Design Of The Rear Spoiler For Road Vehicles Engineering Essay

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ABSTRACT

When objects move through air, forces are generated by the relative motion between the air and surfaces of the object. Aerodynamics is the study of these forces, generated by the motion of air, usually aerodynamics are categorized according to the type of flow as subsonic, hypersonic, supersonic.............................................

The spoiler is also a part of the aerodynamics. The function of the spoiler is to spoil the unfavourable flow of air flowing through the car and generate a proportional downforce. This work describes the design and the performance of the rear spoiler.

Content

1. Introduction............................................................................................................6

2. Literature Review...................................................................................................7

2.1 Principle......................................................................................................7

2.2 Working of Rear wing.................................................................................8

2.3Criteria for efficiency of the spoiler..............................................................9

3. Material.................................................................................................................12

3.1 Material Types..........................................................................................12

3.2 ABS Material.............................................................................................12

4. Design calculation...............................................................................................13

5. Applications.........................................................................................................15

6. Conclusion...........................................................................................................16

Figures

Fig. 2.1.........................................................................................................................8

Fig. 2.2.........................................................................................................................9

Fig. 2.3.......................................................................................................................10

Tables

Table 1.......................................................................................................................13

Table 1.......................................................................................................................14

Chapter 1

Introduction

A spoiler is an aerodynamic device which is used to spoil the unfavourable air (unwanted movement of air like turbulence) of a car which is in motion. Basically this device fitted to the front and rear of the vehicle. The front spoiler also called as front wing or air dam, and the rear spoiler is also called as rear wing.

From historical point of view, post-world-war 11 automobile racing was initially dominated by developments related to engine technology, and later to tire advancements.

During the 1960s, race car aerodynamics evolved as an important and relatively inexpensive technology that could place less well-funded teams.

Over time, the cars aerodynamics on vehicles has become more refined as cars are now tested in expensive wind tunnels as part of continued development process.

The biggest jump in speed occurred in the 1972 with the first efficient use of front and rear wings. It has even nearly become one of the only aspects of performance gain due to the very marginal gains that can currently be made by engine changes or other mechanic component development.

Chapter 2

Literature review

2.1 Principle:

The main principle of the spoiler is to reduce the rear end lift that means to increase the downforce and to spoil unfavourable air movement across a body.

Lift is one of the main aerodynamic forces imposed on a vehicle, but unlike drag, lift can be manipulated to enhance the performance of a car. Lift is the force that acts on a vehicle normal to the road surface that the vehicle rides on.

Lift usually has the effect of pulling or lifting the vehicle away from the surface it drives on.

However, by manipulating the car geometry it is possible to create negative lift, or down-force.

Down-force enhances vehicle performance by increasing the normal load on the tires. This increases the potential cornering force which results in the ability of the vehicle to corner faster.

This down force can be compared to a virtual increase in weight, there by pressing the car down onto the road and increasing the available frictional force between the car tyres and the road, which in effect enables higher cornering speeds.

Drag is the aerodynamics force that resists the vehicles motion through a fluid and points backwards. Drag is detrimental to vehicle performance as it can limit the top speed of a vehicle and increase the fuel consumption, both of which are negative consequences for vehicles. Its size is proportional to the speed differential between the air and the solid object.

What this wings or spoilers does is it prevents the separation of flow and thereby preventing the formation of vortices or helps to fill the vacuum in the rear end more effectively thus reducing drag. So what actually this wings does is that, The wing works by differentiating pressure on the top and bottom surface of the wing. As mentioned previously, the higher the speed of a given volume of air, the lower the pressure of that air, and vice-versa. What a wing does is make the air passing under it travel a larger distance than the air passing over it (in race car applications). Because air molecules approaching the leading edge of the wing are forced to separate, some going over the top of the wing, and some going under the bottom, they are forced to travel differing distances in order to "Meet up" again at the trailing edge of the wing. This is part of Bernoulli's theory. What happens is that the lower pressure area under the wing allows the higher pressure area above the wing to "push" down on the wing, and hence the car it's mounted to.

The way a real, shaped wing works is essentially the same as an airplane wing, but it's inverted. An airplane wing produces lift, a car wing produces negative lift or in other words what we call us, downforce. That lift is generated by a difference in pressure on both sides of the wing.........................................................................  

Well, if you look closely at the drawings, you'll see that the upper side of the wing is relatively straight, but the bottom side is curved. This means that the air that goes above the wing travels a relatively straight path, which is short. The air under the wing has to follow the curve, and hence travel a greater distance. Now there's Bernoulli's law, which basically states that the total amount of energy in a volume of fluid has to remain constant. (Unless you heat it or expose an enclosed volume of it to some form of mechanical work) If you assume the air doesn't move up and down too much, it boils down to this: if air (or any fluid, for that matter) speeds up, its pressure drops. From an energetic point of view, this makes sense:

if more energy is needed to maintain the speed of the particles, there's less energy left do do work by applying pressure to the surfaces.

In short: on the underside, air has to travel further in the same amount of time, which means it has to speed up, which means its pressure drops. More pressure on top of the wing and less on the underside results in a net downward force called downforce.

2.2 How the rear wing works:

The rear wing assists the front wing and rear diffuser in the overall downforce setup of the car. The angle of the wing is adjusted according to each specific Grand Prix circuit, depending on the amount of downforce required, the weather and the amount of mechanical grip available.

The end plates located at the sides of the wing are designed to smooth the meeting of two different airflows - the high pressure air above the car tries to switch places with the low pressure air below the car and it is this that causes the spinning flow of air behind the car. When these two forces meet they form a 'vortex', a spinning flow of air which is very turbulent.

2.3 The efficiency of the wings is based on following criteria:

Aspect Ratio

The amount of downforce produced by a wing is determined by its size. The length to width ratio is called the aspect ratio; the larger the wing the greater the downforce. As the higher the Aspect ratio more efficient the wing will be. The higher the aspect ratio, the less air resistance created by the vortex at the wing tips. The aspect ratio is the span of the air foil (the long dimension perpendicular to the air flow) divided by its dimension parallel to the airflow.

The angle of attack

Fig 2.1

The efficiency of the wing is the downforce to drag ratio. The amount of downforce generated depends upon the angle or tilt of the wing. The greater the angle of attack the more the downforce will be created.

While increasing downforce a wing also increases unwanted drag. Drag increases with the angle of attack as already stated. The downforce generated by the wing acts in vertical downward direction, while drag acts in the opposite direction to the air flow.

Fig 2.2

From the above two graphs of coefficient of drag VS angle of attack the coefficient of drag can be assumed, if the angle of attack is 80 then the coefficient of drag will be 0.07.

The height of the wing:

The third thing is the height of the spoiler. The height also affects the performance of the spoiler. The gap between the trunk lid and the wing can make air to pass easily. The fig shows the effect of the height of the wing on the car. So the height is taken as 130 mm.

Fig 2.3

Chapter 3

Material

3.1 Material types

Spoilers are usually made of:

ABS plastic - Most original equipment manufacturers create spoilers produced by casting ABS plastic with various admixtures, which bring in plasticity to this inexpensive but fragile material. Frailness is a main disadvantage of plastic, which increases with product age and is caused by the evaporation of volatile phenols.

Fibreglass - Used in car parts production due to the low cost of the manufacturing process. Fibreglass spoilers consist of fibreglass filler fastened with synthetic tar. Fibreglass is sufficiently durable and workable, but has become unprofitable for large scale production.

Silicon - more recently, many auto accessory manufacturers are using silicon-organic polymers. The main benefit of this material is its phenomenal plasticity. Silicon possesses extra high thermal characteristics and provides a longer product lifetime.

Carbon - fibreglass based on carbon fibre is the youngest material on the automotive aftermarket. Carbon is light weight, durable, but also a very expensive material. Unlike ordinary fibreglass, solidification of the connecting tar takes place in a pressure chamber using high temperatures. Due to the very large amount of waste during the manufacturing process, large scale producers cannot widely use carbon fiber in automobile parts production currently.

3.2 ABS Material

ABS Resistance:

Excellent resistance (no attack) to Glycerine, Inorganic Salts, Alkalis, Many Acids, Most Alcohols and Hydrocarbons

Limited resistance (moderate attack and suitable for short term use only) to Weak Acids

Poor resistance (not recommended for use with) Strong Acids and Solvents, Ketones, Aldehydes, Esters, and some Chlorinated Hydrocarbons

ABS Quick Facts:

Maximum Temperature: 80°C

Minimum Temperature: -40°C

Autoclavable: No

Melting Point: 221°F 105°C

Tensile Strength: 4,300 psi

Hardness: R110

UV Resistance: Poor

Translucent

Rigid

Specific Gravity: 1.04

ABS Fabrication:

It can be thermo-formed, pressure formed, blow moulded, sheared, sawed, drilled, or even "cold stamped"

Joints can be ultrasonic welded, thermo-welded, and chemically bonded

Impact resistant

Commonly used for telephone bodies, safety helmets, piping, furniture, car components, TV casings, radios, control panels, and similar

Chapter 4

Design Calculations

The design of the rear wing or spoiler of the car is totally dependent on the coefficient of the drag. Higher the coefficient of drag, greater the performance of that spoiler. The coefficient of drag is directly proportional to the angle of the spoiler where the air attacks. As the angle increases the drag coefficient increases.

To design the spoiler the width of the car should be necessary to consider the wing span that means the total length of the spoiler. The following table shows the most common width of the cars:

Table 1

Honda City

Toyota Corolla

Kia Forte

Mitsubishi Lancer

Width

1715

1710

1775

1770

By considering the common width in the range of 1700 - 1780 mm, the length of the spoiler 1700mm can be acceptable to create the max drag force.

The formula for downforce of a wing is given by:

Where:

D is downforce in Newton

WS is wingspan in metres

H is height in metres

AoA is angle of attack

F is drag coefficient

ρ is air density in kg/m³

V is velocity in m/s

The data for the calculation of the spoiler is as follows:

Table 2

WS/ Length

1700mm

1.7 m

Height

100mm

0.1 m

Angle of attack

150 (15 x π/180)

0.26 radians

Coefficient of drag (according to AOA)

0.015

0.015

Density of air (𝝔)

1.2

1.2

Max. Velocity (V)

200 km/hr (200/3.6)

55.55 m/s

The angle of attack is taken as 150, because the car should have some downforce but in some limit. So if the angle of attack increased the drag will increase which can affect the performance of the vehicle like less fuel economy. Therefore the angle 150 is the perfect angle for the road vehicle spoiler.

Also to design spoiler the second thing is the speed of the vehicle. To achieve the best performance max speed required that is 200km/hr for a normal road vehicle.

The third thing is the height of the spoiler. The height also affects the performance of the spoiler. The gap between the trunk lid and the wing can make air to pass easily. So the height is taken as 100 mm.

Therefore by substituting the above values in the formula,

D = x (1.7 x 0.13 x 0.26) x 0.015 x 1.2 x (55.55)2

D = 1.595 N

Therefore the downforce created by the spoiler is 1.2275 N. This could be acceptable for a normal road vehicle.

Chapter 5

Applications

Cars have spoilers to increase their grip on the road. Normally the weight of a car is the only thing that forces the tires down onto the pavement. Without spoilers, the only way to increase the grip would be to increase the weight, or to change the compound the tire was made out of. The only problem with increasing the weight is that it doesn't help in turns, where you really want to grip. All that extra weight has inertia, which you have to overcome to turn, so increasing the weight doesn't help at all. The way the spoiler works is like an airplane wing, but upside down. The spoiler actually generates what's called 'down force' on the body of the car. 

Chapter 6

Conclusion

The design of spoiler described in this project can be used for any road vehicle having width at least 1700mm. The main aim to design this type of spoiler is to improve the overall performance of the car with respect to dragforce. The designed spoiler could be give the best result if it will be in use.

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