My friend and I were arguing on whether it was worth it to have a spoiler on a car. Eventually, I agreed with him that the cost of a spoiler might make the whole thing not worth it, particularly in the general sense. However, I disagree with him wholeheartedly when he says that spoilers have no effect at all. Who is right?
It would sort of depend on what “effect” you were expecting, wouldn’t it? 
On your average car it’s not really effective because you never get going fast enough that the downforce generated is worth the drag tradeoff. At higher speeds, though, the spoiler is essential. In some notorious cases (like the original Ford GT40) the cars were actually a mightmare to drive at high speed because they generated lift rather than downforce. The spoiler, properly designed, counteracts that.
On your typical street car (like an Escort or some other compact car that teenagers like to “rice out”) they’re purely cosmetic. Most of those cars couldn’t go fast enough to make it matter in any case. It’s just the ostentatious version of the “Type-R” sticker.
They can have some effects. That is why they are put on some types of real racing cars. They are like an upside down wing that improves the traction and handling in the rear at high speeds. What type of effects were you disputing?
98% of the cars you see on the street with aluminum bleachers bolted to the rear decklid are just hurting their gas mileage by increasing drag and adding weight.
As far as OEM spoilers like you see as part of “sport packages,” they don’t do anything.
Spoilers and wings are for the race track, not the street.
On most cars they are just for cosmetic reasons. On supercars like a Ferrari or Porsche, the whole body is designed so it generates downforce without the need of a spoiler (eg. ground effects).
On the other hand, there are cars like the Subaru Impreza or Mitsubishi Lancer that are almost as fast as a supercar but are based on the body of a compact/mid-size sedan. Those cars really need a spoiler.
Another car that apparently needed a spoiler is the Audi TT. After many customers reporting that the car was unstable at high speeds and several accidents, Audi decided to change the suspension settings and add a spoiler. http://www.autointell.com/News-2003/January-2003/January-2003-5/AuduTT-Spoiler-sm.jpg
Yes, this tiny piece of plastic is a functioning spoiler.
Airman Doors: Funny you mentioned the Escort. The Ford Escort RS Cosworth homologation special version was equipped with a huge, unsightly but fully-functioning spoiler
http://www.escortpower.org/forums/gallery/files/5/ford.escort.rs.cosworth.ronnie01_original.jpg
I remember when Saturn offered a spoiler, their literature said, “It’s not going to have the slightest effect on your driving, but it looks cool.”
At least they are honest 
To get a good idea of how ‘important’ spoilers are look at the BMW M5.
As fast and powerful as any car you are likely to see, and in a normal ‘boxy’ shape. Yet it doesn’t require large spoilers, just a small upturn at the trunk, and a lip at the front. As the Audi TT story shows, these tiny spoilers are valuable for car stability and downforce at high speeds, but they don’t need to look anything like the rear end of a Neon SRT-4.
Always entertaining are inverted-wing spoilers on the rear of front-wheel drive cars. If they are effective (questionable), they act to increase the downforce on the rear wheels (which don’t do anything but support the rear half of the car) and act to lift the front wheels (small moment around the rear wheels). :dubious:
If it was much of an effect at all, it would be counter-productive to both handling and power.
Hell yes. Look how many cars having them are sold.
So the effect is to increase the marketability of the car when sold! 
Without commenting on the effectiveness of the spoilers in any of these cases, your comparison isn’t really fair. The M5 isn’t particularly boxy, weights 4000 odd lbs compared to 2800 odd lbs for the SRT-4, and is a RWD V10. The SRT-4 is faster than the Audi TT in most respects, so perhaps it needs a spoiler?
Wow, this was almost exactly the type of argument that my friend made, and I argued against it. He said exactly 98% of the cars out there don’t require a spoiler. He said that they make the car heavier and hurt my beloved standard of gas efficiency.
I said that more than 2% of the cars out there need them. And, I argued that “high speed” is anything over 65 mph, or slower, depending on the wind speed. Two cars that I brought up were the Subarus and the Lancers. It’s good to know about the Audi TT, however, I’m sure the Saturn doesn’t help my case.
Is there a general statement that I can make about the effectiveness (or usefulness) of a spoiler? That they are needed after XX mph, or into XX-type headwind? Or, possibly, to say: if the car is XX low off the ground, they are needed for certain ground effects or to reduce drag coefficient?
Let’s put it this way: If you need a spoiler for a car to drive at highway speeds your car is so poorly designed that were I you I would take it back to the dealer and demand my money back.
Motor Trend agrees with me:
If you’re driving your Chevy Cavalier on the Autobahn, you might could justify bolting a row from Section 38 of the local football stadium to the decklid, but for normal driving, you don’t need it. Same for those ground effects body kits.
Those gignormous hunks of fiberglass on the backlids of the Evo and the WRX do nothing but generate appeal for the rice-boy crowd. (My Legacy Spec B–with the same engine as a WRX STi–is sans spoiler.)
There is no reason any street-legal car driven at normal driving speeds requires a spoiler. (And as someone already pointed out, the entire line of BMW performance sedans, including the M3, M5, and M6, lack any significant spoiler.)
On the previous car, the spoiler did perform one service; I used to lock my deck-mount bike rack to it. Now, I have to take it off and put it in the boot if I’m worried about it being stolen.
Stranger
In a nutshell a spoiler is a wing that pushes down when air flows over it. This creates drag but at high enough speeds creates more foce pushing down on the car giving more traction in high speed cornering. In the US you are probably not going to find more than a handful of roads that you can legally operate car where a spoiler would have any effect on handling. You aren’t going to be cornering at 80-100mph anywhere in any typical US city, at least not for long.
Why do they call it a “spoiler”? What does it spoil (besides the looks and performance of a car, I mean)?
Any downforce you generate incurs a penalty in drag (backward force) so if the spoiler has it’s claimed effect, it is reducing available acceleration and hill performance. Google “induced drag” for cites to your hearts content.
Street cars tend to be power limited at anything faster than bicycle speeds, thus
a spoiler will at best do nothing for perfomance, and in all likelyhood, harm it.
If you have oodles of power, and are on a low traction surface, then a spoiler could be helpful to overall performance (at the cost of fuel economy). Street cars don’t operate in such conditions, but these guys do. Those wings are so large because that is what is requred to be effective at sub-100mph speeds.