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General E46 Forum
This is the place to get answers, opinions and everything you need related to your E46 (sedan, coupe, convertible and wagon) BMW!

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Old 12-15-2010, 01:41 AM   #1
raptordavedave
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Talking Aerodynamics.. ASK AWAY!

based on the amount of responses from my last thread i have decided to open a new thread so that you guys can ask anything you guys want regarding aerodynamics (dont worry i searched to make sure this is a original thread) ..oh yeah and if anyone is wondering who i am please see my last post. ( http://forum.e46fanatics.com/showthread.php?t=810610) ...ask away! and i will answer every question fully
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Old 12-15-2010, 01:45 AM   #2
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Do the CSL style splitters found on front bumper serve any purpose i.e. downward force? Or are they just cosmetic?

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Old 12-15-2010, 01:59 AM   #3
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Do the CSL style splitters found on front bumper serve any purpose i.e. downward force? Or are they just cosmetic?

they do serve some purpose but they are mostly for looks on our cars...what it does is extactly what its name states... it splits the air traveling underneath the car minmizing the amount of air and thus reducing the amount of lift..The splitter does not actually create downforce. it instead makes a greater area for the pressure to build up. The more high pressure there is above the splitter, the greater the net downforce there is because air will always travel from a area od high pressure to low pressure... this results in some downforce on the bumper Additionally, if there are openings in the front bumper (above the splitter or lip) for brake ducts or radiator cooling, this increased air pressure will encourage more air to flow into these openings if you look at the picture below you can see what the spiltter is doing.. it is pushing the air to the above the car into the vents for the brake ... a splitter combined with a diffuser would actually help aerodynamics of the car at high speeds of 120+.... this reduces the amount of lift... however one would also have to lower the car as well to truly make this a noticeable aerodynamic change.the concept behind is that if a car is practically sitting on the ground the amount of air flowing underneath the car is minimized and in addition the air traveling underneath the car is increased in velocity creating a more stable car since their is less upward force on the car. the lower the better... but going down to 1/2 inch like a enzo is not practicable for us because we have curbs rocks etc ... i kept this simple... if you want some more detail i am more than willing to do so
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Old 12-15-2010, 02:10 AM   #4
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So would a combination of aftermarket aero parts have a significant effect? Say a Carbon Fiber CSL Trunk, CSL Bumper with splitter, rear diffusor, lowered properly, heck even rounded mirrors? Say on the ammount of downforce generated..would it help with grip in a track event?
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Old 12-15-2010, 02:13 AM   #5
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How can wheel design have an impact in air flow over it, surrounding it, and through it, if any?
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Old 12-15-2010, 02:13 AM   #6
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So would a combination of aftermarket aero parts have a significant effect? Say a Carbon Fiber CSL Trunk, CSL Bumper with splitter, rear diffusor, lowered properly, heck even rounded mirrors? Say on the ammount of downforce generated..would it help with grip in a track event?
Dumb question is dumb.

What do you think.
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Old 12-15-2010, 02:16 AM   #7
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I think to fully benefit from aero our cars need a full under body floor panel and proper rear diffuser that originates from the floor panel at the proper angle. The angle of the diffuser from the floor panel plays an important role. The diffusers on M's and Mtech2 bumpers are really only cosmetic. Additionally a splitter up front with the car at a low enough ride height would be nice. I think most of the lips/splitters/and diffusers we add are solely cosmetic and play no major role in adding any beneficial down force.
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Old 12-15-2010, 10:13 AM   #8
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Bernoulli's Principle is in effect.
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Old 12-15-2010, 11:18 AM   #9
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So would a combination of aftermarket aero parts have a significant effect? Say a Carbon Fiber CSL Trunk, CSL Bumper with splitter, rear diffusor, lowered properly, heck even rounded mirrors? Say on the ammount of downforce generated..would it help with grip in a track event?
yes it would... but around a track.. no... the reason being is that first of while no matter how many parts we slap on it our cars body is not the most aerodynamicaly oriented body yes it would help with grip on a track but you would have to be moving at high speeds of 90+ to actually notice an effect. answered how a diffuser and csl trunk affect aerodynamics in my last thread so i will just retype it for you

1) diffuser (f1)
okay the simple way to put how a diffuser works is that it accelerates the air inderneath the car... imagine a wing.. the air going underneath that car is traveling faster than the air going over the car creating a low pressure zone under the car... to make this as simples as possible im going to use a example... lets imagine that the wind going above and underneath the car are like strings... you dont want the strings above and underneath the car to move when you turn or at high speeds... the diffusers act as anchors for these "strings". this provides greater stability at high speeds...thus when these cars turn at very high speeds the flow of air underneath the car has energy which is harnesed by the diffuser thus allowing the car to maintain its speed better through the turns and it also keeps it more stable the diffuser directs the flow of air underneath the car in a stable path reducing entropy... i hope than answers your question. In addition.. if you look closly you may notice that one sports cars the undercarriage of the car is entremely smooth.. a good example is the lfa.. this reduces the drag underneath the car ..heres a example of the lfa using the same concept as the F1 cars use... notice channels underneath the car

2) CSL Trunk
it does very very slighly... it more for looks if you ask me.. heres why... when designing a car you wanna make it cut through the air... in other words.. you want it to mimimize the disturbance in the flow of air.. thats why all of the exotics sports cars are very slippery and are very low to the gorund since it reduces the amount of air disturbed.. on a csl trunk.. the drag behind the car is reduced since it directs the air back into the main flow of air.. imagine air behaving as strings as in this picture .. a csl trunk reduces drag slightly since when the air reaches the trunk it is directed upwards back into the main stream of air reducing the low pressure zone behind the car and reducing drag slightly... hope that answers your question.. i tried to keep it as simple as possible

PS: a M3 "spoiler" does something similar but on a smaller scale... but on these cars its more for looks than functionality because for most of us our cars cant reach 150+mp

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Old 12-15-2010, 11:31 AM   #10
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How can wheel design have an impact in air flow over it, surrounding it, and through it, if any?
at high speeds this the place of greatest problem because the air wraps around the wheel creating a invisible ring around the wheel pushing the car up.. which is what we dont want .. the way to eliminate this is by making the air travel over the wheel and out... dont let it circle... this is achieved by vents placed around the wheel.. so now the wind is traveling directly over the wheel is actually acting as a "strap" holding the wheel down.. so now we are creating downforce rather than upward force .. the is achieved by vents in the wheel. the faster the car goes now.. the more down force that is created on the tires. so not so much wheel design but more of how the wheel size is accommodated with vents can make a difference here is a great example of wheel vents
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Old 12-15-2010, 11:32 AM   #11
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Judging by the first Porsche picture I would imagine in our e46 that a roof spoiler may add more downforce than a trunk lip spoiler would. Thoughts??
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Old 12-15-2010, 11:42 AM   #12
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Judging by the first Porsche picture I would imagine in our e46 that a roof spoiler may add more downforce than a trunk lip spoiler would. Thoughts??
i honestly have never been able to understand why people put roof spoiler on their cars because most of the time it is only for looks the point of a roof spoiler is to direct the air onto the spoiler thus generate a lot more downforce since the faster the velocity of the air the more downforce that is created (f=ma) below are two pictures of a car with a roof spoiler and one with not roof spoiler... the roof spoiler is supposed to act as a guider for the string(imaging air traveling in strings) onto the spoiler thus generated more downforce than it would normally. a roof spoiler and a lip spoiler on the trunk wouldnt do much aerodynamics as a couple acting as a unit but they do look good i hope that answers your question.. if you want more detail feel free to ask
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Old 12-15-2010, 11:44 AM   #13
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More downforce = more drag = poorer fuel economy.
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Old 12-15-2010, 11:46 AM   #14
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I see. So basically roof lip and trunk lip spoilers are useless. Unless you install rally style wings. Anyone want to go offroad?
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Old 12-15-2010, 11:51 AM   #15
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More downforce = more drag = poorer fuel economy.
more downforce doest not mean more drag... more downforce means that there is less drag because for most cars the air travels below and above the car creating upwards force since a low pressure zone forms above the car. however,, with the gift of aerodynamics if one were able to miminze the force of air below the car to practically zero then their is only air traveling above the car which is then acting as a "strap" holding it down onto the ground creating more downforce as the speed increases. people usually see air as a negative when driving at high speeds because it creates drag.. but air has energy that be harnessed to push the car down as it goes faster generating an awesome amount of downforce
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Old 12-15-2010, 11:53 AM   #16
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More downforce = more drag = poorer fuel economy.
Are you really worried about fuel economy at 120? Lol
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Old 12-15-2010, 11:57 AM   #17
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BMW Roof Rack aero question. Too ramp the rack or not ramp the rack?
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Old 12-15-2010, 12:13 PM   #18
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Concidering that car companies new car designs are 4 to 6 years in development and now that you are in that development culture, where do you see the future design of the next generation of N. American, Japanese and European cars leaning to?
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Old 12-15-2010, 12:21 PM   #19
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BMW Roof Rack aero question. Too ramp the rack or not ramp the rack?
by ramp im assuming you mean like a little wind deflector right before the roof rack... well if you are gonna have a bike up their or something dont go fast because the faster you go the more drag that you are going to create but i would suggest go for it because that way you can minimize the amount of "strings" coming into contact with the roof rack. because without it you are basically carrying a "drag magnet" up there. the only downside that i can think of it is that it wont look that good but you can be creative!
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Old 12-15-2010, 12:29 PM   #20
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Which is more aerodynamic a tear drop shape or a conical shape?
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