fLat bottomed (girls) make the race car go round!!!
 

Its begun. Time for a flat bottomed car and a rear diffusor.

So far the car has a front splitter that goes back about 4 feet covering where the old shield used to be. Now ive got another 2 feet about to go on with this template i made (just need matterial i wanna use). The rest of the way back should be pretty simple cutting out spots like where the exhuast flanges hang a little low and going out far past the side skirts for some side splitter action.
http://i39.photobucket.com/albums/e1...snzy0vbjj.jpeg

http://i39.photobucket.com/albums/e1...s9fcad9nl.jpeg


Now onto the rear diffusor. Im still not sure what to do but this is what i have so far. The diff cooler bracket i made to catch air under the car and force it up into the cooler has what seems to be a 15.2* up angle. A 458 i tested had a 17* up angle but ive also seen flat diffusors. Guess this is as good as its gonna get without a wind tunnel. Now on a rainy day on the highway a huge rooster tail can tell a lot about a diffusor. Im also not sure how many fins to use or if they should be straight or taper out like ive seen after the center 2 like this / | | \.

http://i39.photobucket.com/albums/e1...pscvn0cdqe.jpg

What i did since the car was up in the air, i ran a string from the front to the back touching points of the car. This insured the flat bottom will be flat and that the fins are parallel to the ground. So far ive made 2 fins that are 10" apart. I dont know if ill have 4, 6, or 8. These 2 are straight as a arrow.

Any thoughts from here????

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Awesome!!

God damn, what angle is the diffuser?

15.2 like I said. Also the 458 is 17* so...

Yea but the 458 doesn't have the downforce your spoiler has. Your car has come along way in a few yrs. For some weird reason the pics and some of the words didn't show up on my work computer.

So less angle with a spoiler? I found a few posts from some forums but I can't pretend to understand it all. Here's one...

Rear Diffuser angles - TenTenths Motorsport Forum

This subject is really hard to solve without a full scale wind tunnel. Too much angle the airflow going out of the diffuser it disrupts the spoiler but you can get away with the angle by extending the mouth out away from the rear of the bumper. The greater the angle increases the total volume of diffuser. You might not need that much because the spoiler works with the diffuser. Too much of one makes the other less efficient, balance is the key. I would make the diffuser mounts adjustable so you can tune it rather than making it with one specific angle.

Here is a good place to read about different designs and the physics behind it all.
Diffusers | Engineering basics | Aerodynamics - Racecar Engineering

I ran across this video when watching one of their other shows awhile back. But it explains some more variables and it might help out.

Glad to see you have another how-far-can-I-go project. Sub'd to see how things progress.

If I read your description right, you plan on making the underside of the car as flat as possible. Will that provide any down force?

That's correct and I have no idea. I think it just ensures proper air flow under the car and around the tires and channels it to the diffusor so down force can happen. Besides that I'm waiting for style smart people to chime in.

Yeah I saw that. Only need $2000-$4000 to test everything. I also need to rake the car I think. I won't do that though till the flat bottoms done and I measure everything. If the flat bottom tapers up and back the whole way and I have this AOA on the diffusor I don't think I'll need rake because the under bellys doing it for me. Since this car had no diffusor though or flat bottom I may be at 100% rear DF.

Keep the strakes parallel all the way. If you were doing full cf mold you might benefit from molding them together to a point in the center, but unlikely. The point is to have the flow as smooth and linear as possible while accelerating the airflow under and out of the car.

Unless you have one crazy massive wing mount hanging it back several feet from the rear of the car, you're not getting any activation of the wing from the diffuser, so it's all about drag and air speed underneath the car. I'd put some small strakes at the end of the blades or just to the inside of the tires as well. Keep the air underneath from exiting along the doors and wheels.

As I said on Facebook. In all my models I lost attachment after 12 degrees or so, but your mileage may vary with real world testing. You can always use the string and GoPro test method :)


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And like that video, I'd be surprised if you're downforce wasn't very rear biased.

Fwiw, I could tell a significant difference at high speed just by having the airflow under the car smoothed and sealing off the flow into the rear bumper cavity. That was effectively a massive air brake at speed.


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You mean like this..... | | | | and not like this / | | \??

With string and a go pro what I am I looking for? What's it look like if you have lost attachment?

The plan right now is a full flat bottom and the flat bottom will extend past the side skirts maybe 4-5 inches. The diffusor will have 8 fins and so far all are parallel to each other and parallel to the ground. I'll finish today and post a pic.

What do you small strakes at the end on the blades?

In a way I actually don't mind it! My weight ratio is like 58/42. The rears to light weight from all the weight removal. In turns it sucks but I understand at high speed in a straight it can be and feel bad.

cool project. I always like to follow these custom parts threads! Keep posting up pics!

350/370's react quite well from an aero perspective to lots of rake....

Well heres 15.2* and 8 fins then! Centers 10", next 2 are like 5.375", and outer 4 a like 7 something. Just something i picked. All are 100% parallel with each other so we'll go full scale with alumilite and test with string i guess.

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You should just leave it all cardboard.

I like the Carbotech stickers on the bumper too. Just in case I didn't see one, there's another one to remind me.

When you taking this thing to the track? You've been building it for 2yrs now, and haven't tested it on the track. I would have done one thing at a time to see if it made any improvements. ;)

Because its not done.

And im building this to be safe like all my past cars ive had on the local tracks.

And these projects cost nothing vs having to buy seats, belts, almost 2k wing, cage etc. rome wasnt built in a day and ive got expensive other hobbies.

Rusty ill make it to Pitt this summer. That 1:30's i saw must be killing you. You can tag along down into the teens with my draft. Ill slow it down some.

Just wait until you get of of those spec Miata's running you down. :icon17: You couldn't run with Bruce Sheldon and his Ford Pinto @ 01:10:584. :icon17:

I can't wait to run the new section of track they built last year.

Same here. Last time i was there was on my bike at a 1:03's. Im assuming unlike mid ohio where im faster in a car than on my bike, Pitts gonna be the opposite. Of course mid ohio does change the key ohio.

Diffuser needs a curve, not flat-plane. You are trying to replicate the top surface of an aircraft wing. You "need" outside fences aligned fore/aft with the longitudinal centreline of the vehicle - just inboard of the tyres, so that "wash" around the inside of the tyre does not create turbulence in the diffuser

Would probably help to install maybe 3 or 4 more strakes to channel air as well.

I'll post a photo of mine to make this verbiage clearer.

Yes please do

I think he means something like these. These designs isolate turbulent air from the rear wheels, keeping the air under the car moving as fast as possible.

http://speedhunters-wp-production.s3...57-800x533.jpg

http://speedhunters-wp-production.s3...6-1200x800.jpg

http://speedhunters-wp-production.s3...09-800x533.jpg

http://speedhunters-wp-production.s3...96-800x533.jpg

https://youtu.be/pdAEHB5-CPU

You could make your own wind tunnel

Yeah if you want a good car to study, under suzuki's s15 is one of the best.


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Flat diffusers work fine because we are interacting with the ground plane. Airfoil shapes are required when setting up relative pressure differences, but here we can work on a local pressure.

Now for angles:

http://www.the370z.com/attachment.ph...1&d=1453492708

From https://www.linkedin.com/pulse/how-d...eNoSplash=true

Enjoy

Little update...

Next 2 feet are done bringing me back passed the front tires. Also finished up one side blade that runs all the way back to the rear tire. Now just gotta copy this piece to the passenger side and install a center section.

Sadly the W brace hangs lower than anything. If I installed the panels to the brace the flat bottom wouldn't be flat. Instead of tapering up going back it'd actually taper down which would make the bottom 100% want to take off like a airplane and that wouldn't be good. So everything will be flat down there but the W brace. It'll just have to stick out some.



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Well the outside fins do run up to the tires, does that count?

No idea what I'm seeing but I'll get a ground height measurement once the flat bottoms done and I go back onto the ground.

Looking good .............

BTW, I have some photos of the diffuser I will be using for my Z34 here as well ......

I don't have images of the front splitter yet but will post them in my engine thread in due course

Nice!

So as long as the fins closest to the wheels run close to the wheels thats good?

Ill also make this diffuser piviot where its attached to the rear spash guards if you will and make it height adjustable with aluminium plate screwed to the rear bumper beam. Ill use some 1-2" x 1/8" plate with holes drilled it in like every 0.5" so if 15* dont work ill just move the plate down 0.5" at a time.

Takjak, that article was really good, but went over my head a few times. The key points I took from it (correct me if I'm wrong) were: volume of air entering the diffuser dictates optimal rake; to reduce delaminating and/or increase efficiency, create vortices; and the suspension setup can create the same effect as a diffuser - without using a diffuser.

As for the turbulent air from the tires, as I understand, ideally you want to isolate it so it doesn't slow down the air from the diffuser.

Did it say how to do that?

where's the pics of flat bottomed girls?

Don't see any. Reported for lying.

:icon23:

A couple things that come to mind are:

1) You probably want to attach the rear diffuser REALLY well or it will just get pulled right off. Overbuild the attachment points like you wouldn't believe. A lot of rear diffusers are only there for cosmetics, or as a cover to smooth the underside of the rear of the car. But with your design, you're hanging it as a wing, so really make sure it's securely mounted.

2) If you clam-shell the whole bottom of the car, you'll have to make sure you have enough room for hot air to leave the engine compartment. A vented hood might do it, depending on the vents, otherwise you might want to look at venting the front fenders. I suspect the factory aero blows the hot air under the car at the firewall, and you'll be blocking all that off. So with less throughput through all your coolers, they will be less efficient.

3) Don't forget you need to be able to get air to the brakes. There isn't much from the factory, but when you really streamilne the evacuation of the air under the car, ducts are a necessity. Make sure you have room for them under the clam-shell.

These are pretty rudimentary points that you probably already considered, but that's what's popping up in my mind when I see this. But great work so far! Can't wait to see how it turns out!