COEFFICIENT OF DRAG

[smoseley]

Originally Posted by cards2468

the higher the CG, the better the acceleration in a straight line. Of course that ruins your cornering, and like I said, aerodynamics becomes a factor too.

Such conventional logic! CG height has nothing to do with cornering ability!!!

A motorcycle's CG relative to its width is the equivalent of like a 40 foot tall car! It still corners ok!

A human's CG is the equivalent of like an 80 foot tall car! Ever seen an NFL running back corner?

[Lancia]

I think you are forgetting cambering.
There are a few unusual vehicles designed to camber on more than two wheels, but it's rare.

[Bavarian Motorist]

Originally Posted by cards2468

There's more to aerodynamics other than the coefficient and the frontal area. The height would effect the drag, especially considering more tire is exposed when raised up. However, the higher the CG, the better the acceleration in a straight line. Of course that ruins your cornering, and like I said, aerodynamics becomes a factor too.

That's my point. Lowering it would reduce the surface area exposed to air resistance. I was just wondering if it'd make a noticeable difference

[smoseley]

Originally Posted by Lancia

I think you are forgetting cambering.
There are a few unusual vehicles designed to camber on more than two wheels, but it's rare.

Thanks, I totally didn't realize that motorcycles lean into turns. My point was tha tthe generalization "tall means poor cornering" is off.

[TimMullen]

Originally Posted by cards2468

You'd be surprised how similar they are. You are right about the idea behind the golf ball, but at low Reynolds numbers, laminar flow has a tendency to separate off smooth flat surfaces. Tripping the airflow to create turbulent air allows the flow to "stick" longer. The same thing occurs on smooth wings of an airplane. As mentioned before, quite often aircraft designers will position vorticie generators to prevent separation

Here you can see the results of oil spread on a wing placed in a wind tunnel. The dark area is where the laminar flow separated.

There's a lot more to it than just that. Angle of attack makes a huge difference. I would suspect that the angle of attack of the wing in the example above was pretty high - around normal stall speed/angle to need to keep the air flow attached. Otherwise, the air would stay attached and laminar of the "backside" of a wing.

Also, keep in mind that automotive aerodynamics is actually much more complicated (in some aspects) than aircraft aerodynamics. The aircraft wing acts in "free air" where the underside of the car is "constrained". The air flow under a car has a "hard" boundary on one side (the ground), and that affects the air flow along the bottom of the car. Additionally, the bottom of the Elise is flat, so there is no reason that the flow will not remain laminar until it encounters the diffuser. Still, at the speeds encountered, the diffuser could continue the laminar flow - no need for vortex generators.

[cards2468]

Originally Posted by TimMullen

There's a lot more to it than just that. Angle of attack makes a huge difference. I would suspect that the angle of attack of the wing in the example above was pretty high - around normal stall speed/angle to need to keep the air flow attached. Otherwise, the air would stay attached and laminar of the "backside" of a wing.

Also, keep in mind that automotive aerodynamics is actually much more complicated (in some aspects) than aircraft aerodynamics. The aircraft wing acts in "free air" where the underside of the car is "constrained". The air flow under a car has a "hard" boundary on one side (the ground), and that affects the air flow along the bottom of the car. Additionally, the bottom of the Elise is flat, so there is no reason that the flow will not remain laminar until it encounters the diffuser. Still, at the speeds encountered, the diffuser could continue the laminar flow - no need for vortex generators.

I did a lab test with air flow over a flat plate having 1 side being perfectly smooth and the other side being sand paper. The smooth surface detached very quickly and the rough surface held the air much longer. But you are right, automotive is much more complicated than aircraft. So many more parameters. I'd have to see tests done to see at what Reynolds number the laminar flow would separate, because like you said, having the ground inches from it makes a difference.

As for CG height, on a motorcycle it's not as relevant, but for a 4 wheeled vehicle, it is very critical with camber and body roll. In Formula 1, they typically design the cars as light as possible so they can position ballasts low and where ever they need them to balance the car the way they want. Even the engine is positioned ridiculously low. The top of the cylinder housing is roughly at the height of the top wishbone on the rear suspension.

[glennsmooth]

Drag isn't such a bad thing. Formula 1 cars are insanely fast and have a rediculous drag coefficient of 1. I only wish for more power as Ford GTs and Ferraris go flying past me on the straights. If you want low drag get one of these. It's only .11 Aptera

[sleepe]

On the non-sport pack optioned Elise it has one oil cooler on the passenger side.
I have always thought that closing the driver's side oil cooler-less air inlet would
reduce drag and therefore allow the car to maintain higher speeds with less motor effort. I don't know of any block-off plates sold for this purpose though.

I read an article in Road and Track that showcased a top-speed test.
The cars included the MB mclaren, Saleen s7, z06, viper, veyron, and ruf Porsche. All the cars were modded except for the Saleen and the Veyron.
The Saleen blocked off most of its air ducting with translescent tape to reduce drag. I think the Elise would benefit from smaller air apertures. I've always thought the air inlets for the radiator and the oil coolers were oversized for such a small motor. thoughs?

[cards2468]

Originally Posted by glennsmooth

Drag isn't such a bad thing. Formula 1 cars are insanely fast and have a rediculous drag coefficient of 1. I only wish for more power as Ford GTs and Ferraris go flying past me on the straights. If you want low drag get one of these. It's only .11 Aptera

It's not a good thing either. F1 has such a high Cd mainly because of the tires, which also generate lift! The 2 other big factors are induced drag from the wings and the air inlets (side pod, air intake, etc.)

I've only been here a few days, but I must say I'm very impressed with the intellect and knowledge on these boards.

[xtn]

Originally Posted by TimMullen

Additionally, the bottom of the Elise is flat, so there is no reason that the flow will not remain laminar until it encounters the diffuser. Still, at the speeds encountered, the diffuser could continue the laminar flow - no need for vortex generators.

Riiiiiiiiiiiight.

The likelyhood that there is any significant, true laminar flow under an Elise/Exige is about none. Sure, it's flat compared to most cars, and as such offers a nice benefit, but it is not anywhere close to being a truly smooth, flat, undisturbed surface from the splitter to the diffuser.

In regards to the golf ball dimples, the only place they might be useful at all is across that line where the air would begin to curve up into the diffuser. It would be higher drag to use it on a flat plane that is parallel to the airflow, where you aren't worried about separation. It is only lower drag where it gives the benefit of retaining airflow attachment around a curved or bent surface that would otherwise stall.

xtn

[holmz]

Originally Posted by Bavarian Motorist

It's been pointed out before that CD does not account for over-all surface area or the area resisting the air. The Elise and Exige are obviously very low in that measurement compared to other cars.

A better metric is the Cd*A which is then makes it all equal.
(Bad Cd time small area ~= good Cd with a large area)

Originally Posted by Bavarian Motorist

Does anyone have any idea how much lowering the car would reduce drag?

I think that the area is about the same looking at the front of the car whether it is on the ground, or on a pole.

You might have slightly less drag under the car with it lowered, or maybe it is higher... Dunno.
But the more wheel that is showing would probably hurt the drag more than anything, so lowering would be better.

Lowering might help the downforce though.
If you had a bit of rake so that the front was lower than the back by 1 degree, then the drag would be sin(1-degree)*downforce.
You can get a lot of downforce without a whole lot of drag under the car, but that doesn't help if the car is a road-rally or road car as those heights are typically less than 2", which doesn't allow for much other than a smooth track.

[apexdc]

you will probably find these interesting.

Can someone explain what a diffuser does?