Correct Rear Deck Spoiler Angles.

[Austin Vert]

Hi Folks,

Don't know if this one has been discussed before, but i was wanting to clarify the correct rear spoiler angles on our 123 cars. I'm no expert on this subject, but i was always under the belief that the spoiler angle should have a gentle forward downward sloping angle with the back edge raised higher when mounted on verts and sportsroofs.

This downward angle ensures that the air flow coming off the roof hits the spoiler and helps force the rear end down more with downward pressure on the top surface of the spoiler. If the spoiler is higher at the front edge and the back edge is lower,wouldn't that defeat the purpose of having a spoiler on in the first place, because you're just creating drag at that point?

The instructions that came with my rear spoiler told me to tilt the front edge down slightly and have the back edge raised.That mounting angle has been my understanding of the whole thing as well, or have i missed something here along the way? I've seen a lot of Mach's with the rear spoilers higher at the front edge, and thought that they were incorrectly angle adjusted.

This article backs up what i always thought was correct.

http://www.gmecca.com/byorc/dtipsaerodynamics.html

Your thoughts.

Greg.

 

[will e]

Interesting article. It clarifies that our spoiler is really a wing. It also notes that wings are most effective if there is nothing between them and the ground. It specifically mentions how the trunk lid limits their effectiveness.

 

[71mach351]

My theory is that it should be flat. It's a nice level surface to put your beer.

 

[Austin Vert]

Bar's open Rick! What are you having?

Greg.

 

[Kit Sullivan]

They probably work better if tilted forward...if they work at all. They will obviously have some effect, as anything bolted to the outside of a car will have some effect. How much of an effect is open for debate I think.

I remember reading somewhere that Larry Shinoda claimed an extra 100 lbs of rear downforce at 60 MPH with the rear wing installed, but no mention of what angle it was set at. That could be just an legend though...who knows if he really ever claimed that?

For me, since most of my car is about the "look" and how it drives around at normal street speeds, I keep the rear spolier angle horizontal to the angle of the top surface of the trunk lid. It looks best to me that way. I may be cheating myself out of a few pounds of "downforce" on my way to the cruise night, but that's the price I gotta' pay for being stylish, I suppose!

And since the spoiler was only ever available on the sportsroof/ mach 1 bodystyle, if there is an "official" adjustement spec from Ford for the rear spoiler, it certainly would not apply to the convertible or the coupe, since it was never offered on those.

 

[Don C]

The rear spoiler does act as a wing, and can not only add down force, but also be used to change the balance of the car and reduce the pressure at the rear of the car to pull more of the air out from under the car.

The angle of attack of a wing is in relation to the direction of the flow of the air. The turbulence of the air flow also affects the effectiveness of a spoiler. For a fastback car it's fairly easy to visualize the direction of the airflow, it basically follows the line of the roof and rear window. When the front of the spoiler is angled below that air flow line (negative angle of attack) down force begins to be produced. It becomes more difficult with coupes and convertibles, as the air does not flow as smoothly and it is more difficult to determine what the air flow line is, plus the air is more turbulent. The line of the airflow likely will change based on speed. Then, for convertibles you add in the additional factor of whether the top is up or down.

The greater the angle of attack (negative in our case), the greater the drag is that will be produced. This drag can be used to benefit the evacuation of the air from under the car, and used to change the balance of the car, though, in effect adding weight to the rear of the car. If you place a car on 4 scales and then pull directly back on the car, some of the weight will shift to the rear of the car, and be shown on the scales. The rear design of our cars already helps in this regards.

The only way to accurately determine the best angle for the rear spoiler is wind tunnel testing at the speeds that the car will be driven.

 

[jbojo]

Agree that the only way to tell is with wind tunnel testing. My thoughts about the effectiveness is that you will need to be going pretty fast before they start to work. Also they were used to maintain laminar air flow coming off the rear of the car. If the flow is a smooth one coming off the rear drag is reduced. If the flow starts to break up and is no longer laminar drag starts to be created. If you look at wind tunnel tests you can see that it varies with different cars and at the rear or the trunk the air starts to circulate and not continuing the flow smoothly. Its at the point where the air starts to circulate where drag is being created.

An interesting example of this is with trucks. You see a lot of people removing the tailgates and putting in the web to reduce drag. In reality the are decreasing their fuel mileage instead of helping because they are creating so much air turbulence behind them. A tonneau cover works much better here. You see on the newer trucks in that the tailgate is wider at the top and flat because at that point the air coming off the top of the cab is hitting that point and the tailgate is straightening out the air again. The rear spoiler on our cars are probably more suited to reducing drag than creating down force in everyday use.

 

[4Vforever]

Hi Greg,

Just have a look at the XY GT-HO in 71, they used the rear wing as Al Turner was in charge and claimed how great these wings were (probably with Shinoda's figures) about downforce at high speed and so on. They were still used in 71 when Howard Marsden took over from Turner, but he wasn't convinced that they actually done much. So a lot of testing was done after Bathurst and after the 71 season. You will notice in 72 that no GT-HO has a rear wing or front spoiler on it, as the conclusion was that after all the testing, was that it did nothing beside keeping the boot/trunk closed at high speed and the front one use to bend backwards at speeds over 100 m.p.h. This is why the planned XA GT-HO were to built with no front spoiler or rear wing, as the cars performed better and kept oil temps cooler without them. Even when the factory went to hardtops in mid 73, they still had no front spoilers or rear wings. So realistically Greg, how ever you want to set your rear wing up is up to you as there is no real benefit anyway, and if there were, we can't drive on the roads at the speeds which would make a difference anyway.

 

[Paul of MO]

"I remember reading somewhere that Larry Shinoda claimed an extra 100 lbs of rear downforce"

Don't they weigh about 100 lbs? LOL

- Paul

 

[Austin Vert]

Thanks for all your comments and feedback guys.

What i didn't mention before was that my general knowledge of rear spoilers over the years, has told me that they were really not very effective unless you were doing speeds of over 100 miles per hour per say. A bit of a gimmick you might say.

Jason summed it up well when he said, when it's all said and done, it doesn't really matter about angles too much as the spoiler ain't doing much anyway at any angle.:s

I don't regret putting one on my '73 Vert, as i like the look of the thing on there. I know some vert guys don't, and fair enough too. I remember back around in the Seventies here in Australia they ended up being called ironing boards and were made fun of by some. I think that had a lot to do with the fact that they were known to be a white elephant way back then as well.

I guess that nails the whole thing then.Thanks again.

Greg.

 

[Kit Sullivan]

The spoiler is just like a lot of other performance-inspired adornments put on a whole variety of cars: all for looks and style, no real functional value.

The hood pins are another example of something that is more "show" than "go". Totally unnecessary on a steel hood with the standard hood-latch mechanism.

The hockey-stick "racing stripes" are another example: How many pitmen or flagmen need to spot you coming down the freeway in the morning on your way to work?

Our cars were made in an era of much styling and design inspired from other exciting "activities"...lots of styling derived from racing and track cars being the most obvious.

But this lead to the polar-opposite styling "themes" of the 80's: true "form follows function" without anything on a car that doesn't legitimately perform a function of some sort. This sure did result in a lot of plain-jane cars with very little style.

How many Plymouth Lasers, Ford Fairmonts, 80s-era Malibus do you ever see at cruise nights?

If you put a car-cover over one of them, can you even tell what kind of car it is from the shape?

Like most of you here, I prefer the crazy, care-free styling of cars from the 60s-70s to the later utilitarian-only look.

And, the rear spoiler DOES work: it makes the Mustang look that much more bad-ass!

 

[will e]

For the coupe and convertibles it depends on the color combination and other trim. My light blue, no stripe, chrome rocker, chrome wheel guards, chrome wheels, chrome door guards and chrome body protection wouldn't look right with a black deck spoiler. This even though I have a black front spoiler.

Painted blue to match the body would work better.

 

[Austin Vert]

Well said Kit,

I agree.

I see your point there Will and would go along with what you say.

Greg.

 

[69 Rustang]

My thought is that the rear wing induces drag/slows the car down at high speeds--hurts more than helps in other words. But it looks cool, both my Mach 1's have beer holders because I like the look.

 

[midlife]

One needs to ask the question as to what the performance gains or losses are with the spoiler installed backwards...

 

[Jeff73Mach1]

I hooked a strain gauge to the underside of my deck lid and drove 150mph in forward and 55 in reverse. The results were inconclusive.

 

[mrmach1]

I've always wondered about this subject. I put mine at the angle that was even with the slope of the roof line. I asked this same question to the forum at the MACH1 CLUB.

 

[jbojo]

To really determine this you would need a wind tunnel to adjust it for maximum performance as each cars stance/shape would be different. The speed required to realize the benefits is high so it's more for the cool factor than functional. The goal is to keep the air flow as laminar as possible coming off the rear of the car and keeping the point where the air becomes turbulent as far back as possible. The point where the air becomes turbulent is where drag is induced and the further back from the car the better. Also the shape of the airfoil would make a difference. On a plane you want lift and on a car you want the force in the opposite direction to plant the car on the ground so the wing shape should be reversed. You could adjust the angle of the wing to create the downward force but you start to induce drag as the angle increases. I have never looked at our cars wing shape too closely to see how its really designed. Here is a video that explains this on a general level but it explains the concept. I had the opportunity to test wing designs in a wind tunnel long ago and its an interesting science. Maybe someone here works in this area now and can provide their thoughts here too.

 

[mrmach1]

To really determine this you would need a wind tunnel to adjust it for maximum performance as each cars stance/shape would be different. The speed required to realize the benefits is high so it's more for the cool factor than functional. The goal is to keep the air flow as laminar as possible coming off the rear of the car and keeping the point where the air becomes turbulent as far back as possible. The point where the air becomes turbulent is where drag is induced and the further back from the car the better. Also the shape of the airfoil would make a difference. On a plane you want lift and on a car you want the force in the opposite direction to plant the car on the ground so the wing shape should be reversed. You could adjust the angle of the wing to create the downward force but you start to induce drag as the angle increases. I have never looked at our cars wing shape too closely to see how its really designed. Here is a video that explains this on a general level but it explains the concept. I had the opportunity to test wing designs in a wind tunnel long ago and its an interesting science. Maybe someone here works in this area now and can provide their thoughts here too.

Thanks for the video it was very informative.