[gameson]

If it does give downforce, which one perform the best downforce? The M performance? M4?

Thanks.

[im_an_alien]

To create down force the spoiler needs to be an airfoil, so air needs to flow above and below it. It is the pressure differential above/below that creates down force. It also needs to be raised and in the air stream, so mounted flush with the trunk won't do much except for looks.

[Billfitz]

A ducktail is better than nothing. It doesn't have the shape of an airfoil, but it has some effect working like aircraft elevators. That said, the speed at which it would have any effect would be very high. The standard speed setting for deployment of the spoiler on the F34 is 68 MPH, as below that it would have no effect.

[visionaut]

Yeah, they work (at least the BMW engineered ones).

Aerodynamic downforce and drag increase with air speed squared. At speed they can be significant. Because the rear of these cars already has a very aerodynamic tapered roof/window shape, it actually produces a clean, high energy flow field for a spoiler.

A spoiler isn’t a wing, but it also generates downforce (reduces lift) based on its horizontal length and deflection angle. It produces downforce in two ways. It slows the airflow ahead of it, which increases the air pressure on the trunk and it deflects air upward as it flows past the end of the car reducing lift. Spoilers also increase drag (same reasons).

BMW recommends their M Perf front splitter & rear spoiler be used in tandem to balance their aero effects on the chassis, which have been tested to increase downforce by ~20% front and rear. (F32 M Perf CF Spoiler was like 150lbs downforce at 125mph).

[McGyv3r]

Originally Posted by im_an_alien

To create down force the spoiler needs to be an airfoil, so air needs to flow above and below it. It is the pressure differential above/below that creates down force. It also needs to be raised and in the air stream, so mounted flush with the trunk won't do much except for looks.

This is completely wrong, see the video below for proof. In essence, the new Nissan Z with a duck bill type spoiler is limited to 155 mph whereas without are limited to 136 mph due to lift.

[n55david]

The answer is yes. I always try to get people I know to get the largest spoiler they can so it slows them down

[jsouth]

Downforce creates drag. Unless you are doing cornering at very high speeds, you'll be slower. Unless, of course, you have so much torque that you are spinning the tires at high speeds. THEN you might see a benefit.

Other than that, it's just poser crap. There is a reason that Indycars lower drag for qualifying and increase it for the race. And you aren't approaching ANY part of that equation in a street car.

[Billfitz]

Down force does create drag, but it's an acceptable trade off when the alternative is taking to the air. That's not an exaggeration, as the shape of a car is the shape of a wing. Of course this doesn't apply to normal driving speeds, but in the realm of professional racing it certainly does. And yes, big wings on street cars are just poser crap. It's what guys do.

[FaRKle!]

Originally Posted by visionaut

which have been tested to increase downforce by ~20% front and rear. (F32 M Perf CF Spoiler was like 150lbs downforce at 125mph).

Where's the source on that? Those numbers make the M4 GTS aero look ineffective (210lbs at 183mph for the GTS wing vs that tiny spoiler doing 150lbs at only 125mph...)

[visionaut]

Originally Posted by FaRKle!

Where's the source on that? Those numbers make the M4 GTS aero look ineffective (210lbs at 183mph for the GTS wing vs that tiny spoiler doing 150lbs at only 125mph...)

I don’t have an specific test result but I did a rough calculation based upon 2 datapoints I’d picked up. (I’m an Aero engr and was digging into BMW’s aero tech, which is quite good). The first was from a source knowledgeable of the F32’s EfficientDynamics development (and wind tunnel use), that cited that the M Perf CF front splitter-spoiler added 20-30% downforce to the unadorned already-efficient front end design of that car. The second was an AC Schnitzer wing test that cited the OE aero rear downforce. I recall that as ~125lbs, but I forget if it was at 125 or 140mph.

So knowing the M Perf aero enhancements were looking for balance front-to-rear, and more for high speed road balance (vs using it to add understeer for improved slow speed performance), and knowing that it takes considerably more engineering effort to add downforce from aero to the front of a car than it does at the rear by adding an effective wing/spoiler, I thought 150lbs was a decent estimate for the M Perf rear spoiler.

Big wings like the GTS one are typically capable of adding almost 100% downforce to the rear of car, if they’re deployed in a free-flow field. (even more with a Gurney flap).

I’d expect the F-series M4 to have superior aero out of the box. If the big GTS wing puts out those numbers at top-speed, I suspect that quoted number is all added downforce, not total downforce, as my M Perf rear spoiler calculation was.

Tom

[FaRKle!]

Does it make sense to state the total downforce though rather than just the additional contribution from the spoiler (to me it doesn't)? Saying "F32 M Perf CF Spoiler was like 150lbs downforce at 125mph" seems a bit misleading and crediting all of that to the spoiler versus just saying "F32 M Perf CF Spoiler was like 25lbs downforce at 125mph."

The GTS numbers come from the BMW press package (and thus put into print under many review articles).

The GTS numbers were stated at two speeds (for the splitter and spoiler)
-124mph: 26.5lbs F, 88lbs R
-183mph: 63lbs F, 210lbs R

[visionaut]

Sure. It was just a quick mental calc, from a known total rear downforce number, and it’s probably conservatively low too (just used 20%, it’s likely higher).

The GTS downforce data F & R, with the rear being that much larger shows what I mentioned about it being technically easier to add downforce at the rear than the front. My estimate for the rear M Perf spoiler downforce might need to be raised a bit to reflect BMW obviously applying a more rearward-oriented downforce ‘balance’ than 50-50.

Most folks don’t have any grasp of aerodynamic forces, or how quickly they grow with speed. Telling them the M Perf aero adds 20-30% to the car is too abstract. Telling folks it’s like a person standing on your trunk lid at speed helps them relate/understand that properly engineered aero isn’t just aesthetics (though a lot of the eBay type stuff hardly looks engineered).

BMW’s aero works. The combo of their front splitter and rear spoiler are noticeable to the driver at highway speeds, the car is more planted and glued to the road as speed hits triple digits, and the increased stability translates as firmer steering feel.

[floridaorange]

Originally Posted by Billfitz

A ducktail is better than nothing. It doesn't have the shape of an airfoil, but it has some effect working like aircraft elevators. That said, the speed at which it would have any effect would be very high. The standard speed setting for deployment of the spoiler on the F34 is 68 MPH, as below that it would have no effect.

I swear on take off under WOT the car feels more pressed to the ground post spoiler install. That just butt dyno talking?

[Billfitz]

Confirmation bias. It takes a lot of air pressure to exert downward force, which requires a lot of speed. Remember as a kid sticking your hand out the window and changing its angle like an airplane elevator? At 10 MPH it does nothing, at 60 MPH it does a lot.