Rear spoiler/roof extension design

[JDear]

I am a drag racer with a Mitsubishi Mirage hatchback that is a sh!t box in the wind and I have been on several other forums and been asking for advice and have not gotten any helpful answers. So hopefully someone on here will be willing to help me out

How much does a rear roof spoiler/extension effect rear down force and aerodynamics? I am preparing to instal a parachute later this season and I won't run a launcher so I figured I should take that into account and make sure the spoiler design with aid in the chute deploying, and I also have increased the front aero with blocking off all holes and directing air into ic and radiator, and lip on the bottom of the bumper 2.5" off the ground, and finishing the full belly pan this week. Also replacing the rear 3 windows with lexan so it will be even lighter in the rear and it already gets light at the top off the track during deceleration. So I want a little more downforce in the rear but I also want to decrease drag at the same time.







and this is kind of the design I am going for, depending...

[manta22]

JDear;

Make SURE your belly pan is well attached-- it can generate enough force to rip itself loose and scoop up the pavement. For the rear spoiler, a single sheet of aluminum won't be stiff enough unless you make it out of something like 1/4" plate. You'll need some adjustable braces on each side to vary the angle of incidence. If you think everything on your spiler is strong enough, go ahead and sit on it-- the aerodynamic loads will be of that order if it is working properly. Experiment with the size & angle until you are satisfied with the results.

Regards, Neil  Tucson, AZ

[JDear]

very cool. it will for sure be attached well for the same thinking about the force it will see! It will be bolted on and easily removable for quick work at the track.

Most of the hot rods with spoilers have them made out of pretty thin aluminum with gussets/diffusers welded or riveted down it. Would that be good enough? I will def have to brace the end of it to the body, prob 4 braces, 2 on the ends and 2 near the middle. Should I just rivet it to the body so it can flex up and down slightly or how should I do that?

[hotrod]

That style of spoiler is more to reduce drag than it is to produce down force.
It provides an extension of the roof line that allows the airflow to remain attached to a surface then reach a sharp edge were it is easy for the airflow to separate cleanly.

On the natural roof the airflow will try to follow the rear curve of the roof onto the rear glass but it will not be able to turn that sharply. It will then separate from the surface and create a large wake behind the car. This large wake means high drag.

By having a slight downward taper to the spoiler the air is able to manage the small change in direction and begin to pull down behind the car before it cleanly separates at the rear edge of the spoiler. This results in a smaller wake, and less drag.

Do some study/reading of Kamm rear end, he is the engineer that came up with the concept of a slightly tapered rear surface that cuts off cleanly.

Look closely at the rear treatment of the Prius and Honda Insite on this web page, that is what you want to match as best you can.
What you are doing is tricking the air flow into behaving like the rear of the car has a very long taper to it by getting the airflow moving along that path then cutting it free cleanly at a sharp surface that does not upset the smooth flow.
What happens is you form a stable bubble of air that follows the car and creates a shape that acts like a long tapered tail, allowing the airflow to smoothly fill in behind the car instead of tumbling uncontrolled into the big hole you just punched in the air.

http://en.wikipedia.org/wiki/Kammback

http://www.cyclingnews.com/news/photos/trek-speed-concept-preview/77587

http://www.gerrelt.nl/section-aerodynamics/roofspoiler-explanation.html

If you need some downforce at the rear of the car put a small upturned "gurney lip" at the end of the Kamm tail, that will create positive pressure on the top of that Kamm tail extension and give you down force with relatively little drag.

Larry

[SPARKY]

If you are not going to have a launcher  I would mount the chute over to one side and at an angle that will MAKE the pilot chute get into the slip stream

[JDear]

Wow!! Thanks a lot for your excellent response sir!

Yeah my thought for creating downforce was the little lip at the end of the spoiler, but I also thought that extending the roof line would increase the "torque" on the rear wheels because of how far past the wheels it will make the roof line. True maybe?

Yeah I work for Toyota and that is what started my whole aerodynamic research and thoughts for my car. I finally got my new book, called aerodynamics of a race car, by joseph katz. I want to learn as much as possible!

What about those little "diffusers" on the spoiler? Or is just having the spoiler going to give me the benefit that I am looking for?

I really think there is a lot to gain from these aero mods on my car, because I don't make near the HP that my buddies and competitors do, but I am going as fast as them because of my power to weight ratio. Most of my friends are in the 700-800hp range and I am only on a small displacement 1.6l engine and 550-600hp... So I really appreciate all the help guys!

[JDear]

If you are not going to have a launcher  I would mout the chut over to one side and at an angle that will MAKE the pilot chute get into the slip stream

wouldn't that make the chute pull to one side though....?

[hotrod]

Yeah my thought for creating downforce was the little lip at the end of the spoiler, but I also thought that extending the roof line would increase the "torque" on the rear wheels because of how far past the wheels it will make the roof line. True maybe?

True, since you are front wheel drive you do not want any more down force behind the rear axle than absolutely necessary, as it will "lever up" the front of the body and unload the front wheels.

wouldn't that make the chute pull to one side though....?

The attachment point would still be in the center of the car, he is just talking about kicking the pilot chute and the canopy into the air stream, they will be pulled back behind the car long before they fill with air, but you want to avoid ejecting the chute into that dead air bubble behind the car, you can have a lazy chute opening if that happens as the canopy will just drag along the strip for a while in that dead air before it starts to fill when it gets fully extended behind the car.

Larry

[JDear]

But extending it with the spoiler will both aid in the lack of drag in the rear of the car as well as give a little downforce to the rear tires. So I think it should be win win.

Does anyone have pictures of their spoilers? How do you attach them to the body? What did you make the adjustable braces out of?

For mine I was planning on making the rear spoiler equal length to the rear bumper in hopes that it would create an even flow of air out of the rear of the car and also hopefully help the chute to deploy from the center. Does that make sense? I wish I could draw it out to show you... Kinda like a > coming of the back of the car, top air will be forced down and the bottom air will be sucked up I think and if I put the chute where those two streams meet, it should propel the chute out.

[Blue]

For mine I was planning on making the rear spoiler equal length to the rear bumper in hopes that it would create an even flow of air out of the rear of the car and also hopefully help the chute to deploy from the center. Does that make sense? I wish I could draw it out to show you... Kinda like a > coming of the back of the car, top air will be forced down and the bottom air will be sucked up I think and if I put the chute where those two streams meet, it should propel the chute out.

That's going to be about two to five car lengths behind the chute bag.  The air flow in the first car length behind a blunt-backed body is fully reversed.  Whether this recirculation is stable or unstable depends on more factors than I can list.  Tape some yarn tufts on the back of your car and drive down the highway.  Have a friend video you from about ten car lengths back as he approaches to tail gate distance.  When the character of the tufts changes due to him climbing up your butt, that's where the recirculation stops and "the two streams meet".

Put the pull point on centerline and vertically located to straight through the CG.  Look up the Gallardo on the Texas Mile thread for an example of too low of a pull point.

Although a side mount would work, it's not worth having to explain the difference between deployment and pull point to everyone who questions it.  Put the chute pack up high and angled up so that the drogue goes into the upper clean air flow as soon as it leaves the pack.

[JDear]

oh wow. So I should mount the chute up high, but still mounted to an actual lower point on the car? Like the hole in the hatch in this picture, but have the mount come out of the hole and come up a little higher still?



What about spoiler design? Like does it matter if it is the same length as the rear bumper or how should I judge the correct length?

[Blue]

Spoiler size and shape are often restricted by the ruling body you are racing under.  Check your rule book and make friends with a tech inspector.  They won't let you do anything illegal, and asking them these questions can save a lot of time and misunderstanding if you don't pester them too much.

The pull point on that Honda you have pictured looks pretty good: top of the bumper.  Make it stout.  Put the bag under the spoiler (~18" above where the Honda has it) and angle it at 45 degrees or better to shoot above the spoiler line when released.

There's lots of REALLY good people on this board.  Look up the threads on parachute pull point and pay attention to guys like Hot Rod, JL222, and more than I can list.  There are experienced people and knowledgeable people; look for the ones who are both and take their advice.

[JDear]

OOOO good point! I didn't even think about the regulations for that kind of thing. I will try to find the rules for that.

If I mounted it that high wouldn't that put it like a couple inches below the spoiler? The 45 degree angle on the chute is a good idea, I had been thinking about doing something like that, most guys running them here have them parallel to the ground and they often do drag on the ground even just for a short period of time. I don't want that to happen.

Should the spoiler extend past the chute opening section so that the pilot chute gets caught by the air?

[SPARKY]

Read between the lines on what Blue said mount it where the Pack is just below the spoiler extension mount it some where between vertical and 45 deg so that the pilot shoots well up into the slip stream. Then anyone can pack your chute an it will deploy on time every time.

On my old lakester I had to Cock the Pilot just right to get it to shoot out sideway enough to get in the slip stream.  I learned the hard way---I almost went out the back door at El M draging my pilot before I went to my back up chute.  My new lakester will have on the primary  one of the new pilot only LAUNCHERs for my primary chute.  I can swap chutes in tubes to determine the high speed or the low speed is "PRIMARY"

[JDear]

I was more asking how far out from the spoiler it should be if it really matters... in looking more closely at some of others hatchback chute pics most are parallel to the ground and now I notice that some are angled up like you guys are saying. Now I understand why! Thanks!

I read through some of the rules for NHRA drag racing and they state that any spoiler is allowed. It didn't seem to be any restrictions on size at all.

Do people sell universal chute mount kits that can weld it? In your opinions, what are some of the better brands of chutes?

I will make a new template for the spoiler and post pictures so I can get your advice on it.