Will this spoiler hurt fuel economy?

[03wgn5spd]

The top of the liftgate is dented up after removing a broken up Votex spoiler which was hiding dents. All that is easily achievable is the Latvia/Protuning/Ebay spoiler. It looks pretty flush from the top but the reality really shows in one of the pictures.

I have removed my roof rails in the name of increasing fuel economy and I don't want to negate that.

Thoughts?

Thanks

Ebay link to Protuning spoiler:

ROOF SPOILER for VW GOLF MK4 IV Universal Variant TAILGATE Cover WING MK 4 R32 | eBay

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Taped mockup below...

 

[Nuje]

There are aerodynamic engineers that design spoilers for specific purposes, which could include improving air flow / aerodynamics off the back of the car, or generating downforce to help improve traction. I doubt an aftermarket bolt/stick-on spoiler will have had that amount of engineering.
Short of wind-tunnel testing, I think you'll have trouble getting an accurate answer here.

 

[03wgn5spd]

While true, I was hoping this could be discussed in generalities. To me, any obstruction to wind hurts fuel economy.

 

[Lug_Nut]

Shorter wiper blades at the high pressure base of the windshield are likely a more effective Cd reduction than any removal or addition or substitution of a spoiler (whether for imagined down force, or a rear window air wipe, or just another 'marking-my-territory' differentiation) at the far rear.
If you said you removed the rails to make washing or snow removal easier I'd understand. The rails are in a low pressure region, air flow is up above the roof. That's why the sunroof can be used as a vent. Removal of the rails for a purported drag reduction doesn't make sense.

 

[03wgn5spd]

Shorter wiper blades at the high pressure base of the windshield are likely a more effective Cd reduction than any removal or addition or substitution of a spoiler (whether for imagined down force, or a rear window air wipe, or just another 'marking-my-territory' differentiation) at the far rear.
If you said you removed the rails to make washing or snow removal easier I'd understand. The rails are in a low pressure region, air flow is up above the roof. That's why the sunroof can be used as a vent. Removal of the rails for a purported drag reduction doesn't make sense.

Thanks, removal of the rails for drag reduction...perhaps based upon my limited understanding or previous threads here saying to do it?

 

[Nuje]

Yeah - I'm guessing what those roof rails contribute to drag would be minimal (as in almost unmeasurable; mud flaps would probably add more drag).
And to your earlier comment on the spoiler, I think you're on the right track: Anything spanning the width of the car that gets in the way of air flow will increase drag.

 

[TDeanI]

Roof rails are a huge drag. Removing will leave holes. I used gorilla tape. Haven’t leaked yet.

That spoiler looks like it would help. U want a nice trailing edge on roof for separation of flow off of the car. It would be better without a gap between spoiler and roof. All new SUVs have a spoiler there now. The roof spoiler prevents a large swirling vortex from forming higher than the roof that increases the frontal profile and drag.

 

[03wgn5spd]

Nuje and Tdeanl..you guys have opposing perspectives. Which one is right?

 

[Nuje]

I think we agree, actually.
With the gap between the spoiler and the roof line, you're interrupting the airflow so drag will increase (how much....we'd need a wind tunnel to determine that).

I do want to see the numbers that roof rails running parallel to the length of the car add "huge drag", though - given the shape and and width, seems to be they'd be relatively inconsequential.
Horizontal roof bars for sure add significant drag (12% worse fuel economy according to C&D here), but something much narrower and in line with the flow of air....I'd guess it'd be closer to 1.2%.
With all of that said, I don't work at Boeing (I noticed @TDeanI is near Seattle and he used fancy words like "swirling vortex" ), so he'll probably come in here and smite me for my uninformed opinion.

 

[03wgn5spd]

I think we agree, actually.
With the gap between the spoiler and the roof line, you're interrupting the airflow so drag will increase (how much....we'd need a wind tunnel to determine that).

I do want to see the numbers that roof rails running parallel to the length of the car add "huge drag", though - given the shape and and width, seems to be they'd be relatively inconsequential.
Horizontal roof bars for sure add significant drag (12% worse fuel economy according to C&D here), but something much narrower and in line with the flow of air....I'd guess it'd be closer to 1.2%.
With all of that said, I don't work at Boeing (I noticed @TDeanI is near Seattle and he used fancy words like "swirling vortex" ), so he'll probably come in here and smite me for my uninformed opinion.

I think you guys disagree here though:

"That spoiler looks like it would help."

Not starting a war. He did say it could be better if it were closer but can't get around that part.

I can't get around the idea of a horizontal object above the roofline not causing additional drag.

If I am wrong, this bad boy is going on! I want to be wrong.

 

[Lug_Nut]

A rear mounted 'spoiler' is a downforce producing reversed air foil used to add traction to the rear drive tires when the vehicle is operated at high speed. The increased downforce was more beneficial than the cost of the added air resistance.
What's that? Your TDI is front drive? Why do you want more rear drive traction?


Reducing the low pressure drag immediately following the decidedly non-Kamm rear section will be of greater benefit.
Forget Boeing. What station wagon did they design? This is from that air craft builder that also built automobiles:

Saab 95 (1959 – 1978) - Press release and images - SaabWorld

Unveiled in May 1959 as Saab’s first station wagon, the 3-door 95 was developed from the 93 series as a versatile load carrier, able to carry up to 500 kg of cargo. It retained the [...]

saabworld.net

"From 1961, a rear ‘cheese grater’ spoiler was added to the roof, primarily to keep the screen clean by channelling airflow over it."