Interior Exterior
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A well designed rear wing with proper wing tip design known as a Gurney flap increases the effective camber of the airfoil Yo, Tim... The rear diffuser are sometimes referred to as venturi channels. As the flow excelerates under the car, the pressure decreases as dictated by Barnoulli's Relation. This low pressure air then slows down through the channels at the same time air enters the channels from the side of the car. If the outer walls are close to vertical a strong vortex will form in the channel. This vortex will keep the flow attached within the channel while helping to stabilize the flow beneath the entire vehicle. Therefore these channels increase the downforce and decrease the drag of the vehicle. Because we do not have channels under the car, a rear slant is added behind the rear axle. This slant generates the same effect as the underbody channels, only to a smaller degree. A well designed rear wing with proper wing tip design known as a Gurney flap increases the effective camber of the airfoil. The result is greater downforce with only a small increase in drag. For example, look at the wing tips of newer and more efficient aircraft. Or, look at the Benetec wing..........Hell, but then, that could just be my opinion.... The airdam is designed to decrease drag by keeping air from the underside components aka suspension.......Now I have to go all the way to 13..
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Now, for a car, we flip the aerodynamics upside down. In the case of the Elise, the underside of the car is flat until you get to the rear diffuser. At this point it curves upward, and like a wing, causes the air flow to speed up and consequently generate "downward lift".
The bottom of the car doesn't act like a wing, actually it is best represented as a duct. The purpose of the diffuser is to reduce the velocity of the air that flows under the car (not to speed up the air), ideally expanding the air back to the pressure before the front of the car. The diffuser does not create down force, its purpose is to maximize the efficiency of the "duct" and to reduce drag at the rear of the car. Maximizing the efficiency of the "duct" means maximizing the flow of the higher velocity air under the car and as it was correctly stated, the the higher velocity air is what creates the low pressure area. This low pressure area sucks the car down to the road.
The strakes on the diffuser are... to keep the air from "spilling over" (or in this case from sneaking in from the sides).
Correctamundo! The idea of the strakes are to block air from entering the sides of the diffuser and interfering with the flow of "ducted" air from under the car.
Nope. It was Aerospace Engineering - with lots of aerodynamics and fluid flow (lot of structures too...)
Tim, which area of engineering represents the majority of your professional competence?
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The bottom of the car doesn't act like a wing, actually it is best represented as a duct. ... Maximizing the efficiency of the "duct" means maximizing the flow of the higher velocity air under the car and as it was correctly stated, the the higher velocity air is what creates the low pressure area. This low pressure area sucks the car down to the road.
Isn't this basically how a wing works? Varying the air pressures on the two sides causes lift, and if inverted, causes downforce?
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Isn't this basically how a wing works? Varying the air pressures on the two sides causes lift, and if inverted, causes downforce?
Basically, yes that is how wings work. Although similar, airfoils (wings) and ducts are different applications of Bernoulli's principle.
The top surface of an airfoil works best in free air, turn it upside down, put it close to the ground and the airfoil shape will not be as effective pulling downward. The proximity of the ground is what makes the flat bottom of the car and diffuser work like air flowing in a duct. Also, an airfoil is a continuous smoothly contoured shape where the bottom of the car is featureless with an abrupt transition to the diffuser. Flip the car upside down and the bottom surface and diffuser won't make an effective airfoil shape.
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Thanks
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You're Welcome.
BTW, your car looks awesome!
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You're Welcome.
BTW, your car looks awesome!
Thanks.. I thank Lotus (and Sector111.com) everytime I look at it
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Hey Sleepless...if you really want to get faster lap times with your "Track Only" Elise, you could lighten her up by removing your street plates and passenger....10 pounds = one horsepower.
The passenger is my 64-year-old father who likes to go for rides with me. We make a trip twice a year together to spend 3 days running around Thunderhill Raceway; father-son bonding thing... He is the reason I bothered to strip 100lbs off the Elise. Too bad he weighs 175lbs... I'm still 75lbs off ... oh well... At least I get a couple of sessions without him in the car
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This stuff is touchey to set up. IF you want to optimize performance. Things like ride height and rake greatly affect matters too. And the lower you go with your ride height, the more important it is to keep it stable. So that the resulting benefit is stable.
Ronin has noted that he dropped his car 1 1/2 inches (~38 mm) IIUC. This is lower than most Euro guys suggest. Not sure if the car was bumpsteered, performance aligned and corner balanced. At what cornering speeds is the car being used?
Here are some tidbits from Lotus, noting aspects of the S1 & S2...see how you can read about their tests...ride heights..and the need for testing and balancing the car out?:
"The Elise S2 has a Cd of 0.407 and a CdA of 0.651 with the roof on. The Elise is more aerodynamic with the roof on. The front and read lift co-efficients are Clf -0.02 and Clr -0.04 assuming a 130/130mm F/R ride height and roof on. These coefficients need to be multiplied by the appropriate surface area before the downforce in kg can be calculated. These compare well to the equivalent figures for the standard S1 car which are Cd 0.408, CdA 0.653, Clf -0.030 and Clr +0.053. This assumes a standard ride height of 140/140 F/R and the roof on. Lotus reduced the front downforce and the rear lift on the S2 to achieve almost neutral balance which delivers consistant handling balance at all speeds, while the S1's high front downforce and rear lift contributed greatly to the cars tendancy to oversteer at high speed. It could be argued that reducing downforce is a retrograde step and that leaving the front in S1 configuration and then increasing the rear downforce further to achieve a Clr in the region of -0.3 would generate more lateral rip, but testing showed the drag penalties associated with doing so were unacceptable. The roof of the S2 was lowered to also improve the quality of air flow over the rear of the car and hence make any rear spoiler more efficient. The Elise S1 111S and Sport 160 spoiler efficencies were reduced by the higher roof line.
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Hey Sleepless, The top of my Elise will never come off ,{ unless sucked off by the preasure difference at the speeds I drive on public roads} oh wait that was a dream...........So to Every One reading these posts remember.....
10lbs = 1 hp....so we will all be driving much slower cars this weekend......
adjust your driving style accordingly.......I'm 55 and I have to say I laugh out loud every time I get out of this thing............Then I realise why I bought it..
......Be SAFE and FAST.......
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"Truce"........Frank..... I am one opioninated M F&*&#@........take care....
Truce, take care...
Where is the group hug icon???
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I'm so glad I took my meds yesterday and didn't get involved.
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Posted for Ronin
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Posted for Ronin
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Posted for Ronin
''WING BLING
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Wow, Frank those are some nice cars
Do you still have all of those?
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Tim - For your information, Vertical wing fluid dynamics winged keel in action.
aka Ronin
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Posted for Ronin
Not much down force on the Gullwing .
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Not much down force on the Gullwing .
I don't know about that it looks like it has two very large wings
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What happened to the wing on the Elise?.......looks like the ends went limp!