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It's correct to view it either way. A surface that reduces lift ("upforce") is, in effect, producing "downforce."
Can't agree with you there. First, presumably the spoiler is reducing lift, not cancelling it, so you probably still have less force on the rear tires as speed (and reduced lift) increases.
Second, even if it cancelled all lift, that still isn't the same as adding downforce. Your grip would increase with speed in the case of adding downforce, rather than just staying the same in the case of no lift.
This is from the perspective of physics, not car knowledge (which I mostly lack.) Perhaps the real world performs differently.
Can't agree with you there. First, presumably the spoiler is reducing lift, not cancelling it, so you probably still have less force on the rear tires as speed (and reduced lift) increases.
Second, even if it cancelled all lift, that still isn't the same as adding downforce. Your grip would increase with speed in the case of adding downforce, rather than just staying the same in the case of no lift.
This is from the perspective of physics, not car knowledge (which I mostly lack.) Perhaps the real world performs differently.
In building and driving race cars I have on many occasions tried to defy the laws of nature, but to no avail.
The spoilers on the F-Type coupes are not wings, so they cannot create a downward force. They can only reduce the lift created by the wing-like shaped surface of the roof line. I have not closely inspected the surface of the of the extending vert panel to determine if it can actually generate down force.
Can't agree with you there. First, presumably the spoiler is reducing lift, not cancelling it, so you probably still have less force on the rear tires as speed (and reduced lift) increases.
Second, even if it cancelled all lift, that still isn't the same as adding downforce. Your grip would increase with speed in the case of adding downforce, rather than just staying the same in the case of no lift.
This is from the perspective of physics, not car knowledge (which I mostly lack.) Perhaps the real world performs differently.
I know what you're saying, and as I said last night you are correct, but so am I. You're defining downforce differently, but look at it this way--gravity IS downforce.
If you view it that way, a stationary car has "downforce" applied to it. A speeding car has lifting forces applied, so the effects of gravity ("downforce") are lessened. The purpose of a spoiler is to reduce lift so that gravity can (hopefully) keep it on the ground, and in that sense, it is increasing downforce when deployed.
As an airplane pilot since 1982, I also understand that if I'm flying straight and level at a given nose attitude and power setting just above stall speed, if I reduce my throttle setting and hold the nose attitude constant, I WILL go down. Thus, reduction of lift by any means (speed or spoiler) creates more "downforce."
In building and driving race cars I have on many occasions tried to defy the laws of nature, but to no avail.
The spoilers on the F-Type coupes are not wings, so they cannot create a downward force. They can only reduce the lift created by the wing-like shaped surface of the roof line. I have not closely inspected the surface of the of the extending vert panel to determine if it can actually generate down force.
You keep referring to the lifting body characteristics of the coupe roof line, and it does resemble the top of an airplane wing which is shaped to produce a low pressure zone.
So, why did the JLR engineers decide the convertible spoiler needed to deploy at 60 mph vs. 70 mph. I'd have to have several drinks and squint very hard to see an airplane wing there.
The spoiler is not a wing. It is an obstruction at a 45 degree angle. 50 % of the wind force striking the spoiler will be translated to a downward force which will eventually be felt by the tire and increase traction. As to whether the coupe roof acts as a wing creating lift, only a wind tunnel would show. As to the difference between US and UK, if it is a difference in physics and not government regulation, it can only be due to driving on the left vs right.
I would think that if the spoiler had enough rake so that the incoming wind deflected off it enough to great meaningful downforce, it would create very significant drag too.
But now I'm in the world of speculation, where anything is possible...
Yes. Some drag depending on the angle but that is a trade of vs losing traction on the rear tires.
Ok...let's assume that the airflow against the spoiler is causing the "downforce". According to Jag, 264 lbs of downforce is created at 186 mph. The coupe spoiler is at a 30 degree angle meaning that 1 lb of drag is needed for every half pound of down force.
That means that 310 horses at the crank would be sacrificed to create the specified down-force. I'm not buying.
And I also find it hard to believe that the spoiler could come close to handling a 500 lb rearward load.
But...I could be wrong.
There is certainly some down-force occurring in this manner, but I find it easier to believe that most of that 264 lbs comes from reducing the lift across the top surface of the car.
No, 1 horse= 550 ft-lb/sec. You have to divide that by your speed in ft/sec to determine how many lbs of drag will consume a horse. (186 mph=273 ft/sec)
About 1 horsepower wasted for every pound of down-force generated if one were to assume that the wind force on that spoiler is solely responsible for the down-force.(which is not what's happening)
HP is a work term like Watts. Ft/sec is a velocity term. Apples and oranges. You simply cannot divide to unlike quantities and derive lb/sec. The car would never move according to your calculations
And I also find it hard to believe that the spoiler could come close to handling a 500 lb rearward load.
Harder to believe is it handling 264 lbs. of downward force.
Originally Posted by Unhingd
There is certainly some down-force occurring in this manner, but I find it easier to believe that most of that 264 lbs comes from reducing the lift across the top surface of the car.
I think it's easy to get caught up in either the net effect or the mechanism by which that effect is achieved and end up in a pointless semantic argument; however, the internet turns out to be the ultimate (so far) venue for pointless semantic arguments.
Is "downforce" the same as "not upforce?" If air flows on both sides of the device, can it still be a spoiler? Is a cocktail all that's necessary to turn "breakfast" into "brunch?" These questions and many more when we return ...
Is "downforce" the same as "not upforce?" If air flows on both sides of the device, can it still be a spoiler? Is a cocktail all that's necessary to turn "breakfast" into "brunch?" These questions and many more when we return ...
HP is a work term like Watts. Ft/sec is a velocity term. Apples and oranges. You simply cannot divide to unlike quantities and derive lb/sec. The car would never move according to your calculations
Power is measure of force applied at a given velocity.
P=constant x Vel x Force (or thrust)
1hp =550 ft-lb/sec
"Watt determined that a horse could turn a mill wheel 144 times in an hour (or 2.4 times a minute). The wheel was 12 feet (3.6576 meters) in radius; therefore, the horse travelled 2.4·2π·12 feet in one minute. Watt judged that the horse could pull with a force of 180 pounds. So: Watt defined and calculated the horsepower as 32,572 ft·lbf/min, which was rounded to an even 33,000 ft·lbf/min."
33,000 ft-lb/min=550 ft-lb/sec.
My whole point is that if the deflection of air were solely used to create the down-force, the parasitic effect on power would not allow the car to come close to the claimed top speed. Hence the down-force is instead created by eliminating the lift across the top of the car.
To quote Commander Data, you're making an unwarranted extrapolation. No idea how much of the extra drag comes from the force directly on the spoiler, but I would guess most of it comes from it disturbing the airflow and creating a larger low-pressure area behind the car. That's one of the side effects of reducing lift.
But I'm just guessin'.
Originally Posted by Unhingd
Ok...let's assume that the airflow against the spoiler is causing the "downforce". According to Jag, 264 lbs of downforce is created at 186 mph. The coupe spoiler is at a 30 degree angle meaning that 1 lb of drag is needed for every half pound of down force.
That means that 310 horses at the crank would be sacrificed to create the specified down-force. I'm not buying.
And I also find it hard to believe that the spoiler could come close to handling a 500 lb rearward load.
But...I could be wrong.
There is certainly some down-force occurring in this manner, but I find it easier to believe that most of that 264 lbs comes from reducing the lift across the top surface of the car.
I think it's easy to get caught up in either the net effect or the mechanism by which that effect is achieved and end up in a pointless semantic argument;
That's why I checked neither the math nor the methodology, and opted to add some levity (upforce?). If you want real fun, go to nasioc and jump into the Mobil 1 debate. Better yet, start a brand new thread on it ;^)
I think it's been going for at least a decade. I haven't checked in years, but I'm sure you'll get a whole horde jumping to the bait.
01-28-2016, 11:48 AM
Watt defined and calculated the horsepower as 32,572 ft·lbf/min, which was rounded to an even 33,000 ft·lbf/min."