Aerodynamics
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Estates tend to be more vertical at the rear than saloons and hatchbacks, and one thing smooth air flows don't like doing is going around corners, i.e. going smoothly along the roof then going vertically down the back of the car. Directing the air down the rear screen of a hatchback or saloon will tend to produce lift, that was part of the reason that people had accidents with the first Audi TTs.
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I think im right in saying that drag co-efficient refers to the efficiency in which an object travels through the air in relation to its front on cross sectional area-therefore a big car with a drag co-efficient of 0.34 will take more energy to travel at a given speed than a small car with the same drag co-efficient of 0.34. Thing is its easier to make a big car have a low drag co-efficient as the passenger area and position makes up a smaller area of the total car so the shape can be for aerodynamic reasons rather than to fit the passengers in comfort. I remember reading that the AX has a drag coéfficient of .27 but I may be wrong, which is impressive for a small car. Im not sure what part lenght plays but I know estate cars always have a slightly lower top speed when you read the back of car manuals.
Anyone remember the Volvo 850 btcc touring cars?? http://www.supertouring.co.uk/the_cars/ ... ate_2.html
According to this site, there wasn't much between the estate and the saloon, but the estate had better downforce at speed.[:D]
According to this site, there wasn't much between the estate and the saloon, but the estate had better downforce at speed.[:D]
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My knowledge and understanding of fluid dynamics, Drag, Cd, Cl, CFD, flow divergance/convergance, etc.etc. is just fine!
Personally, I think you may be referring to reynolds number, and boundary layer thickness, which is indeed a function of length. Above Re=2000, when the boundary layer becomes turbulent, drag is indeed reduced, although this is more in reference to skin friction rather than turbulence and eddys generated behind a non-convergant surface.
After all, in aircraft basics, the larger the chord of the wing (from leading to trailing edge), the greater the drag. (and usually the greater lift too).
You don't see it these days, but when car manufacturers used to quote fuel consumption figures in Urban/56mph/75mph it was always noted that in steady-speed situations, the saloon/hatch version of a particular model would always achieve 3-4mpg more than the estate. I have the figures for a 1.7TD 405 somewhere.
Personally, I think you may be referring to reynolds number, and boundary layer thickness, which is indeed a function of length. Above Re=2000, when the boundary layer becomes turbulent, drag is indeed reduced, although this is more in reference to skin friction rather than turbulence and eddys generated behind a non-convergant surface.
After all, in aircraft basics, the larger the chord of the wing (from leading to trailing edge), the greater the drag. (and usually the greater lift too).
You don't see it these days, but when car manufacturers used to quote fuel consumption figures in Urban/56mph/75mph it was always noted that in steady-speed situations, the saloon/hatch version of a particular model would always achieve 3-4mpg more than the estate. I have the figures for a 1.7TD 405 somewhere.
Umm yeah not sure where I got that figure from yangreen- does sound a bit silly looking at it now- Im still not sure what the overall conclusion is here- all these theories of drag are great but at the end of the day estates are slower than saloons with the same engine in general, and like fastandfurious says, use a bit more fuel- top speed wise this can only be down to better aerodynamics of the saloons-surely?
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Hi Guys,
if you look at the spec's for the base model, they are alway quite a bit more slippery. eg: The CX super with narrow 175/80 tires is considerably more slippery than the GTi counterpart with wide tires and spoilers ... Mainly I'm guessing due to the tire width.
I remember reading about this previously the Renault 25 was very good from memory, the GS was exceptional for it's time (and apparantly the only car around the gave good results in everyones windtunnels --not just the manufacturers) ... Amazing facts, the Etype Jag had a cd - .44 ... More than a VW Kombi [:0]
seeya,
Shane L.
if you look at the spec's for the base model, they are alway quite a bit more slippery. eg: The CX super with narrow 175/80 tires is considerably more slippery than the GTi counterpart with wide tires and spoilers ... Mainly I'm guessing due to the tire width.
I remember reading about this previously the Renault 25 was very good from memory, the GS was exceptional for it's time (and apparantly the only car around the gave good results in everyones windtunnels --not just the manufacturers) ... Amazing facts, the Etype Jag had a cd - .44 ... More than a VW Kombi [:0]
seeya,
Shane L.
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<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by adamskibx</i>
Umm yeah not sure where I got that figure from yangreen- does sound a bit silly looking at it now- Im still not sure what the overall conclusion is here- all these theories of drag are great but at the end of the day estates are slower than saloons with the same engine in general, and like fastandfurious says, use a bit more fuel- top speed wise this can only be down to better aerodynamics of the saloons-surely?
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
You need to take gearing into account as well.
Umm yeah not sure where I got that figure from yangreen- does sound a bit silly looking at it now- Im still not sure what the overall conclusion is here- all these theories of drag are great but at the end of the day estates are slower than saloons with the same engine in general, and like fastandfurious says, use a bit more fuel- top speed wise this can only be down to better aerodynamics of the saloons-surely?
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
You need to take gearing into account as well.
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<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by DoubleChevron</i>
Amazing facts, the Etype Jag had a cd - .44 ... More than a VW Kombi [:0]
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
true indeed, But:
The actual Drag force on a car is the product of Cd, Air density, Frontal area, and the squate of the speed. The Cd of an e-type is higher than that of a VW combi (and of a large number of other cars too) but it's frontal area is tiny. Thus, if you do the calculation from D=0.5xCdxPxAxUxU (that should be "half Cd Rho a U squared") the total drag for any given speed on an E-type is substantially less than that of a bigger vehicle with a better drag co-efficent.
The best description of Cd I've heard is that it's the relationship between the frontal area of an object, and the area of turbulence it leaves behind. This is why any object with a smooth taper at the back (like say an aircraft), which leaves very little turbulent air behind it, has a low Cd.
Amazing facts, the Etype Jag had a cd - .44 ... More than a VW Kombi [:0]
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
true indeed, But:
The actual Drag force on a car is the product of Cd, Air density, Frontal area, and the squate of the speed. The Cd of an e-type is higher than that of a VW combi (and of a large number of other cars too) but it's frontal area is tiny. Thus, if you do the calculation from D=0.5xCdxPxAxUxU (that should be "half Cd Rho a U squared") the total drag for any given speed on an E-type is substantially less than that of a bigger vehicle with a better drag co-efficent.
The best description of Cd I've heard is that it's the relationship between the frontal area of an object, and the area of turbulence it leaves behind. This is why any object with a smooth taper at the back (like say an aircraft), which leaves very little turbulent air behind it, has a low Cd.
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Whilst our discussions have concentrated on the visible body shape, there are two other major hidden factors:
Firstly the underbody shape. There have been some real horrors with footwells, spare wheels, exhaust systems and all sorts of rude mechanical bits hanging down and upsetting the smooth airflow. I remember that when the DS was first introduced, the Motor Show stand featured a DS body suspended to show its smooth undersurface, this must have contributed to the low Cd figure. Nowadays, some manufacturers fit a "chin" below the front bumper to deflect the air around the sides of the underbody.
The other hidden factor is the engine cooling air supply which has to find its way through all sorts of obstructions before exiting under the car.
As far as aircraft are concerned, the generation of lift automatically produces a disturbed airflow behind the aircraft, referred to as the wake. The wake behind a Jumbo has been known to flip a light aircraft onto its back ten miles behind it. And on final approach, with flaps and wheels down, it's even worse!
Firstly the underbody shape. There have been some real horrors with footwells, spare wheels, exhaust systems and all sorts of rude mechanical bits hanging down and upsetting the smooth airflow. I remember that when the DS was first introduced, the Motor Show stand featured a DS body suspended to show its smooth undersurface, this must have contributed to the low Cd figure. Nowadays, some manufacturers fit a "chin" below the front bumper to deflect the air around the sides of the underbody.
The other hidden factor is the engine cooling air supply which has to find its way through all sorts of obstructions before exiting under the car.
As far as aircraft are concerned, the generation of lift automatically produces a disturbed airflow behind the aircraft, referred to as the wake. The wake behind a Jumbo has been known to flip a light aircraft onto its back ten miles behind it. And on final approach, with flaps and wheels down, it's even worse!
The "E" type Jags windscreen was very upright, I imagine that made things far worse. [V]
Re Tyres, doe the fact that the they are rotating, and thus creating their own tubulance reduce the drag they create? [?]
Useless piece of information.. Hulls.. Most powerboat hulls have a severe step in them (step up toward the stern) so as to create a mixture of air and water (bubbles-foam) under the hull to reduce suction.. [8)] [:)]
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Re Tyres, doe the fact that the they are rotating, and thus creating their own tubulance reduce the drag they create? [?]
Useless piece of information.. Hulls.. Most powerboat hulls have a severe step in them (step up toward the stern) so as to create a mixture of air and water (bubbles-foam) under the hull to reduce suction.. [8)] [:)]
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