Aerodynamics is the science of how air flows around and inside objects. More generally, it can be labeled “Fluid Dynamics” because air is really just a very thin type of fluid. Above slow speeds, the air flow around and through a vehicle begins to have a more pronounced effect on the acceleration, top speed, fuel efficiency and handling.
Vehicle Dynamics Simulation, Simplified To make OptimumLap, our engineers reduced the vehicle to its most fundamental components. They found that a vehicle can be defined with only 10 parameter—and each parameter represents a specific aspect of the car (such as engine, tires, or aerodynamics).
Therefore, to build the best possible car we need to understand and optimize how the air flows around and through the body, its openings and its aerodynamic devices.
Aerodynamic Principles
Drag
No matter how slowly a car is going, it takes some energy to move the car through the air. This energy is used to overcome a force called Drag.
Drag, in vehicle aerodynamics, is comprised primarily of three forces:
- Frontal pressure, or the effect created by a vehicle body pushing air out of the way.
- Rear vacuum, or the effect created by air not being able to fill the hole left by the vehicle body.
- Boundary layer, or the effect of friction created by slow moving air at the surface of the vehicle body.
Between these three forces, we can describe most of the interactions of the airflow with a vehicle body.
Frontal Pressure
Frontal pressure is caused by the air attempting to flow around the front of the vehicle as shown in diagram D1 below.
Diagram D1. Frontal Pressure is a form of drag where the vehicle must push air molecules out of the way as it travels through the air.
As millions of air molecules approach the front of the car, they begin to compress, and in doing so raise the air pressure in front of the car. At the same time, the air molecules travelling along the sides of the car are at atmospheric pressure, a lower pressure compared to the molecules at the front of the car.
Just like an air tank, if the valve to the lower pressure atmosphere outside the tank is opened, the air molecules will naturally flow to the lower pressure area, eventually equalizing the pressure inside and outside the tank. The same rules apply to any vehicle. The compressed molecules of air naturally seek a way out of the high pressure zone in front of the vehicle, and they find it around the sides, top and bottom of the vehicle as demonstrated in diagram D1.
Rear Vacuum
Rear vacuum is caused by the “hole” left in the air as a vehicle passes through it. To visualize this, let’s take a look at our demonstration car in diagram D2 below. As it drives down a road, the blocky sedan shape of the car creates a hole in the air. The air rushes around the body as described above.
At speeds above a crawl, the space immediately behind the car’s rear window and trunk is “empty” or like a vacuum. These empty areas are the result of the air molecules not being able to fill the hole as quickly as the car can make it. The air molecules attempt to fill in to this area, but the car is always one step ahead, and as a result, a continuous vacuum sucks in the opposite direction of the car.
Diagram D2. Rear Vacuum (Also known as flow detachment) is another form of drag where the air the vehicle is passing through cannot fill the space of the hole left behind by the vehicle, leading to what amounts to a vacuum.
This inability to fill the hole left by the car is technically called Flow detachment.
Flow detachment applies only to the “rear vacuum” portion of the drag forces and has a greater and greater negative effect as vehicle speed increases. In fact, the drag increase with the square of the vehicle speed, so more and more horsepower is needed to push a vehicle through the air as its speed rises.
Therefore, when a vehicle reaches high speeds it becomes important to design the car to limit areas of flow detachment. Ideally, we give the air molecules time to follow the contours of a car’s bodywork, and to fill the hole left by the vehicle, its tires, its suspension and its protrusions (i.e. mirrors, roll bars).
If you have witnessed the Le Mans race cars, you will have seen how the tails of these cars tend to extend well back of the rear wheels, and narrow when viewed from the side or top. This extra bodywork allows the air molecules to converge back into the vacuum smoothly along the body into the hole left by the car’s cockpit, and front area, instead of having to suddenly fill a large empty space.
The force created by the rear vacuum exceeds that created by frontal pressure, so there is very good reason to minimize the scale of the vacuum created at the rear of the vehicle.
![Race Car Vehicle Dynamics Program Suite Race Car Vehicle Dynamics Program Suite](/uploads/1/2/6/3/126347951/373392374.jpg)
Diagram D3. Turbulence is created by the detachment of an air flow from the vehicle. The final unavoidable detachment at the very rear of the vehicle leaves a turbulent wake.
When the flow detaches, the air flow becomes very turbulent and chaotic when compared to the smooth flow on the front of an object.
If we look at a protrusion from the car such as the mirror in diagram D3 above, we see flow detachment and turbulence in action. The air flow detaches from the flat side of the mirror, which of course faces toward the back of the car.
The turbulence created by this detachment can then affect the air flow to parts of the car which lie behind the mirror. Intake ducts, for instance, function best when the air entering them flows smoothly. Wings generate far more downforce with smooth flows over them as well. Therefore, the entire length of the car really needs to be optimized (within reason) to provide the least amount of turbulence at high speed.
Drag Coefficient
To enable the comparison of the drag produced by one vehicle versus another, a dimensionless value called the Coefficient of Drag or Cd was created. Every vehicle has a Cd which can be measured using wind tunnel data. The Cd can be used in drag equations to determine the drag force at various speeds. In his comprehensive book “Race Car Aerodynamics: Designing for Speed“, Joseph Katz provides a table of common vehicles and their Cds and Frontal Areas. Here is an excerpt from that table:
Car Drag Coefficients (Excerpt from “Race Car Aerodynamics” by Joseph Katz. © Bentley Publishers)
Vehicle Type | Drag | Frontal area | |
Coefficient Cq | A[m2] | CDA [m2] | |
Ford Escort 1.3 GL | 0.39-0.41 | 1.83 | 0.71-0.75 |
Nissan Cherry GL | 0.39-0.41 | 1.83 | 0.71-0.75 |
Volvo 360 GLT | 0.40-0.41 | 1.95 | 0.78-0.80 |
Honda Accord 1.8 EX | 0.40-0.42 | 1.88 | 0.75-0.79 |
Nissan Stanza SGL 1.8 | 0.40-0.42 | 1.88 | 0.75-0.79 |
Mazda 323 1.5 | 0.41-0.43 | 1.78 | 0.73-0.77 |
Nissan Sunny | 0.41-0.43 | 1.82 | 0.75-0.78 |
Talbot Horizon GL | 0.41-0.44 | 1.85 | 0.76-0.81 |
Alfa Romeo Giulietta 1.6 | 0.42-0.44 | 1.87 | 0.79-0.82 |
Toyota Corolla 1300 DX | 0.45-0.46 | 1.76 | 0.79-0.81 |
VW Golf Cabrio GL | 0.48-0.49 | 1.86 | 0.89-0.91 |
Full-size sedans | |||
Renault 25 TS | 0.30-0.31 | 2.04 | 0.61-0.63 |
Audi 100 1.8 | 0.30-0.31 | 2.05 | 0.62-0.64 |
Mercedes 190 E (190 D) | 0.33-0.35 | 1.90 | 0.63-0.67 |
Mercedes 380 SEC | 0.34-0.35 | 2.10 | 0.71-0.74 |
Mercedes 280 SE | 0.36-0.37 | 2.15 | 0.77-0.80 |
Mercedes 500 SEL | 0.36-0.37 | 2.16 | 0.78-0.80 |
BMW 518i (520i, 525e) | 0.36-0.38 | 2.02 | 0.73-0.77 |
Citroen CX 25 Gti | 0.36-0.39 | 1.99 | 0.72-0.78 |
BMW 323i | 0.38-0.39 | 1.86 | 0.71-0.73 |
Alfa Romeo 90 2.0 | 0.38-0.40 | 1.95 | 0.74-0.78 |
Mazda 929 2.0 GLX | 0.39-0.44 | 1.93 | 0.75-0.85 |
Saab 900 Gli | 0.40-0.42 | 1.95 | 0.78-0.82 |
Volvo 740 GLE | 0.40-0.42 | 2.16 | 0.86-0.91 |
Volvo 760 Turbo w/intercooler | 0.40-0.42 | 2.16 | 0.86-0.91 |
Peugeot 505 STI | 0.41-0.43 | 1.97 | 0.81-0.85 |
Peugeot 604 STI | 0.41-0.43 | 2.05 | 0.84-0.88 |
BMW 728i (732i/735i) | 0.42-0.44 | 2.13 | 0.89-0.94 |
BMW 745i | 0.43-0.45 | 2.14 | 0.92-0.96 |
Ford Granada 2.3 GL | 0.44-0.46 | 2.13 | 0.94-0.98 |
Sports cars | |||
Porsche 924 | 0.31-0.33 | 1.80 | 0.56-0.59 |
Porsche 944 Turbo | 0.33-0.34 | 1.90 | 0.63-0.65 |
Nissan 300 ZX | 0.33-0.36 | 1.82 | 0.60-0.66 |
Mazda 626 Coupe | 0.34-0.36 | 1.88 | 0.64-0.68 |
Opel Monza GSE | 0.35-0.36 | 1.95 | 0.68-0.70 |
Renault Fuego GTX | 0.34-0.37 | 1.82 | 0.62-0.67 |
Honda CRX Coupe | 0.35-0.37 | 1.72 | 0.60-0.64 |
Audi Coupe GT 5E | 0.36-0.37 | 1.83 | 0.66-0.68 |
Chevrolet Corvette | 0.36-0.38 | 1.80 | 0.65-0.68 |
Chevrolet Camaro Z 28 E | 0.37-0.38 | 1.94 | 0.72-0.74 |
Mazda RX-7 | 0.36-0.39 | 1.69 | 0.61-0.66 |
Toyota Celica Supra 2.8i | 0.37-0.39 | 1.83 | 0.68-0.71 |
VW Scirocco GTX | 0.38-0.39 | 1.74 | 0.66-0.68 |
Porsche 911 Carrera | 0.38-0.39 | 1.78 | 0.68-0.69 |
Honda Prelude | 0.38-0.40 | 1.84 | 0.70-0.74 |
Mitsubishi Starion Turbo | 0.38-0.40 | 1.84 | 0.70-0.74 |
Porsche 928 S | 0.38-0.40 | 1.96 | 0.74-0.78 |
Porsche 911 Carrera Cabrio | 0.40-0.41 | 1.77 | 0.71-0.73 |
Jaguar XJ-S | 0.40-0.41 | 1.92 | 0.77-0.79 |
From this table and our knowledge of the body shape of some of these vehicles, we can conclude that the best Cd is achieved when a vehicle has these attributes:
- Has a small nose/grill, to minimize frontal pressure.
- Has minimal ground clearance below the grill, to minimize air flow under the car.
- Has a steeply raked windshield (if any) to avoid pressure build up in front.
- Has a “Fastback” style rear window/deck or sloped bodywork, to permit the air flow to stay attached.
- Has a converging “Tail” to keep the air flow attached, and to minimize the area against which flow detachment eventually occurs
If it sounds like we’ve just described a sports car, you’re right. In truth though, to be ideal, a car body would be shaped like a tear drop, as even the best sports cars experience flow detachment. However, tear drop shapes are not conducive to the area where a car operates, and that is close to the ground. Airplanes don’t have this limitation, and therefore teardrop shapes work.
The best road cars today manage a Cd of about 0.28. Formula 1 cars, with their wings and open wheels (a massive drag component) manage a minimum of about 0.75.
If we consider that a flat plate has a Cd of about 1.0, an F1 car really seems inefficient, but what an F1 car lacks in aerodynamic drag efficiency, it makes up for in downforce and horsepower.
Aerodynamics How-To Tips (1/4)
Cover Open wheels
Open wheels create a great deal of drag and air flow turbulence, similar to the diagram of the mirror in the “Turbulence” section above. Full covering bodywork is probably the best solution, if legal by regulations, but if partial bodywork is permitted, placing a converging fairing behind the wheel provides maximum benefit.
Minimize Frontal Area
The smaller the hole your car punches through the air, the better it will accelerate, the higher the top speed, and the lower the fuel consumption it will have. It is usually much easier to reduce FA (frontal area) than the Cd (Drag coefficient).
Converge Bodywork Slowly
Bodywork which quickly converges or is simply truncated, forces the air flow into turbulence, and generates a great deal of drag. As mentioned above, it also can affect aerodynamic devices and bodywork further behind on the vehicle body.
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![Program Program](http://racingcardynamics.com/wp-content/uploads/2014/12/Post1.2Filler1.jpg)
The suspension on a vehicle serves multiple purposes:
- It provides a stable platform from which to control the vehicle
- It provides a way to isolate the chassis and driver from the shocking jolts that the tires experience going over anything but a glass-smooth surface.
- It provides a way to keep all the vehicle’s tires in contact with an uneven surface.
- It provides damping of oscillations that rubber tires, springs and uneven surfaces naturally create.
Many versions of suspension have been created over time to resolve deficiencies, but in general they all seek to control the movement of the tires in three ways:
- Laterally – Controlling side-to-side movement
- Longitudinally – Controlling forward/backward movement
- Vertically – Controlling up and down movement
Suspensions accomplish this using links and structures that locate the wheels/tires in a specific “geometry” relative to the vehicle. The geometry dictates the behavior the tires/wheel and chassis exhibit when accelerating, braking and turning.
Suspension Components
Let’s have a look at the components that make up a suspension.
Tires
As the first point of contact with the road, the tires work in conjunction with the suspension geometry and weight transfer dynamics to provide grip. Many different types of tires exist, but every tire relies upon its contact patch with the road (Shown in diagram T1 below) to create the friction needed. Generally, the larger the contact patch, the larger the amount of friction created.
Diagram T1. Tire contact patch which contacts the road surface
The grip provided by a tire is also based on the coefficient of friction (Cf) of the rubber compound and the tire’s construction (Radial/bias). This coefficient indicates the lateral grip the tire is capable of providing for a given weight being placed on it. A Cf of 1.0 means it is capable of providing 1 lb of lateral grip for 1 lb of vertical load on it.
Racing slicks (tires with no tread) are very high Cf tires, in the range of 1.0 or more. Street (treaded) radials, on the other hand, rarely even approach a 1.0 Cf. If you were to place 500 lbs weight onto a tire with a Cf of 1.0, you could expect 500 lbs (actually a little less) of lateral grip. Without aerodynamic aids to add to apply further weight to the tire, the vehicle could almost achieve a 1G turn.
Wheels
The wheel is what the tire mounts on and each type of wheel has its own particular characteristics depending on its width, diameter and construction materials.
The primary types of wheels used on cars are alloy and steel.
Alloy wheels can be constructed to very minimal weights, as alloying materials such as aluminum and magnesium can be used. They are also generally much more expensive than their steel counterparts, but they also lack the dent resistance of steel wheels. An alloy wheel, when struck by a curb will sometimes shatter and crack. Nonetheless, for most motorsports series and street vehicles, alloys are the choice.
Steel wheels can also be constructed to very low weights and their cost is quite a bit less than the alloys, due mostly to lower cost construction. Steel wheels are deformable when struck, and will usually allow air to leak out of the tire, as opposed to shattering. NASCAR and the general stock car scene use steel wheels due to the extreme forces encountered.
Wheels, aside from their width and diameter, have an important design characteristic called “Offset”.
In the wheel/tire cutaway diagram WH1 below, the sample wheel shows a red line that represents the mounting face for the wheel—the face with the lug holes that we bolt onto the hub of the vehicle.
The yellow dotted line represents centerline of the wheel and “Zero offset” from the centerline. If we move the mounting face toward the vehicle, as show on the left in the diagram, we create “Negative Offset”. If we move the mounting face away from the vehicle, as shown on the right in the diagram, we would create “Positive Offset”
Offset is important in relation to the design of the upright/knuckle, as it determines scrub radius (see more info below).
Diagram WH1. Wheel offset is the distance, positive or negative from the wheel center line when viewed from the front.
Brakes
It goes without saying that while the gas pedal on your car is the preferred pedal to push, the brakes are of vital importance as well.
Two types of brakes are available—Disc and drum. Both types use friction to turn the kinetic energy of the moving vehicle into heat. What makes one type of brake better than the other is the effectiveness of each type in dissipating or shedding the heat generated. Too much heat and the brake pad/shoe material will generate less friction, leading to what is termed “Brake fade”.
The disc brake, shown in diagram B1 below, produces more reliable stopping power under racing or hard-driving conditions because its rotor (the surface against which the brake pad generates friction and heat) is exposed to the air flow. This dissipates heat to the open air quickly.
A disc brake system works as shown in figure B2 below. The driver presses the brake pedal, which forces a piston in the master cylinder to compress the brake fluid (Yellow). The fluid runs inside a brake line to the caliper (Green) where two pistons (Blue) with attached brake pads (Red) are forced against the spinning brake rotor (Grey), generating friction and slowing the brake rotor and its attached wheel.
Diagram B2. Hydraulic disc brake system showing a cross-section of the master cylinder and caliper.
The drum brake, shown in figure B3 below, utilizes semi-circular shoes that are forced against the inside of the brake drum by a slave cylinder.
With the brakes released, there is a small air-gap space between the shoes and the drum as shown in figure B4 below.
Diagram B4. The drum brake, when released, leaves an air gap between the shoes and drum.
Diagram B5. The drum brake, when engaged, pushes the shoes against the drum creating the friction that turns kinetic energy into heat energy.
With the shoes engaged as shown in figure B5 above, the brake creates high levels of stopping power through large amounts of friction. However, because the drum is “closed” compared to the exposed rotor on the disc brake, more heat is retained, which leads to brake fade sooner.
Diagram B6 below shows a drum brake in a hydraulic system. The driver presses the brake pedal, which forces a piston in the master cylinder to compress the brake fluid (Yellow). The fluid runs inside a brake line to slave cylinder (Blue) which contains two pistons (Pink). These pistons are attached to the brake shoes (Red/light blue). The pistons force the brake shoes against the drum (Green), generating friction and slowing the brake drum and its attached wheel.
Diagram B6. Hydraulic drum brake system showing a cross-section of the master cylinder and drum assembly.
Drum brakes are cheaper to manufacture and are generally used in conjunction with a live axle. However, disc brakes are the preferred for any type of race or sports car where they can be fitted as they have less mass and better cooling.
Suspension Design Tips (1/4)
Minimize Unsprung weight
Unsprung weight, or the weight comprised by tire, wheel and suspension affects how well the tire follows the bumps and dips in the road surface. Using lighter wheels, tires, and suspension components will reduce the weight. The weight of these suspension components by itself is not so critical as the ratio between the vehicle’s sprung weight (chassis, driver, engine, etc) and the unsprung weight. The lower the unsprung weight in relation to the sprung weight, the easier it will be to control the tire/wheel via the springs, dampers (shocks) and anti-roll bars.
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