Rear wings on hatchbacks: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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Jan. 30, 2012 6:36 a.m. wclark New Reader

I have been searching the web on this subject for a while but keep coming up short on answers.

My question is: For a rear wing, Is there an optimal height above and position relative to the rear edge of the roof of a hatchback?

It seems most racing body rules dictate wings cannot be above the roof line (or severely limit how far above the roof they may be), which I assume is why most rear wings on hatches I see in racing pictures are mounted at the roof level and trailing the rear edge.

The rules for the racing I do has no such limit so I want to position the wing I am adding to balance the front splitter optimally.

From what I have read about air flow, the boundary layer of air over a flat surface is under an inch, and given the fact that my A2 Golf roof is not flat but only has a slight downward taper to the rear before the edge, I would think that a wing greater a 3-4 inches above the roof would be in pretty undisturbed air.

I also read that positioning the wing so its front edge is about in line with the roof rear edge may be optimal, both to increase leverage on the rear wheels and to help with drag. I understand the first part but not the second.

Anyone with experience care to share on this?

Thanks.
Jan. 30, 2012 6:44 a.m. dean1484 SuperDork

Wings serve two purposes. They can either reduce drag by braking up the vaccum at the rear of the car or create down force. Placing a wing close to the rear deck lid at the back will reduce drag. Placing it up high in clean air is better for downforce. The location front to rear is all about how the force is transmitted to the car. The further back behind the rear wheels the more of a lever efect is created and more force is transmitted to the rear wheels.
Jan. 30, 2012 8:15 a.m. Maroon92 SuperDork
Jan. 30, 2012 8:32 a.m. mad_machine SuperDork

you need look no further than the Estates that Volvo used to race. They had a subtle "lip spoiler" on the roof edge of the hatch that fooled the air into thinking the car was several feet longer.. this making it more slippery through the air
Jan. 30, 2012 9:19 a.m. JohnyHachi6 Reader

I expect you will have the best luck placing the wing approximately as shown in Maroon's image, but maybe a couple inches higher.

I think what dean said is true and it's well worth the drag penalty (which is quite small) to have more downforce (so put it a little higher). On some cars you may be able to achieve some benefit to both downforce and reducing drag by lining up the leading edge of the wing with the trailing edge of the roof, but the air flow is very complex in that area of the car, with a lot of recirculation, so I would avoid that by raising the wing up a few inches into clean air if possible.
Jan. 30, 2012 5:01 p.m. wclark New Reader

Thanks everyone.

Since adding the duckbill style lip on the top of the hatch is easy and effective for drag reduction I will do that in addition to the wing.

From what I gather here, if I mount the wing just a few inches above the roof and as far back as I can, that will likely be as effective as I can make it from a downforce standpoint. Based on that I will locate it based on what is needed to clear the hatch when it is opened (I intend to mount the wing via end plates that fasten to the C-pillars and to use a shorter gas strut on the hatch).