How Effective Is The Spoiler
 

I know this will stir up quite a debate and let me state from the beginning that I really like the looks of our spoiler and the fact that it deploys at 60 and retracts automatically, I question the functionality of it.

On a fast track such as Indy, Formula 1, and even sprint car tracks, you need to have a spoiler on the rear and the front of the vehicle. Pushing down on the back only lightens up the pressure on the front and leads to steering problems. Also notice that at the comparatively slower speed of the sprint car you have much larger spoilers.

In the case of going for top speed on a relatively straight course wouldn't the spoiler act as a drag to air flow and affect the top speed and make the front end light and affect steering?

Re: How Effective Is The Spoiler
 

Have you ever noticed how much the silhouette of the crossfire looks like an airplane wing?

Basically, (I'll try to keep this simple) there's more than enough downforce on the front of the car at speed. The deployment of the spoiler just equalizes the downforce on the back of the vehicle to counteract the lift generated by a low pressure zone created by airflow over the car.

If you're curious about it's effectiveness........disable the spoiler and go for a drive on the expressway at speed! You can feel the back of the car try to "hunt" around......it's a less than comforting sensation! Don't say I didn't warn you! :D

Re: How Effective Is The Spoiler
 

The Crossfire's spoiler is functional. It's 80 lbs @ 80 mph functional, if Chrysler is to be believed. I'm sure our rear diffusers have something to do with it too. :cool:

I believe Indy and Formula 1 cars use wings, not spoilers. Second, front wings are required on F1 cars because the rules prevent the use of underbody aerodynamics (raised footboxes are required, etc.)

You assume, incorrectly, that pushing down on the back of a car leads to steering problems. Pushing down on the rear of the car with 80 lbs applies very little lifting force to the front of the car because of the lever arm. Imagine the car as a see-saw pivoting around the rear axle. Our rear spoiler is less than a foot from the rear axle yet the front wheels (and about 90% of the car's weight) are a little less than 8 feet away. Basic physics will tell you that the lifting force on the front wheels will be a maximum of 10 lbs, and when you take into account that 10 lbs of lifting force acts on more than 2500 lbs, you hopefully see that the steering is, for all practical purposes, unaffected.

The spoiler acts as a drag to airflow, but does so in exchange for downforce. There is no free lunch. In this specific case, the more downforce you make with the spoiler, the more drag is necessary to create it. The efficiency of our rear spoiler is unknown. This is known as a lift-to-drag ratio.

Yes, our top speed would be reduced due to the increased drag. A small price to pay for that rock solid feeling you get in our cars at unmentionable speeds, in my opinion.

No, it will not affect steering. The DC Engineers did a great job making sure that the existing steering system took the rear downforce into account.

My name is sonoronos and I love engineering.

Re: How Effective Is The Spoiler
 

My guess is that the spoiler doesn't so much "create downforce" as it "reduces lift" at speed. As mentioned above, the profile of the car is somewhat like an airfoil and therefore it creates some lift, especially as velocity increases...that lift relies on a nice smooth, undisturbed (laminar) flow of air over the top of the vehicle...the spoiler...well... "spoils" that flow and lessens the tendency to create lift.

Bryan
(B.S., aerospace engineering)

Re: How Effective Is The Spoiler
 

Great answers and great discussion NoCones apkano & sonoronos. I hope others will reply.

Re: How Effective Is The Spoiler
 

The owners manual states very explicitly that one shouldn't drive the car over 62 mph if the spoiler does not deploy so the car must get "light" at that speed. Therefore I'm sure it has a use. The old Audi TT had that same problem. When they were originally built they didn't have that bar on the trunk but then too many people were losing control and Audi recalled all that were built until that time to retrofit them. The ones that were built after that time had them already on the trunk.

Re: How Effective Is The Spoiler
 

I was under the impression that a wing's function is the creation of downforce while a spoiler's is to disturb the laminar flow over the top of the vehicle. This would reduce lift, as well as reducing the low pressure pocket behind the vehicle (therefore reducing drag). I imagine that a spoiler may also produce a small amount of downforce, but it is a secondary effect and not the primary purpose of the device. While reducing lift and producing downforce may have the same effect on steering, to me they seem to be different approaches with different engineering solutions (wings vs spoilers).

Re: How Effective Is The Spoiler
 

This is not nessecarily true, I'm at work and dont' have time to go into detail...but via looking around on the net you can find ways to improve downforce without increasing drag. Let me get home and I'll elaborate.

Re: How Effective Is The Spoiler
 

You are correct concerning the operation of both wing and spoiler, but to be clear, both a spoiler and wing can produce downforce. They do so in slightly different ways. A spoiler is used to create a high-pressure area which produces a downforce via pressure differential. This assumes that the underbody is a low-pressure area.

If the area above the car at the spoiler is at 30psi and the area under the car's spoiler is 15psi, then a force of 15psi is exerted on the area of the car covered by the spoiler.

Therefore, I'm not sure it's correct to say that the primary purpose of a spoiler is not to create downforce. The spoiler is an aerodynamic aid, such as a gurney flap or a leading-edge flap. In the end, the literature and car magazines that I have read point to the Crossfire spoiler being designed for production of downforce, so I assume naturally that the primary purpose of the crossfire spoiler is downforce.

Re: How Effective Is The Spoiler
 

I'm looking forward to your reply, but I want to be clear that I never did say that it is impossible to improve downforce without increasing drag. I assume your solution to do so is by improving the lift-to-drag ratio of the car as a whole using flat undertrays, naca ducts, etc.

I was speaking strictly from the standpoint of a fixed lift to drag ratio. Even formula 1 and LMP1 teams know that in the end they hit a limit where ultimately they must trade downforce for drag. This is what creates "high downforce" and "low drag" configurations for different tracks conditions.

Now, the Crossfire's aerodynamics could be dramatically improved upon given enough money and motivation, reducing drag as well as improving downforce. But I interpreted the original poster's question as one regarding the little flip-up spoiler on our Crossfires, which is to downforce what micro-mini bikinis are for beachwear - they do the job they were designed for! :p

Re: How Effective Is The Spoiler
 

Bottom line is "Yes" the spolier is effective for 80 lbs of downforce (or 80 lbs of reduced lifting force) at the rear of the vehicle.

As stated... if you turn off the spoiler and drive on the road (preferrably open, straight and no traffic) you will notice the rear of the vehicle becoming "loose" as you get nearer to 62mph. I would NOT recommend you go faster than this to experience this. If you go faster than 62mph, you do so at your own peril. :o

As others have written very well... the car's rear half is shaped like an upside-down wing and will create lift at speed. The laminar flow being disturbed disrupts this lift enough to keep the rear end very stable at high speeds.

Lastly... look at the amount of design and engineering that went into the mechanism and control system for the pop-up spoiler... you don't really think they spent all the time, money, wind tunnel testing and engineering effort for looks do you? :D

Re: How Effective Is The Spoiler
 

Since I installed an on/off switch to my spoiler motor, I usually drive with the spoiler in the down position most all of the time. I usually cruise on the high way between 70 and 80mph, depending on how the traffic is flowing and I have never experienced any ill handling effects what so ever.
If you do take it above 90mph an audible "alarm" will sound when the spoiler is in its down position.
In the extremely rare occasion when I'm caught in a rain shower, I manually switch the spoiler on so it will go up.
But as MMZ_Timelord pointed out, they didn't go to all that trouble for nothing. I just think the German engineers had "Autobahn" speeds more in mind, and that's why they don't warn you until you reach 90mph.

Re: How Effective Is The Spoiler
 

I didn't have to go into detail, cause you hit the nail on the head. I will now cook dinner instead of re-itterating your already well worded post.

Re: How Effective Is The Spoiler
 

The first Audi TT's (similar aerodynamics as Ours) were extremely dangerous and subsequently decklid spoilers had to be installed.

Crossfire spoiler like Porsche are designed and tested to ensure that maximum safe roadabilty at high speed that we can attain with our cars.

Even SRT6 work like former Porsche Whaletails they used to have on Turbo Carrera's, those German's have aero down pat and why I bought my Crossfire.

Re: How Effective Is The Spoiler
 

You guys are really educating me. I didn't expect so many good responses to my initial inquiry. Thank You all.

Re: How Effective Is The Spoiler
 

This is educating; but, I must ask the question - why does the SRT6 have a fixed spoiler? I can't imagine there is much lift at slow speeds so why not just use the original spoiler?

(By the way, for you young squirts, if you want to see a car with lift, you should try a '59 Chevrolet at 100mph! Now that car was not made for those with faint heart. If you hit a crosswind at that speed, the son-of-a-b***h wanted to crab into the wind! Ah, the good old days! I will take the good new days any time when it comes to cars.)

Re: How Effective Is The Spoiler
 

To look different is my guess.

Re: How Effective Is The Spoiler
 

Are you sure that the SRT6 has a spoiler and not a wing?

My understanding is that the SRT6 has a wing, unlike the spoiler found on the limited and base models. The difference is that the aerodynamic efficiency is much higher on a wing (less drag than a dam or spoiler for a given amount of lift.)

With general approximations of a 1' chord and 5' span, in free air (which the SRT6's wing is definitely not, but I am just assuming this for argument's sake) the downforce at 80 mph is as follows:

0% camber:
-2 deg angle of attack: 18 lbs
-4 deg angle of attack: 36 lbs
-6 deg angle of attack: 54 lbs
-8 deg angle of attack: 72 lbs

-2% camber:
-2 deg angle of attack: 39 lbs
-4 deg angle of attack: 57 lbs
-6 deg angle of attack: 75 lbs
-8 deg angle of attack: 93 lbs

-4% camber:
-2 deg angle of attack: 60 lbs
-4 deg angle of attack: 78 lbs
-6 deg angle of attack: 96 lbs
-8 deg angle of attack: 114 lbs

-6% camber:
-2 deg angle of attack: 81 lbs
-4 deg angle of attack: 99 lbs
-6 deg angle of attack: 117 lbs
-8 deg angle of attack: 135 lbs

I don't have an SRT6, so I can't measure the camber nor the angle of attack. But there are the numbers if anyone is curious and does some measurements.

As for why the SRT6 has more aerodynamic aids than the base/limited models, my guess is this: Acceleration, not Top Speed: The SRT6 has more than 70 ft-lbs torque on the base/limited crossfires. About 25% more torque. And at speed, this additional torque must be applied to the ground without wheel slippage or loss of traction. At a standing start, a lack of traction may at worst causes a low-speed impact. At 80mph, throttle in a low enough gear could be disasterous without sufficient traction.

Re: How Effective Is The Spoiler
 

It looks more like a wing than the regular car's spoiler, but it's just functioning as a spoiler.

Re: How Effective Is The Spoiler
 

Here's an earlier thread with some interesting theories on the subject.