How to decide whether to remove rear spoiler to decrease drag, improve mileage

[Piwoslaw]

Originally Posted by QuickLTD

GM claims these cut outs by the lights dont affect aero either. Me personally.. I think they do but I dont have 4 college degrees and unlimited super computer resources like they do...

Plus, you're probably not sponsored by big oil...

[aerohead]

Originally Posted by QuickLTD

Here is my factory spoiler/wing. Its nice and flat with an 8th of an inch gap across the front. The ends stick up a bit too much over the quarter panels for my liking but that I am stuck with. It also has a big opening in the back. Do you guys think I should fill the gap along the front edge where it meets the trunk and the rear cavity?

If you fill the gap it becomes a bird bath.I don't think it really compromises performance.

[tripp323]

Ok. You say " the lift can not be enough to offset the added drag, by definition" and you are the expert. But then how does a plane take off? Obviously you can generate enought lift using a wing at higher speeds. Your point about loss of traction is well taken. But put that aside for a sec - say we are on the Bonneville Salt flats and I want to lighten my prototype high-mileage vehicle - at some speed couldn't we use lift like a plane about to take off? I may have trouble controlling this - but if I control lift so that I still have enough traction to keep moving forward? Sorry to challenge your paradigm - hope you will keep an open mind for this curious newbie question.

[QuickLTD]

LOL never thought about it that way.. I do suppose that scoop in the top would tend to hold water. I'll Keep the bird bath in the back yard and not in my driveway LOL. Thanks for the reply.

[aerohead]

Originally Posted by QuickLTD

GM claims these cut outs by the lights dont affect aero either. Me personally.. I think they do but I dont have 4 college degrees and unlimited super computer resources like they do...

I agree with GM.It would be like slightly parting your lips on a motorcycle without a face shield.Ram air will "pool" in the indent,however,since there is generous leading edge radii beyond the cut out and its still in a positive pressure gradient,the flow will remain attached around the radius without separation.It's no problem.

[QuickLTD]

From my experience adding lift in general is a no no. I used to have a hot rodded 84 Ford LTD. I didnt add lift but at steady speeds above 90 or so the steering would become very light and you could feel the nose up attitude the car was taking as It tried to push its brick like body through the air. High HP V8 with the stock 6cyl suspension. Lesson learned it made for a very unstable ride. And yes with enough power a cinder block will fly but that becomes more a function of thrust to weight rather than aerodynamic lift if my thinking is correct.

[aerohead]

Originally Posted by QuickLTD

From my experience adding lift in general is a no no. I used to have a hot rodded 84 Ford LTD. I didnt add lift but at steady speeds above 90 or so the steering would become very light and you could feel the nose up attitude the car was taking as It tried to push its brick like body through the air. High HP V8 with the stock 6cyl suspension. Lesson learned it made for a very unstable ride. And yes with enough power a cinder block will fly but that becomes more a function of thrust to weight rather than aerodynamic lift if my thinking is correct.

Yeah,lift is like the plague,to be avoided at all cost.The bummer is,that for human packaging,cars have ended up with shapes,which because of the way they divert flow over,around,and under the car,low pressure areas arise which induce lift,which increase geometrically with speed.------ In the 1970s-80s,Dr.Alberto Morelli developed a car form of low drag and zero lift.So since then,it hasn't been necessary to have lift,although if you don't respect Morelli's work,well then you're stuck with lift.-------- The palliative has been spoilers,front and rear.They would have made a big difference on the LTD.---------- And yes,the cinder block example is thrust-to-weight.You can do it,but today most would argue the folly of it.Some carmakers not only went with flying barn doors,they've gone ahead and put the barn on wheels.You may have seen a few of 'em.Kinda like a cinder block when you think about it.

[tripp323]

thanks for the response
- so you were cruising over 90 mph in your Ford LTD?! That's cruisin!

[ebouwman]

Originally Posted by tripp323

Ok. You say " the lift can not be enough to offset the added drag, by definition" and you are the expert. But then how does a plane take off? Obviously you can generate enought lift using a wing at higher speeds. Your point about loss of traction is well taken. But put that aside for a sec - say we are on the Bonneville Salt flats and I want to lighten my prototype high-mileage vehicle - at some speed couldn't we use lift like a plane about to take off? I may have trouble controlling this - but if I control lift so that I still have enough traction to keep moving forward? Sorry to challenge your paradigm - hope you will keep an open mind for this curious newbie question.

What he means is that the negative effects of drag are more than the gains from having the lift.

Really i can't imagine ANY noticeable good effects from creating lift in a car. Mainly because lift doesn't create a fuel efficient vehicle, and CERTAINLY doesn't do it by reducing weight.

The reason more weight impacts fuel economy is because of energy spent to accelerate the MASS of the vehicle, now remember that when you are adding lift you are only reducing the load on the wheels not changing the MASS of the vehicle, so therefore, no matter how much lift you create (even at a stand still) you are still going to have to move the MASS of the vehicle.

Creating lift should reduce rolling resistance and thats about it, but you are creating a fair bit of drag. Also if you're talking about the bonneville salt flats and doing high speed then you SERIOUSLY don't want lift, in fact you probably want a good amount of downforce to keep you on the ground, especially if you hit a bump.

[brucepick]

Re. "gains from lift".

Oops. So far as I know, the act of creating lift by aero effects by necessity demands power from the system and does not give any benefit, unless you're trying to get off the ground to fly.

Actual weight reduction helps of course - because the engine now has less mass to accelerate. Merely reducing the force of the wheels against the ground by lift (or by a whole bunch of helium balloons) does not reduce the car's mass. So the engine's job is not easier.

Of course it takes energy to lift part of the car's weight off the ground, which ultimately comes from your engine and your fuel. From what I've read here before, that penalty occurs as drag. All other things being equal, a car with more lift will have a higher drag coefficient. Nuttin' comes from nuttin', and NSTAAFL: no such thing as a free lunch.