COEFFICIENT OF DRAG

[charliex]

saw this on the make rss feed today
http://www.instructables.com/id/ELEE7STF5Y3TMFK/

[Beanie]

Nice. I may use some of that to get the rolling resistance. Someone calculated it using my coastdown test results (in Excel) and a math package that I don't have. It (Crr) wasn't that significant, but should help make the HP calculation a little more accurate.

Do it with windows up/down. My car is a convertible, so I did it with the top up and windows and then with both down. CD was definitely affected. It decelerates much faster when you are going, say, 80MPH and there is more drag.

[smoseley]

Originally Posted by gogogo

Im a car designer and engineers always tell me to make the rear shaper for better Cd. They want the air to follow the side and top but they want the air to seperate at the end becauseround shape creates negative pressure behind. Basically an egg with chopped off rear is the best, like a bullet.

What they're asking for is optimal laminar air flow detached as close to the elevation of the roofline as possible. Laminar air flow means less drag (turbulence). The TT is a great example of a low-drag car that is basically like one big laminar airflow magnet. The problem with the original design is that its egg shape never detaches the airflow, and therefore when the car curves back down, the air sticks (laminar = laminated) to the car and pulls it up... the car becomes a giant airfoil (wing). A tapered back end will allow the laminar flow to detach with minimal turbulence. You don't want to just cut off and have a flat back end. That will create a void below the car, and that high pressure above and to the sides will collapse inwards and create swirls of air behind it. That's turbulence, which will also slow your car down, because it sucks the car backwards. It's like trying to pull a capsule out of a vacuum tube... you can pull it 1" no problem. 2" a little harder, but the further you pull, the harder it gets. Same with the back end of a car and top speed.

[EdHahn]

Originally Posted by transio

What they're asking for is optimal laminar air flow detached as close to the elevation of the roofline as possible. Laminar air flow means less drag (turbulence). The TT is a great example of a low-drag car that is basically like one big laminar airflow magnet. The problem with the original design is that its egg shape never detaches the airflow, and therefore when the car curves back down, the air sticks (laminar = laminated) to the car and pulls it up... the car becomes a giant airfoil (wing). A tapered back end will allow the laminar flow to detach with minimal turbulence. You don't want to just cut off and have a flat back end. That will create a void below the car, and that high pressure above and to the sides will collapse inwards and create swirls of air behind it. That's turbulence, which will also slow your car down, because it sucks the car backwards. It's like trying to pull a capsule out of a vacuum tube... you can pull it 1" no problem. 2" a little harder, but the further you pull, the harder it gets. Same with the back end of a car and top speed.

It's not that simple, actually. There are two factors involved: laminar vs. turbulent flow, and separate from that, attached vs. detached flow.

Attached laminar flow is the lowest drag, but is very difficult to achieve. Next lowest is attached turbulent flow, followed by the detached flows.

Turbulent flow has the advantage of being able to stay attached to surfaces where laminar flow would detach - like the roof of most notchback sedans. As such, it is often desirable to use it when laminar flow is not achievable.

For example, the vortex generators on the wing of a B757 create turbulent flow that can stay attached to the ailerons over their entire range of motion. This is sufficiently desirable that the designers of the aircraft were willing to suffer the slight drag penalty when the ailerons aren't deflected.

http://img.photobucket.com/albums/v5...p06/BHX020.jpg

This pic shows the vortex generators out there - they are the black "dots" on the outboard section of the wing.

Vortex generators are also used on the rear roofline of (some?) Evos.

The "vacuum" effect you describe (difference in pressure) is more an effect of detached flow, rather than of laminar vs. turbulent flow.

ed

[apexdc]

I LOVE this forum! To add a little to the two previous posts. The long tails you see on Bonneville cars are optimum for low drag, but obviously impractical on the street. The chopped off (Kamm, named after a German aerodynamicist of the same name.) rearend shape, I believe, was based on the theory that chopping it off clean at a point that still had attached airflow was better than taking the "Bonneville" style and trying to shrink it down to a manageable size using more abrupt tapers which actually induced detatched airflow. I would appreciate any corrections on blunders I have made here.

[LARRY]

Originally Posted by fingolfin

Seriously.....I'd rather take a bubble bath with my Dad than drive a Corvette, spending my life suffocating in that vinyl interior. Good thing aero doesn't matter so much in the twisties.

[Buz]

What about the Cd of an Elise with the top off and windows down. My guess is .65

[holmz]

Originally Posted by Buz

What about the Cd of an Elise with the top off and windows down. My guess is .65

That guessing is a cute diversion while everyone else is talking measurements and math.

[NoFear99]

My 94 RX-7 had a .29 coefficient and I can tell you for sure that my RX-7 was more stable at 200+ km/h. Even at 250 km/h in the RX-7 I had one hand on the steering wheel without problem( I was confident ). I did 200-220 KM/h in my exige S and I can feel the wind hitting on my car. I can imagine what it would look like at 250 km/h in my exige and I would definitively keep 2 hands on the steering wheel for sure.


It explain why my exige S accelerate slower at that speed than the RX-7. There have a big difference in HP also but the aerodynamic of the car must contribute the a slower acceleration at high speed too.

[Elistan]

Originally Posted by Lancia

Who needs the metric system?
My car gets forty rods to the hogshead.

Wow, that's pretty low. My car gets about 483,840 rods to the hogshead.

http://www.google.com/search?hl=en&q...s+per+hogshead

Google is fun.

[Nag]

the most important airo part of a car is the back from what i have been told. I have also hear a pear is the best shape.

so what is the drag of a elise ? and how many poinds of down force is it generating?

[apexdc]

Originally Posted by apexdc

There is an excellent British publication called "Racecar Engineering" that just started a very comprehensive aero analysis of the Exige.

First article is August 2007. The magazine is usually available at places like Borders, Barnes and Noble, etc.

This month they are comparing a box stock Exige with a Hanger 111 racing version. Very interesting stuff.

If you think our cars are bad on drag, check and see what a current Formula One car is. About the same as a barn door!

This article has now expanded over several months. My understanding from some other sources, to my memory, is that the Elise does not generate net lift or downforce. Which means that it generates enough downforce to counteract lifting forces.

[TimMullen]

Originally Posted by apexdc

This article has now expanded over several months. My understanding from some other sources, to my memory, is that the Elise does not generate net lift or downforce. Which means that it generates enough downforce to counteract lifting forces.

Not quite. The Elise generates a bit of "downforce". To some people, it doesn't sound like much, but it's a lot more than most any other car on the road that generates many lbs of lift instead.

The stock Elise generates around 4.5 lbs of "downforce" at the rear and around 8.5 in the front at 100 MPH. The Exige 48 and 42 lbs of "downforce" at the same speed.

[apexdc]

Tim,

Thanks for the correction. I was doing it by memory, but didn't think there was much net downforce.

As a little aside, Phil Hill said the old Pontoon fender Testarossas from the fifties generated so much lift at Le Mans that they would eventually pull out all the suspension travel in the front to the stops and then the steering became very dead and light. Small wonder as the front tires were barely touching the ground.

[trompazo]

I have datalogged an Elise and Exige S at a track with a long straight.

Surprising, the top speeds were not that different.

185km/hr vs 191 km/hr in the Exige S. But the exige got up to 160km/hr much faster. After that, both cars seem pathetically powerless.

I think drag has alot to do with it. These cars don't do well on tracks with really long straights. Amazing everywhere else, but above 160km/hr (100mph) I can't keep up with my wife in her Mini Cooper JCW.

[mathijskok]

Originally Posted by Nag

the most important airo part of a car is the back from what i have been told. I have also hear a pear is the best shape.

so what is the drag of a elise ? and how many poinds of down force is it generating?

There is a documentary about the design of the Elise (think it's even on YouTube) and there you see how the designers and the engineers fight over the shape. The tech guys complain bitterly the shape is a disaster and in the end force the designers to add the little stubby wing on the tail.

Here a nice test.... drive top speed with the top off and shut down the engine (hey there is not power steering, power braking etc, so no problems. Now listen. The amount of noise you hear is a good measure of the drag of the car, the smoother the air the less noise it will make. The Elise makes a racket like few other cars.

Looks great though.

[carlover]

Originally Posted by mathijskok

There is a documentary about the design of the Elise (think it's even on YouTube) and there you see how the designers and the engineers fight over the shape. The tech guys complain bitterly the shape is a disaster and in the end force the designers to add the little stubby wing on the tail.

Here a nice test.... drive top speed with the top off and shut down the engine (hey there is not power steering, power braking etc, so no problems. Now listen. The amount of noise you hear is a good measure of the drag of the car, the smoother the air the less noise it will make. The Elise makes a racket like few other cars.

Looks great though.

If you're looking to find out what kind of drag the car creates, drive with the hard top on, not with the top off

[cpeteod]

So if you remove the 3 front air vents that lead into radiator, and replace that with a solid piece, does that help your DC? Don't mind the fact you may overheat.

[apexdc]

There is an excellent British publication called "Racecar Engineering" that just started a very comprehensive aero analysis of the Exige.

First article is August 2007 and goes on for several issues. The magazine is usually available at places like Borders, Barnes and Noble, etc.

I have mentioned this several times in this thread. If anyone is remotely serious about Lotus aero information, you must get this article. It answers many questions that are posed in this thread.

[Roger]

Originally Posted by Lancia

Who needs the metric system?
My car gets forty rods to the hogshead.

Don't drive far, do you? Not at roughly 500 gallons per mile . . . .


(Sorry, Elistan, I missed your post)