|6th December 2006, 12:43||#1|
Join Date: Nov 2006
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I would like to share some technical stuff about shapes of cars with u guys which i found on net......
Race Car Aerodynamics - Designing for Speed *-* Bentley Publishers - Automotive Books and Repair Manuals
The main aspects of aerodynamics are drag, downforce and then some devices which can be used to change drag and downforce.
Drag at the front and the rear of the car is very different so these will be considered separately.
The frontal pressure relates to how the air molecules move around the outside of the car. As they hit the front end they compress and the pressure increases. The air moving next to the car is at a much lower pressure so the high pressure air moves towards the lower pressure air and thus moves around the car.
Fig.1 The red area indicates high pressure and the green indicates low pressure. The high pressure air flows towards the area of low pressure
This is the reason for sleek edges on performance cars and racing cars as their shapes decrease the pressure at the main pressure points which are the front bumper and the windscreen. On family cars with a primary target of low costs, these expensive aerodynamic designs are not heavily used whereas with a car such as the McLaren Mercedes SLR, every single part of the body is designed specifically designed for optimum performance.
Fig. 2 The McLaren Mercedes SLR. Notice the the aerodynamic shape of the front end and the windscreen
Fig.3 The Hyundai accent. A very cheap 4 door saloon. The front end is very blocky so there will be high pressure building up there. A very obvious difference in aerodynamic design to the SLR
The other main area that is affected by drag is the rear of the car. As a vehicle moves through the air, the air molecules are moving around the body but then they need to fill the space left by the vehicle. This is sometimes known as the rear vacuum. Consider a truck moving along the motorway at 70mph. The large area of the rear end of the truck will need a large number of air molecules to move into it very quickly. However the truck is moving too quickly for the air molecules to continually move into the area so a partial vacuum, technically known as the flow detachment is created. This can happen at any part of the car where a gap can occur behind. The main affected areas are the rear bumper and the rear windscreen. However it can also occur behind wing mirrors and tyres as well.
Fig. 4 A continual vacuum appears behind the rear window and back bumper.
The rear vacuum is more commonly known as the slipstream in racing environments. In order to gain extra speed and overtake the car in front, the car behind will follow very close to the rear of the leading car. From the frontal pressure part we know that with less air molecules to hit into the front of your car the more energy you save. Therefore the driver of the rear car does not have to use as much power to keep up and at the opportune moment can increase his power and accelerate past his opponent.
Using the knowledge of both these ideas you can start to put together a design for a perfect car shape. It should look like the Bentley Le Mans car pictured below. To overcome frontal pressure the nose is like a wedge and the windscreen continues the profile of the front. It overcomes the rear vacuum by having extra bodywork behind the rear wheels to reduce the amount of flow detachment. The wheels are also covered for the same reason. All these factors contribute to the drag coefficient Cd which is a measure of how good the aerodynamic shape is. The lower the number between 0 and 1 means the better the shape. A typical production car will have a coefficient around the 0.3 area. A surprising fact is that a Formula 1 car is seemingly very inefficient as its value is about 0.75. This is due to its open wheels and large side pods and wings. However this inefficiency is compensated by the amount of horsepower it generates.
For an aircraft, lift is exactly what we want. It is explained by the Bernoulli Principle and it states that the faster a gas moves the lower its pressure becomes. This is then translated into lift on a wing as the air running over the top of the wind is moving faster and therefore creating less pressure than the air moving underneath the wing. Downforce is the opposite of this as cars need to stick to the road. Lift can be very dangerous for a car and especially a racing car as is shown by the picture here. The slightest amount of lift can turn a car into a wing.
Fig. 6 The dangers of lift
For standard production cars we would like very little or no lift or perhaps a little downforce. We don't need that extra element of a large amount of downforce because this creates extra drag and reduces economy. In a racing car on the other hand we need both high downforce and little drag. Downforce enables the car to steer round corners at a very high speed because it effectively adds weight to the car and keeps the wheels firmly on the road at very high speeds. Every race is different so the engineers have to decide on a compromise between high downforce and lots of drag or low downforce and less drag.
As the air molecules hit the front bumper of a car we have established that this causes drag. The pressure increase because the molecules slow down. As the molecules move over the car the pressure decreases causing lift to be generated on the bonnet of the car. The pressure then builds again as the molecules hit the windscreen and then more lift is generated over the roof as the molecules accelerate.
Fig. 7 The effects of lift and downforce
As a result of the flow detachment the pressure is still low over the back end so yet more lift is produced. Early in the automotive industry cars suffered greatly from this and at high speeds the rear of the car would become unstable as the lift generated was too great.
There are many devices which can be used on a car to change the amount of downforce and drag on the car. On Formula 1 cars every part is geared to achieving the best lap time for that specific course so even a part such as the suspension arms are aerodynamically shaped so they do not create much drag.
The following devices will be investigated:[/left]
The rear wing of a Formula 1 car is usually comprised of 2 sets of aerofoils connected by 2 endplates. Both the aerofoils work as wings to push the car downwards and they account for about a third of the total downforce on the car. As the diagram below shows, the air passes over and under the wing. The low pressure under each of the aerofoils creates downforce.
Fig. 8 Formula 1 rear wing
The angles on the aerofoils are constantly changed to give a different race setup. For a tight track for lots of cornering a high downforce is needed which results in a lower top speed but ultimately a faster lap time. For a track with lots of long straights, less downforce is needed which results in a higher top speed and faster acceleration.
The end plates not only provide a means of keeping the aerofoils in place but also contain the airflow and extract the most amount of downforce possible.
Spoilers are not quite as effective as rear wings but are more commonplace on sporty sedans and performance cars. Spoilers work by blocking the flow of air as it comes off the rear windscreen. This creates a high pressure area in front of the wing and therefore more downforce.
Fig. 9 The effects of a spoiler
One interesting spoiler is on the McLaren Mercedes SLR where at low speeds (0-55mph) the spoiler is flush against the boot but then as the speed exceeds 56mph the spoiler extends to 10 degrees to increase the amount of downforce. If the driver applies the brakes heavily the spoiler will extend to 65 degrees and act as an airbrake by creating a large amount of drag.
Figs. 8 and 9 The Mclaren Mercedes SLR with the spoiler in its lowered position and extended during driving
Ground devices such as air dams and diffusers create downforce by impeding the amount of air going under the car. This creates and area of low pressure and subsequently produces downforce. These devices are mainly used in races on cars such as touring cars. They will also use side skirts which act in a similar way to the end plates on a wing because they prevent the air from spilling out the side.
Fig 10 Honda Civic British Touring Car. Notice the very low front bumper which blocks a lot of air going under the car and also the side skirts which are also very low to the ground to prevent air spilling out.
Last edited by ported_head : 7th December 2006 at 22:17.
|6th December 2006, 20:33||#3|
Senior - BHPian
Join Date: Aug 2006
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Nice article, though. And I can't stop looking at the SLR's @$$.
Last edited by hydrashok : 6th December 2006 at 20:35.
|11th December 2006, 16:59||#6|
Join Date: Jul 2006
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well nice info..
till date i knew about spoliers and front windscreen advantage, after reading this came to know about the side skirts too.. or else i was thinking side skirts for protection of body and outer look..
thanks vipul for the stuff..
|11th December 2006, 19:33||#9|
Join Date: Jun 2006
Thanked: 34,690 Times
Very very very nice article....
Some thins that came to my mind...
Accent- Very cheap 4 door saloon... Sorry.. couldnt help laughing...
Downforce;- This was very much seen on the Audi TT, first series without the rear spoiler... car had a tendency to lift at high speed...
Was one of the most interesting articles i have come across after joining team-bhp... Great find!!!
|12th December 2006, 08:42||#10|
Join Date: Nov 2006
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Thanks guys....... i will keep on posting technical stuff..........will soon bring in something interesting technical......
|12th December 2006, 14:09||#11|
Join Date: Oct 2006
Location: Bangalore India
Thanked: 6 Times
great stuff Vipul
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