# Why do more cars now have a short "spoiler" above their rear window?

Mentor
Is there some aerodynamic advantage to these short spoilers? Or maybe does it help to keep dirt from accumulating on the rear window? It seems like I'm seeing this feature more and more lately...

## Answers and Replies

It could be for drag reduction.
The plates better define the sectional area of the vehicle, so prevent the eddy that forms behind the vehicle from extending outside that section, which would increase the virtual sectional area, increasing drag.

berkeman
I always see this:

malawi_glenn, AlexB23, OmCheeto and 2 others
Gold Member
Also helps to keep rain and snow off the back window.

But I think the primary purpose is to reduce drag.

(Is it related to the stepped hull of speed boats and seaplanes? Without the step, flow will stick to the contour longer and cause drag).

Gold Member
Heat reduction from overhead sunlight?

Is it related to the stepped hull of speed boats and seaplanes?
Probably not. It is more like a vena contracta.
https://en.wikipedia.org/wiki/Vena_contracta#Explanation

The wake air rising up the rear window to join the faster air stream above, must turn, so it would push the fast air stream upwards where the two flows meet.

Where the two flows merge, they need to be travelling in the same direction, one on each side of the plate. Avoid a 90° corner, where the flows will meet on a diagonal line that increases the profile of the vehicle.

Boat-tail plates are similar in that they partition the circulating eddies behind a boat, or the back of a container on a truck, to remain behind and not rise above the profile. The length of the plate from the rear needs to be sufficient to enclose half the eddy, so the air can turn more than 90°, to become parallel.

gmax137 and berkeman
Gold Member
When well designed, it is used to delay flow separation to reduce the wake behind the vehicle (and the drag it creates), akin to the bumps on a golf ball:

So you want to "trip" the flow to create a turbulent boundary layer. Here is an example of such a spoiler on a trunk:

You want to create something that looks more like the top image than the bottom image in the next figure:

Also, referring to the spoiler in the OP, the long flat surface extending the overhang of a hatchback model can create a greater surface to improve the downforce at the rear wheels, increasing stability. (Of course, if well designed for the vehicle.)

jim mcnamara, Ranger Mike and berkeman
Arjan82
I think there is another reason which is important: to generate a well defined location for the flow to separate from. If this is less well defined, the location of separation may vary and large eddies may form behind the vehicle (Von Karman shedding). This may shake the car around a bit, that wobble may generate the *feeling* that you don't have the car in complete control, or that something is wrong or whatever. (although the actual effect of the shedding on car stability is small I think).

Gold Member
I think there is another reason which is important: to generate a well defined location for the flow to separate from. If this is less well defined, the location of separation may vary and large eddies may form behind the vehicle (Von Karman shedding).
This is what I was getting at. Thank you for expressing it.

Arjan82
I think this chap explains it nicely:

DrClaude and jack action
Gold Member
When well designed, it is used to delay flow separation to reduce the wake behind the vehicle (and the drag it creates), akin to the bumps on a golf ball:

So you want to "trip" the flow to create a turbulent boundary layer. Here is an example of such a spoiler on a trunk:

You want to create something that looks more like the top image than the bottom image in the next figure:

Also, referring to the spoiler in the OP, the long flat surface extending the overhang of a hatchback model can create a greater surface to improve the downforce at the rear wheels, increasing stability. (Of course, if well designed for the vehicle.)
Excellent job! Totally agree...less drag better... GPM fuel economy

Homework Helper
Argh. I can't get this login to work, but here is a published paper on the subject. Specifically hatchback spoilers.

The issue described in that article was primarily lift, not drag.
From the article cited by @DaveC426913 .
Here's what that article says:
"The spoiler effectively reduced the aerodynamic lift at positive inclination angle by causing the surface pressure near the roof-spoiler junction to increase"

You don't want your tail flying - you want traction.

But it also includes drag information.

I can't access the pdf, but I can access the graphics.
This one shows the pressure map for different configurations:

So with no spoiler, you get significant pull (thus drag) on the rear window.
Neutral spoiler (which appears to be what the cars are using) has no significant effect on lift, but cleans up a lot of drag.

Attempting to eliminate lift (last image) creates more drag.

Last edited:
berkeman