What does this 'spoiler' do?

[djones]

I was looking at the spoiler on a friends BMW 3 series and could not work out if it was just for looks or actually does something. Here is a picture of one:



And another one here:

http://www.gashaponusa.com/ebay/xenonsp ... _E46_A.jpg

It's not a wing as such (no gap in or under it) so I don't see how it creates downforce, so what else can it do???

It was an optional extra (standard on the M3) so maybe its just for show.

[Tom]

partly for show but I believe it is called (I call them) a Ferrari lip. The idea is that it is inserted in the lamina flow and boosts the air over the back of the car rather than letting it drop of the back and create drag. I think Ferrari were first to use them.

[dumrick]

Yes, that's right. It detaches the airflow from the bodywork, lessening drag, increasing a bit top speed and decreasing fuel consumption.
Also, because of a small vertical component of the force made in that spoiler, it must allow for a (small) increase in downforce in the rear.

[Ciro Pabón]

It's not a wing as such (no gap in or under it) so I don't see how it creates downforce, so what else can it do???

It seems like a gurney flap to me. Invented by Dan Gurney to race in Indianapolis, (without a wind tunnel, baby).

Let me quote the link:

"... is nothing like as sophisticated (as a wing or flap). Classically it's no more complex than a length of aluminum right-angle rigidly bolted or riveted to a wing's trailing edge."

The link shows also the vortex it creates in the wake, which explains its effect on downforce.

Dan Gurney installed it on Al Unser car as a desperate measure to see if something could be done about the slow lap times they had in 1971. I cannot resist to quote again:

"In 45 minutes or so, the first Gurney Flap was fabricated and attached to the car's rear wing, and Unser went out again. Within a couple of laps it was clear he was circulating no faster than before and everyone in the pit assumed the flap was a failure. But when Unser came in he called Gurney over and quietly asked him whether anyone was around to spy on what they were doing. Once Gurney had confirmed they were alone, Unser told him the rear was now so well planted that the car was pushing (understeering) badly, hence the poor lap times. All they needed to do was restore the aerodynamic balance by adding more front-end downforce and the car would be transformed. For most of that first season, none of the other teams cottoned on to what Gurney had achieved. Anyone who asked was told that the flap was a purely structural feature to strengthen the wing's trailing edge."

Thirty-five years later it still can deceive people that are fixed on the "only a wing works" idea. After all, it was not understood by the entire Indy pitlane for a year...

Besides it shows two elements useful for racing success: an imaginative engineer under pressure and a sensitive pilot that does not brag about his car.

You can find here an F1 Technical thread that touches the subject, out of thread, of course.

After re-reading the thread, I find that DaveKillens gave the same link I am quoting. Honor to the predecessor...

[West]

Taller spoilers like those in NASCAR slow the air down going over the top of the car. It's pretty much a wall in the back of the car. Slower air = higher pressure, relatively speaking. Hence some downforce creation. However, the spoiler may interfere with the undercar aero; Racecar Engineering covered it a year ago but I forgot the results.

Simon McBeath's Competition Car Downforce states that you can keep increasing the height of the spoiler without any real drag penalty, increasing downforce. The spolier on your friend's BMW probably is mostly for show, but usually factory spoilers decrease drag these days.

[manchild]

I don't think that it "boosts the air over the back of the car rather than letting it drop of the back and create drag" or "detaches the airflow from the bodywork, lessening drag, increasing a bit top speed and decreasing fuel consumption".

I'm sure it does the oposite by forcing the air to generate downforce at rear end of the car which increases the drag and fuel consumption decreasing top speed for the sake of downforce.

[Ciro Pabón]

Taller spoilers like those in NASCAR slow the air down going over the top of the car.

I thought those spoilers were too high (over 15 cm) to work only as a Gurney flap. As Manchild suggests they are so high that they behave more like an air brake. At least, when a bodywork is "slow", the rules allow a less higher spoiler.

NASCAR spoiler:


Working on an Indycar gurneyflap:


For example, NASCAR's Fords and Chevrolets have 6.25 inches high spoilers and Dodges and Pontiacs have 6.5 inches ones. This is a concession to Ford and Chevrolet.

[West]

Taller spoilers like those in NASCAR slow the air down going over the top of the car.

I thought those spoilers were too high to work as a Gurney flap and they behave more like an air brake. When a bodywork is slow, the rules allow a less higher spoiler.

That book I mentioned says it like this: the spoiler SPOILS (big surprise) the air over the top, eventually disrupting the laminar flow. Theoretically the air will slow down and hence downforce.

NASCAR does limit the height of the spoilers probably because it's so easy to make downforce with them. Obviously not the level of a wing but it's so much easier to make.

That book however was written nearly 10 years ago and I don't really keep up with aero now.

[djones]

So basically it's used to reduce drag.

Would it increase lift at high speeds due to the air moving slower over the car than under?

If it reduces drag, how much would you expect by? 2%??

[Tp]

The function of the lip is to use the airflow, to physically push the back of the car down, for better high speed stability.

The Audi TT has also a lip, after the engineers found that the profile of the car is pretty much the same as an airplane's wing (i.e. creates lift) . So you can imagine what happen on the derestricted Autobahns......

I think Ferrari were first to use them

Yeah but Ferrari have now moved the "goal post" my adding a diffuser, so the lip works in conjunction with it. For better extraction of the air, underneath the car. Therefore improved downforce.

[dumrick]

It seems like a gurney flap to me.

The Gurney Flap in strictu sensu is only used in wings and wing profiles. It certainly may ressemble one (although the traditional configuration of the Gurney Flap is in a 90º angle from the surface it is attached to), but functionally it cannot be.

[RH1300S]

It's a spolier as correctly stated above.

AFIK a Gurney Flap applies to something attached to a wing - but a brilliant post by Ciro - it's a great story and always worth hearing again. But it works on a similar idea to a spoiler (or so I believe).

Apart from the obvious in that a spoiler can create a high pressure area ahead of it, thus adding some extra downforce (less lift?) - I believe that a spoiler can also delay separation of the airflow coming over the roof and down the rear screen thereby reducing drag (as correctly stated at the start of this thread ).

In some respects the spoiler may be a styling device. It allows the car designer to maintain a good line to the rear of the car while still adding the extra "bit" required by the aero. Imagine how clunky the car would look if it were all metal back there I don't think BMW would be tempted to add something if it weren't doing a job BTW; maybe it balances out a different shaped front end to other models in the range?

[zac510]

It definitely doesn't come on all 3-series. It must be an optional extra.

[manchild]

I believe that a spoiler can also delay separation of the airflow coming over the roof and down the rear screen thereby reducing drag (as correctly stated at the start of this thread )

When I wrote increasing the drag I had in mind what effect it causes behind the car. It certainly directs the air up so there must be greater drag than without it.

[Tom]

not nessicerally. The impression I get reading a book a few months back is that the car pulls some of the air that is thrown off the back. The Ferrari ridge as I should have said, not lip, helps the air 'leap' off the back so that the car pulls less air behind it.