When does the spoiler go down?
 

Dumb question but since I can't see the spoiler when driving, I'm wondering when it goes down. I know it generally comes up at speed but if you are city driving and it comes up but you slow down for a while, does it go back down right away?

Just curious what people behind me are seeing... :cool: :rolleyes:

You wont be able to see it when it riases when your driving... I think it goes up at about 73 or 74 mph and back down in the 40ies

Extends 70-75mph. Retracts below 55mph. Yes, it will rise and fall if you pass those thresholds. If it fails to extend, you'll know it. You won't have to see it, you'll feel it, and not in a good way.

I know it comes up pretty quickly as soon as you hit the speed threshold - just wondering if the down response is as quick.

Owner's manual? :)

I always tell people if you want to see your spoiler go up and down sit in the passenger seat with the top down and have someone else drive while you look out the back. ;)

It does make a big difference at high speed. That is why I have been reluctant to change to other aftermarket spoiler. Not sure how others would function. While it doesn't create downforce it does prevent lift.

Lift and downforce are two different terms for the same thing.

You are right downforce is like negative or reverse lift. I guess what I was trying to say was that the amount of downforce is not positive like with an F1 car where at high speeds many pounds of downforce are generated. The aerodynamics of the Boxster is not designed to add great additional positive downforce but to compensate for the amount of lift inherant at speed. I am not an engineer so I risk getting in over my head by saying anymore.

Downforce and Lift are not the same thing at all.

Lift is created when air passing over the car (moving faster than the air underneath) drops in pressure. That means the slower air running under the car has less counteracting force and so the car 'lifts'.

Downforce is air which is redirected by a Wing upward, pushing the Wing (and the car it's attached to) downward. This keeps the force of the air over and under the car in equilibirum - it counteracts the effect of the low pressure created on top of the car (but does not prevent it).

A Spoiler slows down (or 'spoils') the airflow going over the top of the car so that no low pressure (or less of it) is created. It does not counteract lift, it prevents it.

To many, the differences are subtle, but they are distinct.

Race cars use Wings and downforce. They do this because either a Spoiler or a Wing create drag, which slows the car - it's a compromise between maximum speed and control. But, in order for a Spoiler to be effective at race speeds at preventing lift, it would have to be much bigger than a Wing would be to counteract lift. So it would create more drag than a Wing and have a greater impact on speed. Also, a Wing is more adaptable to changes in Temp, Humidity, Track condition because it's angle of attack can be easily altered. A Spoiler could not.

On a Boxster, the stock Spoiler is effective. The aftermarket Wings are no more so, only look prettier to some people. Also, it allows them to personalize their car. But, they are not better or more effective - some may even be less so.

I'm surprised none of you know the exact mph.
Here it is:
It goes up at 75mph, and down at 50mph unless you override it with the fuse panel or a dash switch.
It's in the owner's manual, for future reference! :cheers:

Owners manual --- PFFFFFT! :rolleyes:

I'm not sure where you're getting your definitions, but lift and downforce are both descriptions of the same force. The difference being the direction of the positive reference. Wings, spoilers, venturis, and whatever else are used to influence this force, but their use has nothing to do with the terminology.

Well, I think we're going to have to agree to disagree here.

Think of a car in terms of an airfoil or wing. It splits the air as it passes through it. Some air is forced under the car, while the rest flows over the car. The path the air must take over the car is longer than the air passing beneath and so the air must flow faster over the car than under it. Air flowing faster, drops in pressure.

So, now, you have the air rushing under the car, and because of the lower pressure on top, you actually have a partial vacuum. 'Lift' is actually the difference between these two pressures - the air flowing over and the air flowing under the car. But, more than the car being lifted upwards, it's actually more like it's being sucked upwards - into this partial vacuum.

Downforce is created when the energy of the oncoming air is redirected upward by a Wing. This imparts a force upon the Wing. Newton's 3rd Law (every action has an equal and opposite reaction) forces the Wing (and the car) downward. But, the Wing operates above the laminar flow of air over the body of the car - air passes underneath it, and so the low pressure area is still being created. It is just overcome by the separate and distinct downward energy on the Wing.

A Spoiler does not allow this Laminar Flow to pass underneath it. It is really an Air Dam which allows the onrushing Laminar Flow to pile up in front of it. This slows it down and so it's pressure rises.

Because there is now no difference between the air above and the air below the car, lift has simply been eliminated, not just overcome as in the use of a Wing. More simply, there is no Low pressure, so no 'Lift' and so Downforce is not necessary.

Again, we may have to agree to disagree. And in practical terms, it is just semantics to many. But, there is a distinct difference between the existance of a Low Pressure area above the car, and a counteracting force produced to overcome it.

So again, Downforce, produced by rushing air and a Wing is used to overcome existing Lift. But, Downforce is not necessary when a Spoiler is used, because no lift exists to overcome.

Your description of the airflow and resulting pressure is essentially correct, although your use of the term laminar is a bit inaccurate.

Nevertheless, you're referring to lift as an inherent characteristic of a car, and downforce as a measure to counter it. These terms are not specific to cars, they apply to the forces acting on any body moving through a fluid and therefore need to be understood in a more general sense. If you refer to these terms without forcing them to be applied to a car (which they aren't) you end up with my description.

Well, yes and no. Laminar Flow of air does exist generally on several points on the top of a car. It is disrupted by such things as the windscreen and such, but often re-established on the rear deck.

As you know, laminar flow really gets the air moving and so it is at these points where the greatest low pressure areas exist.

Generally (and I'm being general because not all cars are the same), laminar flow is re-established on the rear panel of the car (trunk area behind the windscreen) and so it is an area of particular low pressure. This is where the spoiler comes into it's own, breaking up this clean laminar flow and restoring high pressure to that area. It also has the by-product of moving the very turbulent, drag producing air which accumulates right at the rear bumper further back, beyond the car's bodywork.

In essence, I really think we're saying the same thing, but from a different perspective.

Laminar is not really the word you're looking for. Laminar flow is a type of airflow where small eddy disturbances are non-existant. The bulk flow over a body can and will be both laminar and turbulent in different areas, but it's actually mostly turbulent in the case of a car. The terms you really want to use are attached flow and separated flow.

If you have a wing moving through air that forces the air to take a longer path above it than beneath, this will will produce lift (negative downforce), if you turn the wing up side down and move through the air at exactly the same speed, it will produce negative lift (downforce) in the same amount but in the opposite direction. The wing didn't change dimensionally, and neither did the force, just it's direction, which is the only difference between downforce and lift.

Now, depending on how fast that wing is moving, the air flowing over it will either be laminar or mostly turbulent.

OK - Fair enough. I'm far from, nor do I claim to be, an expert on the subject. I'll definitely look into the points you've made. :cheers: