscosity. There are two types of flow possible in a boundary layer: laminar flow or turbulent flow. In nonscientific terms laminar flow is a super thin (thousandths of an inch), super smooth layer of airflow that acts almost frictionless. Race engineers tell me that laminar flow does not exist in race cars or track cars in any quantity to matter, so the discussion in moot.



In nonscientific terms turbulent flow is "rough" where airflow tumbles and rolls as it flows over the surface, causing friction. Turbulence, tumble & friction are tied together … but it’s not all or nothing. You have different levels of turbulence, tumble & friction in different parts of the body. Even though the airflow is turbulent … up to a degree … it can still be “attached”. Imagine a tumbleweed blowing along the ground in a gentle wind. It is “rolling” or “tumbling”, but it is still touching the ground in most spots. Cars have turbulent airflow over the body … to different degrees.

This tumbling & rolling of the airflow creates drag & reduces down force. The less turbulent the air is over the body, the less drag created & the more downforce created. So simply smoothing out the airflow over the existing body shape … so more of the boundary layer of air stays “attached” … reduces drag & increases downforce.

In parts of the body where you have high degrees of turbulence, tumble & friction … the airflow detaches … and drag skyrockets while downforce goes to practically nothing. If you can lower the degree of turbulence, tumble & friction in this part of the body … and the airflow can stay attached … you will reduce drag dramatically & increase downforce significantly … in that part of the body. The more areas of the car you can keep the air attached to the body, the less drag & more downforce the total car will have.

Keeping this simple:
• Smoother airflow = less drag & more downforce
• Rougher airflow = more drag & less downforce

With a good, smooth, aerodynamic body design, aero drag increases at about the square of the increase in speed. With more turbulent airflow over the body, aero drag increases at a higher rate than normal. Anything you do to smooth out airflow … that effectively keeps the boundary layer better attached in more parts of the body … will make your car faster on track. We can see this in the wind tunnel with the smoke wand. The better the smoke follows the contour of the car, the more attached airflow we have. If it’s not, we have flow separation issues to fix.

Flow detachment creates rear drag:
The ideal shape going through the air is a tear drop or rain drop shape. The rear of a tear drop shape allows the airflow going around the mass to meet & merge back together smoothly, like the trailing edge of an airplane wing. This shape produces minimal turbulence. But cars don’t fly through the air … hopefully. Obviously cars run on the ground … which adds a surface into the equation … so cars are not tear drop shaped.

Manufacturers have been improving the aerodynamic shape of cars for several decades, to where now … to me at least … so many body styles of family sedans look similar. They do go through the air better, but many of them look like painted turds. I’ll have to check with some people and see if that is an acceptable technical term for an aerodynamics discussion.

All cars have turbulence behind them as the car punches a hole in the air … forcing the air in many directions. How much air goes which direction is based on the shape of the car. Most PT cars with body styles from the 50’s to the 80’s with more style … have more turbulence behind the car as it punches a hole in the air.



This is because the shape of these cars is not conducive to the air going around the body and rejoining together smoothly. The airflow detaches itself from the rear of body … then tumbles & churns … creating a vacuum effect. This vacuum effect sucks the air back to where it started from. Since the car is traveling at speed, this turbulence behind the car forms a continuous vacuum that sucks on the back of the car as it drives along … creating constant drag from the rear of the car.

While it is not practical to create a tear drop shaped car … it’s been done … you do want the air behind your car to flow back together as smoothly & gently as possible, with the least amount of turbulence & drag. If you do nothing to improve it … at least don’t do anything to make it worse.

Total Aero Drag:
The total aero drag of the car is the combination of rear drag created by flow detachment, frontal area of the nose & greenhouse and surface turbulence. The rear drag created by flow detachment makes up the majority of the drag … with the frontal area of the nose & greenhouse being second in contributing … and the surface turbulence drag contributing the least. The exception to this would be vehicles with a lot of objects affecting the surface airflow.

Understanding Airflow:
Turbulence comes in many variations, with many causes. When airflow changes from smooth tumbling flow which is good … to churning, boiling or crashing … we have excessive turbulence. In most cases, turbulence is our enemy, and our goal is to simply reduce it. Excessive turbulence adds drag, eating horsepower & speed.

Over key parts of the body or wing, where we need to create downforce, excessive turbulence interferes with us achieving optimum downforce. The bulk of aero lift comes from the air traveling under the car … running into objects like oil pans, headers, transmission, exhaust, suspension, etc … becoming turbulent & slowing the air flow speed. This creates lift because the air underneath can’t get out as fast as new air is coming in the front.

Smooth body work & gentle radius curves help air flow smoothly over a car body. Excessive turbulence is often caused by the opposite of smooth & gentle. When you can accurately describe parts of the body as abrupt, sharp, severe … those things will most likely create turbulence. The notchback style roofline of an 80’s Monte Carlo, Cutlass, Grand Prix & Regal all had a sharp trailing edge to the roof. This caused the air to “break away”… churn & crash … therefore create excessive turbulence … over the deck lid.

If the air isn’t flowing over the deck lid smoothly … therefore it is not “attached” … you will have less downforce in the rear. It was so bad the NASCAR teams running these cars convinced GM to make some with the “aero rear window” to help the problem. It didn’t solve the problem. It just made it less horrible. The late 80’s thunderbird had a much smoother transition from roof to back window to deck lid. That car made way more downforce & won a ton of races in Bill Elliott’s heyday, forcing GM to go away from “square cars” & introduce the more rounded, smoother Lumina.

Transitions are important. Transitions from hood to windshield … windshield to greenhouse … roof to back window … back window to deck lid … all need to be smoothed to minimize turbulence. Body gaps, steps, sharp edges … anything protruding up or out … all increase turbulence. If the air is flowing over a smooth surface and runs into a “step” … this step disrupts the smooth flow … causing the air to jump up into another airstream. This airflow running into airflow creates a buffeting effect … and disrupts the airflow past this point.

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[Ron Sutton]

Key concepts:

Bernoulli's Principle is often referred to in aerodynamics conversations. It states that as the speed of a moving fluid increases, the pressure within the fluid decreases. This includes gases & air. Slower air flow, if attached to the surface, creates more force. Faster air flow creates less force. Digest that a bit.

A quick primer about how air speed differential creates downforce or lift:
If the air under the car is flowing slowly … it creates more force … which is lift. If the air over the body is flowing fast … it creates less force … which would be downforce. So this combination creates little downforce & high amounts of lift. Not good.

For increased downforce, we want to speed up the air under the car … to decrease lift … and slow down the air over the body … to increase down force. Make sense?

This illustration is of a race car wing, which is upside down from an airplane wing. The concave design of the top side of this race car wing slows the airflow, which creates pressure. Pressure is force. The convex design of the bottom of this race car wing speeds up the airflow which reduces pressure. This air speed differential creates force pushing down … what we call creating downforce. That is how wings work.




On car bodies, severe angles … like the front nose & grill area, windshield & rear spoiler … slow air flow … building pressure & downforce. Flat or slightly curved, smooth, continuous surfaces … parallel to the airflow … with little or no objects intruding into the airflow stream … help the air flow faster.

While we can have somewhat of an effect of the air going over the body of the car, we can have a greater effect on what happens under the car. The easiest & smartest strategy is to prevent air from getting underneath in the first place. You can’t prevent it all … but you can reduce it a ton. And this creates a low pressure area, reducing lift and creating more net downforce. Downforce is grip … and grip is speed.

You still have some airflow under the car. The better you manage & direct it … and smooth its path to speed it up … getting the air out from underneath quickly, smoothly & cleanly … increases this downforce effect.

How do we speed up the air under the car?
• Reduce the volume of air going under the car at the sources (front & sides)
• Create smooth, flat surfaces
• Eliminate or cover objects that will disturb flow
• Help the air get out from under the car at the rear

Top side airflow crashing into underbody airflow:
In “most” standard production car body designs … especially older models … the airflow coming over the roof & off the decklid is at a angle to the ground. This mass of airflow comes off the rear of the car at a 40° to 60° angle … to the more or less horizontal airflow coming out from underneath the car. Where these two airflow streams “crash” into each other … an airflow boiling effect is created which is very turbulent.

This is very bad in many ways:
1. This is the opposite of flowing together smoothly & cleanly like a tear drop.
2. This extreme turbulence creates extreme drag on the rear of the car.
3. This turbulence slows the airflow underneath the car trying to get out … so lift increases.

The steeper the angle of airflow off the deck lid, the worse the problem. We need to address this problem from the top side & bottom side to redirect the airflow to come together more smoothly, cleanly with less turbulence. This requires a much gentler angle of airflow to meet & merge back together smoothly, like the trailing edge of an airplane wing or a rain drop.

For the underneath, a diffuser … if designed effectively & fed the underbody airflow correctly … will help the airflow come out in a cleaner stream with less turbulence. A flat bottom belly pan is not required for an effect, but helps the process and can increase the effect up to five-fold.

For the top side, we need to redirect the air off the body so it is closer to horizontal, or said another way, at less of an attack angle. This is where the roof, rear glass, deck lid & spoiler (or wing) all come into play. This may surprise you, but the deck lid is the key. Assuming we’re getting good attached airflow over the deck lid … the longer the deck lid is … the more it will straighten out the airflow closer to horizontal. This is key.

But for this to work, we need attached airflow over the deck lid. The roof shaped with a smooth gentle convex curve to the back glass … with no steps, dips or bumps … and the back glass at a gentle convex radius & angle (like a fast back) to the deck lid … also with no steps, dips or bumps … are key to keeping the airflow attached. Race car designers spend more time here than you can imagine.

As long as attached airflow is going over a medium to long deck lid … we have redirected the airflow closer to horizontal … which will help the airflow to come together more smoothly, cleanly with less turbulence at the rear of the car. This produces less drag & more downforce. Short deck lid cars are harder to achieve this with. Pro Stock drag racers have the right idea with the rear spoilers. They run them horizontal, or even a few degrees down at the trailing edge. Basically, they’re just making the deck lid longer … so it positively directs the airflow off the rear of the car … close to horizontal.

These same Pro Stock racers use a short wicker bill (also known as a Gurney flap) on the trailing edge of the spoiler. This helps slow the boundary layer down for increased pressure & downforce … and helps the airflow make a clean detachment … which is key for the upper & lower airflow to merge smoothly.

For cars running wings in the rear, proximity to the body is key. Yes, the wing will have cleaner air up high, away from the deck lid. But then it’s not helping the airflow straighten out off the body. When a rear wing is mounted to the deck lid … at the correct height … the air flowing over the top of the wing surface is making downforce. While the air flowing underneath the wing is helping to direct the airflow off the deck lid at a more horizontal angle. This height varies with body shape. Also, when using a wing, the deck lid needs a sharp edge, or hard corner, at the trailing edge for the airflow to make a clean detachment.

Spoilers with serious angle … say 45° to 70° like used in stock car racing … are a compromise. In stock car racing, often the rules require the spoilers be at a certain minimum angle. For many short track series, that angle is 55° and 70° for others. This is done with the “intent” of keeping the playing field level. LOL

This is an critical concept to embrace. This will be new to some of you & old hat for others with more aero knowledge. The downforce is NOT on the rear spoiler. The downforce is created on the rear deck lid. The spoiler just acts as a big wicker bill to slow the airflow down … which increases the air pressure & downforce on the deck lid.

Spoilers that are taller or angled more upright, slow the air more, which is what increases the air pressure & downforce on the deck lid. But with this … you’re increasing drag … a ton … because the spoiler makes the exiting airflow tumble violently. You’re achieving two good effects & two bad effects at the same time.

The two good effects:
• Adding downforce to the rear of the car
• Straightening the airflow off the deck

The two bad effects:
• The airflow coming off the tall, severely angled spoiler is tumbling violently & very turbulent.
• This enlarges the vacuum effect at the rear of the car … increasing drag exponentially.

So you gain downforce & reduce lift … but the drag goes up a lot! This is why drafting works on high speed tracks. There is a vacuum at the rear of these stock cars sucking them back. If another car can get its nose up in there, the two cars “suck together” and share one frontal area & one rear vacuum drag for the two cars.



Energy Loss:
While we’re on spoilers, the discussion of energy loss needs to be broached. The actual net force that occurs … whether it is downforce, side force or lift … can be affected by energy loss through the structure. I learned about this in my drag race days of the 1980’s. One night in the race shop, while the guys & I were having an adult beverage, we happened upon an important aerodynamic concept.

This was back in the day that Pro Stock cars ran fixed angle spoilers at 15° to 25°. One of the fat crew guys leaned on the lightweight fiberglass deck lid … it flexed in … but the rear of this softly sprung car … nor the rear tires with 4.5 psi in them … didn’t really squat any. Hmmmm. One of the guys wondered if we’re really getting all the downforce that we think we’re getting to the rear tires. We played around & decided to do some R&D.

We reinforced the deck lid structure. We built a new spoiler that hinged at the leading edge where it connected to the deck lid. We reinforced the 12” long spoiler itself. We made an adjustable support on the rear of the car to allow for quick & easy spoiler angle changes. We built a light, but strong, chromoly tubing structure in the rear of the chassis that provided solid support for the adjustable struts off the spoiler trailing edge, pivot point of the spoiler’s leading edge and the deck lid.

In the shop, now when we pushed on the deck lid or spoiler, it pushed down on the chassis & tires. So we went track testing. We started with the spoiler at the same angle as before, which was 17°. Our testing got side tracked because we were having engine problems on the top end. The driver said the engine was bogging down in top gear. We went through all the normal checks, fuel pressure, spark, EGT, etc. No problems.

One guy … a not very sharp guy … said, “do you think we might have added downforce and drag … and it’s making the engine bog?” No. Of course not. It’s a mountain motor making obscene power. Hmmm. Well, let’s try it.

We lowered the rear spoiler angle to 13° … and the engine ran better. The ET got quicker & the mph went up. Hmmm. We lowered it to 10° … and it ran quicker & faster. We lowered it to 7° … quicker & faster. We went to 5° … then 3°, 2° & 1° … all quicker & faster. I asked the driver how the stability was & he said it was great, better than ever. At the end of the day, the car was running quicker & faster than we ever had.

The lesson here is … that I have since experienced a zillion times over the next 25 years of racing … you are wasting force energy … be it down force or side force … if the pressure on the panel gets lost through flex in the body or body structure. And apparently, our “not so sharp” guy was right.

Think of it this way: If you’re pushing on a panel with 80# … and that panel and/or the supports flex … that flex is eating up some or all of that 80#. It’s like electricity running through wire that is too small of a diameter for its length. You’re not getting the same net result at the other end. Another analogy is chassis flex. If you send 800hp through the car, but the chassis & body flex a lot, that flex is eating up a bunch of your power.

Airflow over different shapes:
When designing a car to be more aerodynamic, it’s important to know how air flows over different shapes, so you know what shape to use where. Remember the rain drop or tear drop shape is the most aerodynamic. An airplane wing is simply an elongated tear drop shape … to provide surface area for lift. Or in case of race cars, where it’s flipped over … surface area for down force.

Round is a great leading edge shape, but a horrible trailing edge shape. A piece of round tubing in the airflow stream has a lot of drag. For this reason race car designers of dragsters, Indy cars, GT cars, etc, use aerodynamic tear drop tubing (aka Streamline Tubing ) on struts or suspension pieces exposed to airflow.



Concave surfaces, slow the airflow speed, creating more air pressure & force. Convex surfaces increase the airflow speed, creating less air pressure … even a pressure drop … and therefore less force. Again, think how an airplane wing is designed.

Smooth, flat surfaces are “ok” at creating force. They are less effective than concave & more effective than convex.

Speaking of concave. When you look at roof of cars with two “bubbles” where the passengers heads are … what’s in the middle? A concave area to create downforce. These car designers could have just as easily made the roof flat or domed (convex) … but that wouldn’t optimize the airflow for downforce. Another example is with Trans Am cars & stock cars that are wider at the front & rear fender wells … and narrower at the doors. This concave area creates side force, helping the car carry more cornering speed.

Airflow will follow convex shapes … like a roof or fast back style back glass … as long as the radius isn’t too sharp. This is key in designing or deciding on windshield shape, roof shape & rear window shape when designing & building a car from scratch. Or, important to understand if you’re choosing a car body with the right shapes already on it. I love the 80’s G-body style cars & had a Hurst/Olds style Cutlass drag car in the mid-80’s. But I wouldn’t choose that sharp notchback style for optimum aerodynamics with what I know today.

The decision to make body corners with sharp, hard 90° edges … or gentle corners with a large radius … depends on your goal. Where you want the airflow to stay attached … for side force or downforce … you want to use gentle corners with a large radius. This is what you want on the sides of the car’s nose blending into the side fenders. Where you want the airflow to become quickly detached … to break away cleanly … you want to use sharp, hard 90° edges or even short wicker bills.

.

[Ron Sutton]

Body shapes for decreased drag & increased downforce:

Nose/Front End:
• As small of total surface area as possible.
• Steeply raked back to the top, with the lower valance, spoiler or air dam out as far forward as possible.
• Mount the valance, spoiler or air dam as low to the track surface as possible.
• Get the splitter or spoiler strip “just skimming” or “almost touching” the track surface in full dive.
• All components as flush as possible with the surface of the nose (grille, headlights, trim, spoiler, etc)
• Shape the nose either more rounded, or pointed, in the center … versus flat … so the air splits.
• Smooth radius around sides where it blends into fenders.
• Smooth radius around top where it blends into hood.

Hood/Cowl:
• Hood surface flat & smooth as possible.
• Hood surface lower than fenders or concave in shape.
• If shaped properly, more surface area creates more front downforce.
• Anything protruding through hood detaches airflow at that point.
• A tall front opening hood scoop, even of the perfect shape, adds drag & disrupts airflow.
• If a hood scoop is needed, the cowl induction style is best for aero.
• Eliminate, cover, or smooth vents in cowl.
• Remove windshield wiper arms
• Eliminate, cover, or smooth wiper arm holes in cowl.

Greenhouse:
• As small of total surface area as possible (lower chopped roof).
• Steeply raked windshield.
• Smooth transition from hood/cowl to windshield.
• Flush mount windshield to a-pillars & roof (no trim).
• Smooth, gentle radius of A-pillars.
• Remove rain drip rail & smooth.
• Flush mount door & rear quarter glass to A, B & C pillars.
• Gently sloping rear window, like a “fastback” car model, as opposed to a “notchback.”
• Smooth radius at transition from roof to rear glass.
• Flush mount rear glass to c-pillars & roof (no trim).
• Roof flat or concave when viewed from front or rear.
• Roof flat with smooth radius rear of roof where it blends into rear glass (from side view)
• Spill plates ran along both sides of roof & back glass to keep boundary layer from spilling over sides.

If you run on track with no windows, or the windows down:
• Airflow in the cockpit is drag … with no gain.
• The B-pillars need large, smooth gentle radius from cockpit to body.
• Keep the B-pillars angle to the body less than 90° at its steepest angle. 40° - 60° would be good.
• You can even use a wicker bill sort of vertical trim to keep airflow out of the cockpit.

Rear Body:
• Smooth transition from back glass to deck lid.
• Deck lid surface flat & smooth as possible.
• Deck lid surface lower than fenders or concave shape.
• If shaped properly, more surface area creates more rear downforce.
• Longer deck lid creates more downforce, but more importantly redirects exiting airflow more parallel to ground.
• Fenders curving in from rear axle centerline back, so air can converge together easier.*
• Trailing edges “sharp” 90° angles to bumper & tail light panel … for clean flow detachment & less tumble.

* This is not the route to create side force. To create side force, you want the rear fenders to be as flat as billboards, and act like the side boards of a sprint car wing. If rules permit, a small wickerbill running vertical at the trailing edge of the rear quarter fenders provides cleaner flow detachment & less tumble.

.

[Ron Sutton]

Discussion of Aerodynamic Aids

Wings:
While the exact curves, shape & dimensions of wings vary in design slightly, for the most part the concept of wing shape is already set in stone. Don’t “reinvent the wing.” The leading edge is the part of the wing that first contacts the air and needs to be smoothly rounded. The trailing edge of the wing comes together to a point to aid the air merging back together smoothly. The top of the wing (in race cars) needs to be concave to slow the air & create a high pressure area. The bottom of the wing needs to be convex to speed up the air & create a low pressure area.



As long as you’re buying or making a wing of proven design & shape … surface area is the key to how much down force it is capable of. Simply more surface area is more downforce capability.

Wings can be mounted on the front or rear of track cars. How close, or how far way, you mount the wing to the body surface will play a role in both the wing’s effectiveness & airflow over the body. If you place the wing higher, away from the car’s body, in clean air … it will be more effective at producing downforce because it's out of the car's turbulent airflow. But this does not help the airflow over the car body. As mentioned earlier, if you place the wing closer to the body, the airflow under the wing, can help you direct the airflow over that part of the body.

Wing angle, also known as attack angle, affects the wing’s actual downforce being created. More downward angle in the front equals more downforce … up to the point of “stall”. When you angle the wing downward too far … you reach airflow stall speed … and downforce actually reduces. Drag continues to increase with excessive wing angle, but downforce starts decreasing. For most wing designs this is somewhere around 22° of downward front wing angle.

Side plates:
If we were primarily driving straight, side plates on wings would less needed. Their primary role is to keep airflow on the wing surface, as opposed to spilling off the sides. They do make the wing somewhat more effective in a straight line. But in the corners, is where they really earn their keep. They help airflow stay on the wing surface … while the car is in a state of yaw turning … and the airflow isn’t parallel with the wing. Plus they act as a rudder … helping to hold the car in place.



In sprint cars, the side plates are massive, and called “side boards” by the racers. As mentioned above, veteran sprint car racers will tell you the “side boards” are more important than the wing surface to cornering speed. Realize side plates … and especially massive side boards … do add drag … as will any surface you add that is exposed to the airflow stream.

Wicker bills or Gurney Flap:
Back in the day, Dan Gurney added a strip of aluminum at the top trailing edge of his Indy Car wing for more downforce. As discussed earlier, this slows the air speed across the top of the wing surface, creating a higher pressure area & more downforce. The concept of wicker bills already existed in aircraft, but since Dan Gurney did it in the early days of Indy car racing, open wheel racers called it a Gurney Flap.

The height & angle of the wicker bill all play a role in its effects.
• Taller and/or steeper angles close to 90° … create more downforce … and more drag & turbulence behind the wing.
• Shorter and/or laid back angles less than 90° … create less downforce … and less drag & turbulence behind the wing.

A short wicker bill with a 50°-75° angle lip can be a good tool in a lot of places where you need the airflow to create more force in front of it … and detach cleanly behind it.

Aero Wing Struts:
When building struts to install your wing(s), about the only shape worse than round would be square tubing. Anywhere you have round tubing exposed to airflow … you are adding unnecessary drag. If you design your wing struts with aerodynamic tear drop tubing (aka Streamline Tubing ) the drag in that area will be less than half of what round tubing produces.



Rear spoilers … work like a wickerbill:
Remember, the deck lid is where the downforce is pushing on the car, when you’re utilizing a traditional spoiler at any angle above 45°. The spoiler itself is just a big wicker bill. The taller it is … and/or the steeper the angle … the more downforce will be created … along with more turbulence & drag. If you lay the spoiler down more, the spoiler itself starts to become part of the deck lid surface the airflow is pushing down on.

Just like wings, spoilers can have wicker bills too. It is not uncommon on road race, oval track & drag cars to see wicker bills being utilized to slow the boundary layer of attached airflow down for increased downforce. I am a fan of this strategy, because it allows the main spoiler angle to be less, while maintaining the same downforce. This combination produces less drag & less turbulence behind the car.

Let’s classify the two spoiler strategies as Circle Track & Drag Racing. Circle Track spoilers may be 4” to 6.5” tall, but often at 45° to 70° angles. So they act as big wicker bills to slow the air over the deck lid, but don’t add much to the effective deck lid area. Drag Racing spoilers can be 10” to 14” long and add significant surface area for downforce to be created … due to their low to zero angle … but don’t slow the air down much. They can & do utilize wicker bills to affect the boundary layer & slow the airflow speed.

Side plates work on spoilers just as they work on wings. Depending on design, they keep the airflow on the deck lid and/or spoiler surface & act as a rudder … helping to hold the car in place.

For track cars, I am a fan of running:
• As large of a rear spoiler as I can run
• At a relatively low angle
• Designed for quick & easy spoiler angle adjustment
• Well supported for zero energy loss
• With side plates as large as I can run
• And a removable wicker bill (to run different height & angles)

Roof & rear glass spill plates:
The thin strips of aluminum you often see attached to outside area of roofs & rear windows on race cars are called spill plates. They act similar to the spill plates on wings & spoilers, in that their role is to keep the boundary layer of attached airflow in place. “In place” on the roof & back window achieves two key things. First is additional downforce in those areas. Second is the airflow is kept going the direction needed to apply pressure & downforce on the rear deck lid & spoiler.

While we are in this area of the greenhouse, another goal is to keep the boundary layer of airflow going over the rear quarter windows … attached … so the air flows around the greenhouse and onto the deck lid … for more downforce.

Vortex Generators:
I worked directly with Gary Wheeler back in the 80’s. He is the aerodynamicist that patented the Wheeler Vortex Generators & worked with Kenny Bernstein & other drag racers working on aero racing projects. Vortex Generators are problem solvers. Vortex Generators do what they name implies … they generate vortices (the plural of vortex). Airflow swirling in controlled vortices follows shapes better. This is important.



If the rear glass turns down at too steep of an angle, from the roof, to keep the airflow attached … Vortex Generators placed on the roof just before the steep turn down … creates vortices in the airflow stream … keeping the airflow attached & following the window. Before VG’s … the airflow was separating and coming rearward at downward angle “maybe” hitting the rear of the deck lid or spoiler ... for a “little” downforce … before it crashed into the air coming out from under the car.

Now, with VG’s in place, the airflow follows the rear glass … then the deck lid & spoiler … creating substantially more downforce. This also directs the air off the rear of the car at a more horizontal angle, helping it to merge with the airflow from underneath the car smoother, for less turbulence & drag.



Another key problem they solve, is the normal tumbling, turbulent airflow where it separates from the rear of a vehicle. With VG’s installed, the air exits in controlled swirling vortices … instead of tumbling … which reduces the drag at the rear of the vehicle … and smoothes out the merging of airflow with air from underneath the car.

For years Gary sold his Wheeler Vortex Generators to aircraft companies to put on the underneath leading edge of aircraft wings … to increase lift … and increase the cargo carrying capacity of planes. He sold them to racers that used them to fix aero flow problems or increase downforce. He sold them to normal commuters who used them to increase fuel mileage. Same to the trucking industry. Today, BMW, Subaru & others use vortex generators of their own design to aid aero flow on their sporty cars.

Gary no longer makes or sells the original Wheeler Vortex Generators. He sold the patent to some good guys that run a company called Air Tab. The design is similar & different in key ways. Today, I use the Air Tabs the same way I used the Wheeler Vortex Generators. They must be placed just in front of where you want them to work. See them here.

Air Dams / Spoilers:
A front spoiler is typically an add-on piece partially for style & partially for function. They come in a gazillion shapes & dimensions. Many known as “chin spoilers” mount at a rake angle. An air dam is typically a vertical panel either added on, or built into, the nose bodywork of the car. The shape of this vertical panel typically follows the shape the nose bodywork. In racing, extender lips are often added to the air dam that can be adjusted to just touch the track to better seal off air.

Both air dams & front spoilers are designed to reduce airflow underneath the car, to reduce lift. Air dams are more effective at sealing the car’s bodywork to the track, so they are capable of almost eliminating airflow getting underneath the front of the car in the corners. Spoilers don’t usually get this low, nor cover all the way across the front of the car as well. For this reason, I think of spoilers as “under car airflow reducers” & air dams as under car airflow eliminators.”

A well designed front spoiler or air dam reduces the volume of air getting under the car. This creates a low pressure area & vacuum effect to help hold the front end down through the corner. Front spoilers & air dams don’t create downforce in the typical sense. They do so by reducing lift & creating a low pressure vacuum area, helping to suck the front of the car down to the track.

A well designed front spoiler or air dam … combined with a sizable splitter … reduces lift, creates a low pressure vacuum area … and adds downforce on the splitter … creating more net downforce than the spoiler or air dam would create on their own.

Splitters:
Attach at the front of the car, under the air dam or nose bodywork, and have a flat surface typically ranging from 1-5” running parallel with the ground line. Their role is to split the airflow that goes underneath the car … from the airflow that goes up & over the hood or around the side of the fenders. Very short ones split the airflow to a degree … and reduce lifting force … but create very little downforce. This is a nice, solid gain in front downforce by reducing the lift. Short splitters can be mounted without braces if well attached to the air dam or bodywork.

Longer splitters split the airflow more positively and create downforce on the front of the car. This is a big gain in front downforce, increasing front tire grip, making the car run flatter, improving turning ability, increasing corner speeds. Because the airflow is actually putting downforce on the longer splitter, the splitter needs supports to prevent it from buckling under.



If you want your Pro-Touring car to be a serious track car, adding a quick on/off splitter you can run at the track is a big gain. Take it off to load in the trailer, dive on the street, etc … and just put it on at for track days.

Rake:
Simply lowering the front of the car … relative to the rear of the car … helps reduce lift in two ways. First, it reduces the opening at the front of the car, reducing the volume of air that can get underneath the car to start with. Second, the larger opening in the rear of the car makes it easier for this smaller volume of air to get out.

I’ve seen guys drop the nose ½” in ride height in the front and increase the front tire grip a TON. Partially because it reduced lift & partially because it shifted weight balance, physically loading the front tires more & the rear tires less.

Ground Effects Rocker Panels:
A high volume of air flows down the sides of the cars. For decades, car body styles had the doors & fenders shaped to roll under at the bottom. This contributes to air below the beltline (mid door) to rolling under the car and adding lift. In the 80’s ground effects rocker panels were added to the doors & fenders of cars like the Camaro, Trans Am & many production sporty cars. It was partly for style, but worked functionally to prevent this airflow down the side from rolling under the body & adding lift.

I was in drag racing at that time & if you were building a serious Pro Stock car, you didn’t consider a body style that didn’t come with ground effects. I had a customer who had me build him the new C4 Vette body when it came out in 1984. I tried to talk him out of it for logical, performance reasons, because that car had the worst “curve under” I’d seen in modern cars & no “factory” ground effects package was offered. He loved the look & had me build it anyway. As fortune would have it, we became racing partners down the road. I had to fix that problem & many other aerodynamic flaws in that body design, to make the car drivable & competitive.

Side Splitters / Rocker Extensions:
For cars that don’t have well designed ground effects on the rockers, adding side splitters or rocker extensions, is a very effective tool. A side splitter works similar to a front splitter, except they are on the side of the car. They are placed 90° to the body (or parallel with the ground, like a front splitter) and extend out anywhere from 1-3”. The side splitter prevents airflow from rolling under the body. It splits the air above it, creating a high pressure zone just above it. This reduces lift & adds net downforce, with minimal addition to drag.



Side Skirts:
Don’t confuse side skirts with side splitters or rocker extensions. Side skirts are typically strips of rubber or plastic mounted vertical to the car from underneath. The purpose is similar, to prevent air from getting under the car. This is typically a street car or grass roots racer addition because the car is too high. You don’t see this in most upper ends of racing, because we get the car as low as the rules allow, so there is no room, nor reason to use a rubber or plastic skirt.

Diffusers:
If designed correctly, diffusers help evacuate the airflow out from underneath the car faster, smoother & cleaner. As with everything, any time we speed up the airflow, we’re reducing pressure. When the airflow is under the car … speeding it up greatly reduces lift … adding to the total net downforce achieved with the car.



The diffuser area increases in volume as it exits the car. This venturi effect helps suck the air from underneath the car. As the airflow exits the diffuser it is running slightly uphill … helping to get it away from the turbulent track surface … and easing its smooth transition with the airflow coming over the top of the body.

Proper shape of the diffuser roof is “gradual convex” so airflow will stay attached & follow. If you make the diffuser roof too tight of a radius, the airflow will detach & tumble, negating any gain. The vertical dividers inside help channel the air. On cars where the diffuser has these on the outside, they’re called vertical fences, with the goal of not allowing airflow from the side of the car’s rear body to spill under & disturb the airflow out of the diffuser.

Flat Bottom Belly Pan:
The single biggest strategy to speed up airflow underneath the car to reduce lift, is to run a smooth, flat belly pan every where possible to cover up the obstruction jungle underneath the car. Frankly a diffuser is a band aid … a helpful band-aid … but a band-aid nonetheless … without a smooth, flat belly pan.

What causes the airflow to be slow … and create lift … underneath the car is the airflow running into the oil pan, headers, bellhousing, trans, mufflers, transmission, up-&-down floor pan, etc, etc, etc.

Where ever you can add a smooth flat surface, running parallel with the direction of airflow, you’re speeding up that airflow, and reducing lift. My PT Track car project has a smooth, flat belly pan designed to cover 81% of the underneath. It covers everything mentioned and the rear suspension … and removes with Dzus ¼ turn fasteners for access.

Hood Vents:
The aerodynamic goal with hood vents is to remove some of the pressure & lift under the front of the car. There are pros & cons, so there supporters & detractors. For hood vents to work well at reducing lift, the airflow needs to exit near the front of the hood. This disrupts the airflow over the hood. Does the reduction in lift offset the loss of downforce on the hood?

I don’t think it can be stated one way or another “for sure” without testing the effectiveness of the hood vent design & its impact on airflow across the hood.

Several auto manufacturers have made the hood vent a major & successful part of their race car design, such as the BMW Z4 GTE cars. I believe the key to their success with this design is how they blend the hood vents, hood structure & windshield angle together. That is a very successful car that no doubt took some serious wind tunnel & track time to develop.



I’m positive any of us can make a hood vent system work well & remove air pressure & lift from under the front of the car. I am concerned the project of getting the airflow to work in harmony with airflow over the hood & windshield may be outside the resources for most of us.

Side Vents / Front Fender Vents:
On the other hand … utilizing side vents in the front fenders to remove excess air pressure from under the car … to reduce lift … and to extract hot air from engine compartment … makes a lot of sense. It is much simpler, with less challenges and relatively easy to achieve success with. First, you’re not disrupting airflow that provides downforce … as the boundary layer of airflow over the hood does.



You are causing airflow detachment on the side of the fender, but if the side vent is designed almost parallel with the fender, this will be minor & the airflow can reattach itself as it flows down the side of the door. Regardless, any minor loss of side force is inconsequential compared to the significant gains in down force by reducing lift under the front end.

Tires:
Tires have a horrible aerodynamic shape & they’re rotating at high speeds with disturbs the airflow even more. Having tires exposed hurts air flow. A little F1 trivia: F1 cars have pretty high coefficient of drag numbers. Higher than most people think … around .75+. This is because of the exposed tires, wings & other aero aids adding to the drag. But they don’t run Bonneville … so downforce makes them faster on the twisty tracks they run.

Fender Lips:
In race series that allow, knowledgeable teams utilize short fender lips on the leading edge of the fenderwell to detach the airflow out away from the tires. These look like short, wickerbills curved to match the radius of the fenderwell. They only go on the front & slightly curve up to the top of the fenderwell opening. Some teams install these lips at 90° to the body surface, but 75-80° works better.

In the rear of the fenderwell opening, knowledgeable teams create an inner lip, that curves in from the fender. They extend in several inches with the goal of keeping airflow out of the tires inner fender area. Some teams install these lips at 90°, but again 75-80° works better.

Fender openings:
In race series where teams can’t run any fender lips, the teams often pull the front of the fender outward, to better cover the tire, and guide the air “around the tire.” They push the fenders behind the tires inward, so as to not catch air. This is most evident in short track stock cars, where racers have the fender area just in the front of the front tires pulled as wide as the tech inspector will allow.

The final design tip for fender openings is to not make them any bigger than they need to be. Bigger gaps simply means more air can get in the fenderwell area, create drag & lift, and disturb the air flow down the side of the car.

Aerodynamic hindrances that don’t decrease lift nor increase downforce … just add drag.
• Front opening hood scoops
• Excess airflow through the grille into engine compartment
• Any objects protruding from the surfaces in the airstream: bumpers, irregular shape grilles, headlight rings, hood pins, emblems, drip rail, wing window trim, door handles, windshield wipers, mirrors, etc.
• Anything that makes the airflow boundary layer jump, step or skip: Windshield trim, vent slots in cowl, gaps in body seams, bolt on aero aids like certain spoilers, wheel flares, etc.

NACA Ducts:
When you need to pull air from the outside to inside to cool the driver, rear brakes, etc, placing a NACA duct is an option. For it to work, it must mount on a surface that has good attached boundary layer of airflow, so the duct can scavenge this slower moving air.



Brake Ducts:
The front brakes really need cool air ducted to them. If not, you have to run larger, heavier rotors than you would otherwise need to deal with the braking heat Heavier rotors add to the rotating weight & the larger diameter moves this rotating weight farther out on the rotational axis … increasing the flywheel effect. This hurts performance, so you do not want to run larger or heavier brake rotors than needed.

To keep the brakes cool in track cars, you’ll want to duct some cool air to the rotors. I suggest 3” diameter, smooth inner wall, fire proof ducting. In serious braking applications, race teams will run 2 or 3 ducts per side. Where you pick up the cool air is a decision to be made. Take into account this type of duct works best with low velocity air. Typically, brake duct scoops are mounted behind the grille at some point, behind fake headlights or just above the splitter.



When you mount the brake duct scoop, make sure the air is actually flowing “in” at that point. I have ran across race cars with rounded noses, where the air is only going straight into the grill in the middle 50% of the grill area … and the bow wave was making the air go around the rest of the front end … so their duct scoop out at the end of the grille, under the headlights, was NOT getting air coming in at all. The air was going “across” the scoop opening.

You can always count on the center of the grille area having air coming inward, but you don’t want to interfere with airflow cooling the radiator & oil coolers. I often mount brake duct scoops behind the grille, outward from the radiator air stream, as long as I’m sure the air is coming “in” at speed and not “across”. Another great place, is if you have a vertical air dam with a splitter, is attaching the duct scoop to the airdam, above the splitter.

Lastly, I’ve seen racers effectively pick up air flow from mounting the scoop to the belly pan under the front of the car. I don’t do this, because my cars are high travel set-ups. With the splitter or air dam grazing the track surface … under braking & cornering … there’s not much air to cool the brakes when they really need it. But if you’re not bringing the front end of the car down as far, this may be a viable option for you.

Effects:
If you add aero aids & strategies to an existing car, be prepared to tune on your suspension as you achieve results. If the things you do “work” … it will change the handling of the car, and therefore probably require suspension tuning.

If you increase downforce on the front end of the car, the suspension will compress more during braking & cornering. This will load & work the front tires more ... and more evenly. If the car was neutral before … and the aero change is significant … the car may now be loose in the corners. It may or may not go any faster until you add rear grip to balance the car.

But when you do add grip to the rear of the car … to match the increased grip in the front … the car is going to be significantly faster in the corners, lowering your lap times.

If you design a car with all the aero aids & strategies right from the start, you will simply be tuning the suspension towards the optimum set-up from the start and won’t notice this.

If you make an “aero change” to the car … and there was no noticeable effect to the handling or lap times … what you did either didn’t work, or maybe it did work, but there are conflicting effects. This is sometimes hard to see, feel & measure when your track days are a month apart. But if you can make laps with something “on” … then “off” … then back “on” in the same day … you’ll really see the effects. We call this A-B-A testing.
.

[Bryce]

Ron,

After a quick read through it looks very well written. I am going to dig deeper now. I might even have to pull out some of my old engineering books on aerodynamics.

[Damn True]

Ron, I'm pretty sure I speak for more than a few members here when I say that this level of tech, even if only fractionally applied satisfies the nerd-lust many of us harbor.

Looking forward to the follow-up posts.

[Greg from Aus]

Ron your knowledge and ability to pass it on amazes me. I always look forward to you posts.

Regards
Greg

[Greg from Aus]

Double post sorry

[dontlifttoshift]

Easily, the above are the most interesting posts that have ever been written on this forum.

[Ron Sutton]

Ron,

After a quick read through it looks very well written.
Cool. Thanks !

I am going to dig deeper now. I might even have to pull out some of my old engineering books on aerodynamics.

Uh-oh. If you go deep with engineering terms, you'll lose me. Please keep it "car guy" so I can participate.

[Ron Sutton]

Ron, I'm pretty sure I speak for more than a few members here when I say that this level of tech, even if only fractionally applied satisfies the nerd-lust many of us harbor.

Looking forward to the follow-up posts.

Thanks True. This stuff isn't new or hard to do anymore.

So, I believe many of the aero aids can be utilized on our track cars & PT cars with success.

[Ron Sutton]

Ron your knowledge and ability to pass it on amazes me. I always look forward to you posts.

Regards
Greg

Thanks Greg. Chime in when ever you want to discuss stuff.

[Ron Sutton]

Easily, the above are the most interesting posts that have ever been written on this forum.

Thanks for the kind words. I love all the technical stuff.

Aero is a big deal & easier to do that a lot of folks think. I feel that because we're not aerodynamicists, some folks are afraid to tread into this territory. But even small things that reduce lift, smooth out the airflow or add downforce add measurable performance to our cars.

[parsonsj]

Nice. Lots to read.

Just a quick question: are there sensors available to measure lift/downforce? My first thought (which is 1/1000 as well-formed as your posts) is that without data, much of this is based on lap-times, which brings a lot of other variables into play.

[NOT A TA]

Thank you for taking the time to write the posts above Ron. I learned a few things and the ideas of B pillar mods and fender lips are things I hadn't considered for my aero project upgrades but will now.

I have a 70 Firebird that I use for road tracks, Land Speed Racing, drags, and street. I do not Auto-X the car. I've got a plan of installing many of the aero devices you mentioned in your posts and have the car apart now for these and other upgrades. Your comments on brake duct opening placement have me wondering if I placed mine where they will work well without fans in the tubing. I'm planning on a couple different aero "packages" that I would use depending on the use of the car. So I have 2-3 front dam, spoiler, splitter variations, and couple different rear spoiler setups as well as belly pans, side skirts, diffuser, etc. in mind so I don't want to hijack this thread. Your #7 from above leads me to believe a discussion of all the things I have planned might be better suited to a separate thread. I'll post the duct opening below for discussion here but if you want I'll just start a thread dedicated to my car.

7. If I think your questions … and the answers to them will be valuable to others … I want to leave it on this thread for all of us to learn from. If your questions get too specific to your car & I think it won’t be of value to others … I may ask you to start a separate thread where you & I can discuss your car more in-depth.

I planned on using the original park/turn signal openings for brake duct openings. I used headlights with park/turn incorporated to free up the opening. These will only be used for road track and will be blocked off the rest of the time. Do you think with a spoiler extension and splitter similar (but prettier) to the cardboard one shown the vents would have enough high pressure to function without fans?

[dontlifttoshift]

Smooth body work & gentle radius curves help air flow smoothly over a car body. Excessive turbulence is often caused by the opposite of smooth & gentle. When you can accurately describe parts of the body as abrupt, sharp, severe … those things will most likely create turbulence. The notchback style roofline of an 80’s Monte Carlo, Cutlass, Grand Prix & Regal all had a sharp trailing edge to the roof. This caused the air to “break away”… churn & crash … therefore create excessive turbulence … over the deck lid.

If the air isn’t flowing over the deck lid smoothly … therefore it is not “attached” … you will have less downforce in the rear. It was so bad the NASCAR teams running these cars convinced GM to make some with the “aero rear window” to help the problem. It didn’t solve the problem. It just made it less horrible.

This area is where I feel most 60s cars are lacking. Your 57 is better here than a Mustang coupe. With out redesigning the entire backglass area, what can be done to fix it?

[Damn True]

This area is where I feel most 60s cars are lacking. Your 57 is better here than a Mustang coupe. With out redesigning the entire backglass area, what can be done to fix it?

If I understand Ron's post above and I think I do, vortex generators and spill plates can mitigate the flow detachment over the back of the greenhouse. Fix it entirely? Likely not but it should improve it a bit.

[sccacuda]

Ron, you never cease to amaze me. I've got questions! You've seen my build. It is a PT car, but more road course, with standing mile and Bonneville thrown in. I will be taking it to A2's wind tunnel before any land speed runs. I've calculated my CD at .38. The stock body has 440 lbs. of lift, 125 lbs. of rear down force and 325 lbs of drag. I have modified the body slightly, trying to keep a stock look. I lowered and flattened the rockers 3/4". I can now attach rocker extensions/side splitters depending on the data I get from A2. I actually widened the front fenders in front of the tire 1-1/2" to "hide" the tire from the air. The rear spoiler is a factory 'Cuda trunk mount spoiler that I cut apart and slid an aluminum structure inside of for attachment, prevent deformation, and the ability to run different wicker bills. Again, depending on A2 data. On the bottom, I build the frame connectors flat to run a belly pan. It can start as far forward as the front air dam/splitter. This is where I have a question. Should the pan be dead flat or slightly concave for downforce? My watt's link bracket will pick up the belly pan about 1" higher the main pan and is level to rear valance. Do I terminate the pan in front of the rear axle and then just pick it back up behind the axle to the rear valance? The pan will have to be almost level and flat to keep the rear valance from becoming a parachute. Do you have any better ideas? I will throw in a bottom pic for reference.