Spoilers on cars (physics question)

I saw a Hyundai this morning with an aftermarket spoiler mounted on the rear. They did a nice job matching the color, but it still looked pretty goofy.

I do have a real question, though – or two, actually – before people start making fun of silly customizations and gets shuffled off to MPSIMS.

A spoiler is, as I understand it, essentially an airfoil designed to press the car to the ground at high speeds and prevent it from becoming airborne. Formula One and related vehicles have to have them, else they’d lift off and go careening into the stands. (More frequently than they already do, that is.) This isn’t usually a concern for street vehicles. So…

(1) Given that most cars aren’t going to get the opportunity to go more than 70-75 mph on U.S. highways (let’s ignore the German Autobahn for the moment), how light would the car have to be, including occupants, for the spoiler to be necessary?

(2) Turning the question around, given the average weight of a normal passenger car (usually a few hundred pounds), how fast would it have to be going for the spoiler to have an effect?

Not an earthshattering inquiry, by any means, but I am curious. Thanks much.

In partial answer to 2), the 1990s-era Porsche Carrera 2 (and C4) had a speed-activated spoiler that deployed at about 50 mph. I think that we can trust Porsche when they say that is the speed at which a spoiler begins to benefit that particular car. It weighed just about 3000 lb.

Depending on how you look at it, the negative lift that the spoiler generates is to provide better handling at high speeds by pressing the car against the road even if it’s not ready to fly away. (of course, that simply means you can drive it faster around turns before you do start to fly away.)
Although it seems that your question (1) means ‘necessary’ in the sense of ‘when is the car in serious danger of flying on an average turn on the highway’? The profile is important, so maybe it should be phrased as a car the exact same size as (an average car/a sports car like a Porsche) made of magically ultralight material of any weight desired.
Not that I know the answer.

And I think for the speed at which aerodynamic effects begin to be as important as gravity is around 50-60 mph for most small-average passenger cars. I’m not saying that’s when lift overcomes the weight, just that it becomes noticeable.
panama jack

17 reasons why

Why are cars sort of shaped like airfoils pointing the wrong way? Wouldn’t having the shape of an airfoil with the larger part at the front be more aerodynamic?

You are confusing a ‘spoiler’ with a ‘wing’.

There are essentially three ways to help keep the wheels on the ground aerodynamically - a spoiler, a wing, and ground effects.

A spoiler ‘spoils’ the airflow over the car. Why is this important? Because the car body acts like a wing cross-section. It’s curved on the top, and flat on the bottom. At high speeds, the car itself will generate lift, reducing traction on the tires. A spoiler trips up the airflow and keeps the back end of the car from lifting.

A wing, on the other hand, is an airfoil that puts a downwards force on the vehicle itself. Sprint cars and Indy cars use wings.

How effective a spoiler is on your vehicle depends on its aerodynamics. Some vehicles will benefit greatly from them, and on others a spoiler will have little or no effect at all.

Lift (positive or negative) is not a significant factor in automobile aerodynamics. Indy/Formula One cars travel very fast and are very light, requiring negative lift to maintain traction. However, most private passenger cars are relatively heavy and travel at relatively low speeds.

A spoiler exists to reduce drag. Laminar (smooth) flow is desireable at the front of the car; however, at the rear, it takes some room for the top and bottom streams of air to join up, leaving a low-pressure area behind the car. The spoiler induces turbulence near the rear of the car, increasing the pressure behind the car, reducing the pressure differential between the front and back of the car, and thus reducing aerodynamic drag. This phenomenon underlies the dimples on golf balls as well.

I read an interesting article a few years ago written by someone who had tried to break a speed record at the Bonneville salt flats. This wasn’t for an all-out, unlimited record, but rather a particular class of modified production vehicle. The old record was somewhere in the neighborhood of 200 mph.

The guy decided that a modified Camaro should be able to break the record. The article described two aerodynamic effects that are common to all production cars (Don’t take this as gospel, it’s been a couple years since I read the article and even then it seemed like something that could use some checking). It said that all cars generate lift, which increases as speed increases. And it said all cars are aerodynamically unstable, which I believe means that the center of pressure is ahead of the center of gravity. Think of it like a dart travelling with the fins in front. The car doesn’t spin around because the tires have enough traction to overcome the aerodynamic tendency to swap ends.

Put those two facts together, though, and bad things can happen. In his case they happened at about 190 mph. At that speed the car got so light on its wheels that he couldn’t keep it pointed forwards. That’s why they do those things at the salt flats, there’s nothing to run into, and he recovered to write the article.

I wish to add that at reasonable operating speeds, lift is not an important factor.

Also an automobile is aerodynamically unstable it has no control surfaces to adjust its attitude. Simply put, it is not designed to fly. The relative position of the centers of gravity and lift is just one of the many factors affecting aerodynamic stability in aircraft.

Thanks to Sam for the clarification on the difference between spoilers and wings; sounds like most people have the terminology incorrect, as I’ve never heard Joe Average refer to a “wing” before.


Thanks, but I thought this was tacitly assumed in the OP, that spoilers/wings are basically unnecessary on a road-standard passenger vehicle.

My question is more about idle speculation rather than application to any real-world problem. If spoilers/wings aren’t really necessary for regular cars because they’re too heavy and too slow, then how light and how fast would you need to be to turn the spoiler/wing into more than a fashion accessory? To my mind, it’s less an opportunity to talk about real-world issues and more a chance for someone who knows more physics than I do to flip over an unused envelope and entertain themselves with a little fantasy modeling. Sorry if I wasn’t clear.

Aerodynamically speaking, I’m assuming a standard sedan, like a Civic or Camry (yes, I’ve seen spoilers/wings added to these), as opposed to, say, a PT Cruiser or a Toyota Tacoma.

Sam’s right. I was gonna say that, but he got here first.

This is a wing. Here’s another view of the same car. This one only produced downforce at speeds over 90 miles per hour. Top speed was well over 200 mph (first NASCAR to officially break 200, and unoficially timed at around 240 mph :eek: ), so it was pretty useful. The car weighs about 3500 pounds, but has something on the order of 600-700 horsepower (factory rating for the Street 426 Hemi was an underestimated 425 bhp; this is a full-boogie NASCAR version).

According to the manufacturer, correct? Which means IRL it really had about 250-300 hp measured by today’s standards.

I haven’t wanted to start a thread on this, but you do realize that nearly all hp and torque numbers for American cars prior to 1976 or so were entirely made up, right?

Note that I am not saying the cars were not very fast - there is no doubt of that. Just that IRL 300 hp is a lot.

I’ve read many Hot Rod and Car Craft articles (and an interesting “Mustang World” lately) where they test factory-condition classic cars versus modern cars on an actual drive-up dyno. The Mustang World test was very interesting - most scored more than 100-125 hp less than their “manufacturer rating”, even on recently re-built engines (one scored nearly 200 hp less than it’s manufacturer rating). The winner for most actual road hp of all cubic inch Mustangs tested? The 1999 GT, with 280 cubic inches (beating out cars with 351’s, 429’s, and even 454’s).

Just an observation. BTW - according to Corral.net, the spoiler on my 1998 GT actually reduces the top speed by 5 mph, and gives no measureable benefit in handlability. I would take it off, but it serves it’s purpose - it looks cool! :slight_smile:

Air dams can reduce lift and are a lot more inconspicuous than wings/spoilers. At least to the casual observers. High curbs can find them easily. Smashy, smashy. I found that an air dam with a rubber skirt that was only about 4" off the pavement on my old VW Rabbit dramatically decreased lift and helped stability but it wasn’t apparent at much less than 100mph. Without the skirt the car would get a bit sqirrely at high speeds but with it it would actually become more stable as speed increased. I didn’t do precise measurements to know if there was in fact a net down force but it sure felt like it.

An interesting, and even related, piece of information.

When you set the wings to “full downforce”, a modern Formula One car (which weighs about 600 kilos, empty - so 750 with driver and a bit of fuel) is able to travel upside down in a complete tubular tunnel from speeds 190 kilometers per hour and upwards. Theoretically.

I’m still waiting for an F1 team to sacrifise on of their $1 million cars to test the hypothesis. And film it!

I’ve owned big-horsepower muscle cars from the 60’s, and I’ve driven high-HP modern cars, and I’m here to tell you that the fastest of the 60’s era muscle cars made a phenomenal amount of horsepower and torque. They just weren’t very fast, because of limitations of tires/suspension.

I had a 1967 Camaro that made 425 HP. I also had modern V-rated tires on it, and quite a bit of suspension work. If I floored that car at anything under 40 MPH, the tires would break loose and scream all over the place, taking the vehicle pretty much out of control.

Some of the best big-block engines from the 60’s were actually under-rated, for insurance purposes. The big-block Chevy L-88 and LS-7 engines were probably under-rated. As I recall, Road and Track dyno tested one of them at almost 600 HP, when it was rated at 450.

The big change in horsepower ratings, as I recall, was to go from shaft horsepower measured right at the output to the engine (without exhaust hooked up, etc) to brake horsepower at the rear wheels of the car, which would include all the friction losses from the transmission and the rest of the drivetrain. I think nowadays the BHP is also measured in full stock condition (i.e. no uncorked exhausts, etc.).

That means that 300HP today is quite a bit more than 300 HP from 1967, but not THAT much more. Add 30 HP for exhaust, and maybe another 20HP or so for driveline losses, as a wild-assed guess.

And then there’s torque at low RPM, which is what makes a muscle car FEEL powerful. And for that, you just aren’t going to beat a big-block 60’s era monster engine.

Anth? 460 ci, you mean? Ford didn’t make a 454 that I know of. And a Chevy>Ford swap in a Mustang is relatively uncommon. F1 or F100, yeah, but not a Mustang… Also, IIRC, the numbers weren’t so much made up as they were gained from using incorrect testing and measuring techniques.

Gunslinger: Ahh, the Superbird! Dad had one. Had a Road Runner, too, the death of that car is a nice thread story.

As for personal experience, I’ve had a 1988 Escort Pony, and it would get flighty as heck around 90. I’ve also had a 1987 Escort GT with a largish wing (for a car of that size) on the rear, it didn’t get floaty until about 120. IIRC, they had the same IFS and IRS suspension setups, and body style (save decklid).

300 HP USED to be alot, but now when you can get just about any car with a 200-220 HP V6 in it, or most stock V8s putting out 250-300, perhaps we should up it to 400 being defined as a LOT. Heck, I’ve seen an Escort GT just like the one I had hopped up to 350 hp for less than $1500 worth of work. You can buy a Talon TSI AWD or Eclipse GSX that’ll make 300 with a MBC, 320 with a EBC, plus a new downpipe and wastegate. 400 is easy to hit if you upgrade to a 16G.

It’s getting easier and easier to make more and more HP and torque, down lower, with lesser emissions, using today’s CAD and engine design software. Heck, Ford doesn’t use domed pistons anymore because their CAD program showed them that the domes disrupt the burn dynamics of the chamber, increasing emissions.

My 90 Z24 has a luggage rack/spoiler on the back, but I’m looking for a 92-94 totaled Z24 with a good decklid so I can have the ‘real’ spoiler. My 89 Z24 doesn’t even have a luggage rack spoiler, but it’s not up and running yet, so I can’t take it out and test to see if there’s a difference. They both come with a front chin spoiler, and the 89 has a front air splitter too, I’m going to have to put it on the 90 and see if it makes a difference.

A good rule of thumb is that a factory spoiler will provide limited downforce at speed, on a sports car or sports coupe, and they also are somewhat effective for minivans and the like (the lips at the top of the rear hatch). Don’t expect much from those surfboard shaped spoilers they put on the egg Tauruses, though. An aftermarket spoiler is going to provide negative downforce if it provides anything, since it’s not built or designed for the application, so if you see some gaudy three tiered thing on a Civic, he’s hurting himself. And making it easier to laugh at him. Also, even if it is designed right and provides positive downforce, if it’s a FWD car, how helpful is it? FWD cars would ideally have a spoiler on the hood.

Correct me if I’m wrong, please.


Just two things:

Yes, you are correct. I mis-typed.

Well, the answer is both really. The engineers and marketers knew damn well what they were doing by saying their 300 hp “Super-Ultra-Cobra-Turbo-Jet-Bojangles” versions of their engines were making more than 400 hp.

I have yet to see with my own eyes a stock muscle car of the 1960-1975 era generate more than 350 hp on a drive-up dyno, with pump gasoline, meeting the emissions regs of it’s year of manufacture, and in stock “tune” (no crazy valve or ignition timing, etc).

Like I said, this is always a contentious issue, because people continue to quote these absurdly high numbers, and feel like you are calling them a “wuss” or something if they don’t in fact have 500 hp, “just” 300. I’ve never said those 60’s muscle cars weren’t very fast, and that they wouldn’t smoke me any day of the week without even trying. But 300 honest-to-God-rear-wheel hp is a lot of horsepower.**

This is car talk’s answer to the question.
Spoilers are the manufacturer’s way of saying “Our market research showed that guys think our car is wimpy. … We hope this piece of plastic helps.”

Sam, I’m curious how you know the amount - did you in fact, meeting the conditions I posted above have it tested on a drive-up dyno?

Well, so will my 1998 Mustang, depending on what gear I’m in. Part of the problem is the limited slip, where both wheels break free instead of just one. And I have new 245/45 17’s on it.

I think you are off a bit here, especially with a woefully parasitic Automatic transmission. I’ve read in my Graduate Automobile Engineering book that the Turbo Hydra-matic 400 sapped (including torque converter) nearly 55 hp off of the “flywheel hp”. The differential can take a surprising bit more too, on my Mustang it’s supposedly about 5-10 hp. And, IIRC from my Auto Eng. Prof, who used to test engines at Chevrolet, they also did the following things when testing the engines:

  1. No transmission, no torque converter.
  2. No alternator
  3. No water pump - they had an external, electric pump attached that went to a HUGE reservoir.
  4. The exhaust was attached in a header form, but exhausted to negative pressure, that is, there was a vacuum pump pulling the exhaust.
  5. They ran them at crazy valve and ignition timings, using something he always called “Purple Aviation Gas” (I have no idea what that is, BTW).
  6. The air intake came from an air-conditioned space, and they used chilled gycol coils to cool the ductwork as the air was drawn in. Because the gycol coils created an air friction resistance, they added a small electric fan to “boost” the air pressure at the engine air intake.

If I can believe my Prof (which I really didn’t have a reason not to) all these conditions hardly compare to what my 1998 Mustang was tested under.

True, and true. You are correct, sir.

Err…in the last post, I meant “glycol”, not “gycol”.

Emissions regs? Ha! And crazy valve and ignition timiming was stock on the Mopar Hemi…plus 100+ octane leaded gas, allowing a compression ratio of over 12:1…plus the dual 800 cfm Holley quads, and 425 hp at the flywheel starts to sound at least possible…BTW, the Mopar TorqueFlite 727 is a bit more efficient than the TH400, especially with a lockup converter. All Hemis were available with manual trannies, as well.

Yeah, but I still want a Ford “Cammer” 427…SOHC Hemi, dual 4v carbs, comfortably revs past 7000…banned from NASCAR before it got a chance because nothing Mopar or GM made could even come close. And there’s at least one of them with a Shelby Cobra wrapped around it…700 (ok, that is flywheel :rolleyes: ) horsepower in a 2300-pound open car… :eek: (probably hard to get started moving with that much power…you’d just sit there in a cloud of Polyglas radial vapor :D)