Coefficient of drag

[KnifeEdge_2K1]

anyone know the Cd of the Lancer Evolution MR? i know the VIII is .01 better than the VII and the MR is about .05 (or something around that) better than the standard VIII but i dont know the Cd of the VII gsr so all that previous info is useless

on a second note, Cd is one of the hardest thigns to change about a car right? cornering characteristics acceleration can easily be changed by altering engine output and chassis stiffness and what not but the Cd is dirrectly related to the shape of the car itself

am i mistaken?

[Alastor]

on a second note, Cd is one of the hardest thigns to change about a car right? cornering characteristics acceleration can easily be changed by altering engine output and chassis stiffness and what not but the Cd is dirrectly related to the shape of the car itself

am i mistaken?

I dont know if it is the hardest thing to change but I think fluid dynamics is a rather un-intuitive field of engineering. Maybe that is because you cant actually see the air except in special experiments. Anyway, the coefficient of drag depends generally on the shape. Specifically that includes the frontal area, body profile, undercarriage, surface textures, cooling requirements, down-force requirements, and rear separation characteristics. Plus whatever anyone else thinks is important to the application.

Although changing the Cd is not easy, it certainly is not impossible with some time an effort. As with most things, the more you want to change the Cd the more difficult it is likely to become. Some thought and imagination could probably get you father than you think. However, if every 0.0001 reduction matters your best bet is a costly computational fluid dynamics (CFD) software package or heading over to the local wind tunnel with a can of spray paint ;-)

[GTR Dreamer]

hey, welcome 2 ucp, Alastor!

[Chinky_boi]

Cd can be changed from closing a window to opening it. but if all the windows are closed then the MR can get a 0.28 Cd.

Cd is calculated using a wind tunnel. If wind can get by the object easily then it has a lower Cd. (I don't know how it is calculated.) Imagine a cube and a cone was put into a wind tunnel. The cube will have a really high Cd because no air can go around it easily. The cone in the other hand is curved and very good for wind to get around. That is why you see rockets, jets, and some cars shaped like a cone.

[taz_rocks_miami]

I dont know if it is the hardest thing to change but I think fluid dynamics is a rather un-intuitive field of engineering. Maybe that is because you cant actually see the air except in special experiments. Anyway, the coefficient of drag depends generally on the shape. Specifically that includes the frontal area, body profile, undercarriage, surface textures, cooling requirements, down-force requirements, and rear separation characteristics. Plus whatever anyone else thinks is important to the application.

Although changing the Cd is not easy, it certainly is not impossible with some time an effort. As with most things, the more you want to change the Cd the more difficult it is likely to become. Some thought and imagination could probably get you father than you think. However, if every 0.0001 reduction matters your best bet is a costly computational fluid dynamics (CFD) software package or heading over to the local wind tunnel with a can of spray paint ;-)

Welcome to UCP Alistor! You seem very well versed on the subject, are you an engineer?

Taz.

[Matra et Alpine]

ON production cars there are 2 simple steps which can have quite large impacts on Cd.

First remove the wing mirrors - even on well designed ones, you can drop 0.01Cd :) The A610 is a cited example with this.

Second, underbody DRAG is the largest component you can easily alter.
But as Alastor says it's non-intutitive how to improve it.
Some would benefit from flat bottom, others may benefit from boards directing flow away. Front air-dam and splitter to reduce the airflow under is a usually successful first step - but not taken to excess, the balance of air directed sideways and below can be critical and CAN end up worse.
Then as you play with improving Cd you alter the charactersitcs of the vehicle at speed and may find it can GO faster but you're terrified every time you try :) THEN you have to ad aeor 'devices' to keep the car stable - these add drag and Cd goes up :(

Again an 'old' example on Cd and the 'look'. In the 70s Ford UK produced a car - the Corsair - which had a pointed nose. So to the eye it looked 'sleek. At the same time they produced the first Transit VAN. Big, 'blunt' looking and it had 30% LOWER resistance than the Corsair. So believe the CFD ( Computational Fluid Dynamics ) software over the eye :)

[Alastor]

Thanks for the welcome everyone. Taz I am a last year Mechanical Engineering student, I guess that makes me a "Jack Of All Trades, And A Master Of None".

[henk4]

Some would benefit from flat bottom,

I have been searching the web for a picture of the bottom of the Citroen DS, there is a famous pic of one, being put vertically on the rear side. Could not find the pic though, but it was point in case for the flat bottom, and already in 1955

[Matra et Alpine]

I have been searching the web for a picture of the bottom of the Citroen DS, there is a famous pic of one, being put vertically on the rear side. Could not find the pic though, but it was point in case for the flat bottom, and already in 1955
But as they built a 'double-hull' to get the flat bottom it meant these things rusted faster than most other cars :)

[johnnynumfiv]

The midget has a flat bottom, kind of, but i dont think it adds any aerodynamic advantages.

[henk4]

But as they built a 'double-hull' to get the flat bottom it meant these things rusted faster than most other cars :)


It is about the principle not about the execution :D

[Matra et Alpine]

It is about the principle not about the execution :D
As with all things French :mad:

[KnifeEdge_2K1]

Cd can be changed from closing a window to opening it. but if all the windows are closed then the MR can get a 0.28 Cd.

Cd is calculated using a wind tunnel. If wind can get by the object easily then it has a lower Cd. (I don't know how it is calculated.) Imagine a cube and a cone was put into a wind tunnel. The cube will have a really high Cd because no air can go around it easily. The cone in the other hand is curved and very good for wind to get around. That is why you see rockets, jets, and some cars shaped like a cone.

i know theoretically the best shape is a tear drop which has a cd of .05 but as we all know we cant build a car like that :P

[KnifeEdge_2K1]

the MR can get a 0.28 Cd.


wanna back that up with a reference?

i know the evo VI has a 0.30 Cd and that the standard VIII has a Cd lower than the VII by 0.01 and the MR about 0.05-0.1 better than the standard VIII but i dont know the Cd change between teh VI and VII models

theoretically VII should be lower than the VI because things improve over time but we all know the VII is inferior to the VI in many ways

[henk4]

As with all things French :mad:

and we love it!

[DasModell]

indeed we do :D

for aero .. check www.mulsannescorner.com .. a very god site :D and has a good list of essential reading :D