Flatbottom = Big Difference?

DaewooOfDeath wrote: I get what you're saying, but then why do these guys bother building lift into their machines? Isn't it a fluid dynamics law that higher speed is lower pressure and lower speed is higher pressure?

They are not, they have properly designed front splitters to prevent air getting underneath that a) produces down force at the front and b) prevents enough air getting underneath to be able to induce lift. They also will have either a spoiler or wing at the back to increase down force (or at least reduce further lift) It all works as a system, unfortunately you've possibly jumped to the last stage without covering the basics first.

Guys, I just checked. The flat bottom should make downforce all by itself.

http://www.wisegeek.org/what-is-the-bernoulli-effect.htm http://en.wikipedia.org/wiki/Bernoulli's_principle

Flatbottom = faster flow under the car = less pressure underneath = less lift/more downforce.

I have a really hard time understanding how that plate could have added lift in the front. A hood which slopes downwards from the windshield to nose will produce downforce, not lift, letting air get under the hood produces some lift. A car typically produces lift over the rear glass or roof.

There are some factory performance-oriented cars that come with a flat bottom and no significant diffuser, spoiler, splitter or canards...the Elise and Toyobaru come to mind - although they both have a lower-than-average ride height.

DaewooOfDeath wrote: http://www.wisegeek.org/what-is-the-bernoulli-effect.htm http://en.wikipedia.org/wiki/Bernoulli's_principle

This is basic stuff anyone arguing about flat-bottom aero should understand - and the major difference between a flat-bottom car and a wing. A wing isn't run so close to the ground that it accelerates the air on the underside of the wing.

I think the VW Beetle problem happened because the car had many lift-generating surfaces on top and was severely ass-heavy. It didn't take much to get the nose in the air, and just like an airplane can fly upside down, a car with the right angle of attack can fly too, and a flat bottom makes it easier. And the Beetle, having no downforce to speak of, could do this particularly easily, it was like a symmetrical-wing stunt plane when driving level.

I think Gameboy is correct. If you look at the racecars that go airborne, they tend to be normally high downforce designs. However, when you get a bubble of high pressure air underneath them (spin out, hit a big bump funky, have a crash) then they become stalled airplane wings and shoot the car skyward.

The Beetle is a problem child for this because it has a big, natural low pressure above the front of the nose, a big, natural high pressure zone under the pontoon fenders and a slight, natural reverse rake. It's because of this that the flatbottom makes the car act like an already airborn Mercedes endurance racer under normal driving conditions.

I've been designing my own aero for the Civic. Using used splitters from Trans Am cars, a material called "Polymetal". http://escondidosignco.com/materials/

You can get a massive sheet of polymetal for about $150, make a template and cut.

Going to make a flat bottom floor with a rear diffuser similar to GT2 cars...

DaewooOfDeath wrote: I think Gameboy is correct. If you look at the racecars that go airborne, they tend to be normally high downforce designs. However, when you get a bubble of high pressure air underneath them (spin out, hit a big bump funky, have a crash) then they become stalled airplane wings and shoot the car skyward. The Beetle is a problem child for this because it has a big, natural low pressure above the front of the nose, a big, natural high pressure zone under the pontoon fenders and a slight, natural reverse rake. It's because of this that the flatbottom makes the car act like an already airborn Mercedes endurance racer under normal driving conditions.

Yep the beetle is shaped just like and airplane wing, there was a problem like this with the early Audi TTs too. Most cars have a long hood and then the a pillar windshield bulge that gets rid of the wing effect.

For the spats you can use nylon aftermarket ones, they hold up well. Or you can use polycarb like lexan which is also really tough and can fold in half or close to it and then spring back. Just take it easy and get the adjustment length right, too long and you can get some pretty unsettling scraping. If you cut multiple short slots in the plastic with a router you can use a wing nut and slide them up and down as needed. Go with 1/8" or thicker to keep bowing down at higher speeds. You could always back thinner stuff with aluminum angle too.

foxtrapper wrote: It's one of the reason an old beetle was so exciting as you hit 90mph. Heck yes it can make a huge difference. You just made the missing lower half of the airfoil, or wing.

How did you get the old beetle to do 90. Down a long hill ?

You have basically created an air foil, so like others have said you need some flaps.

DukeOfUndersteer wrote: I've been designing my own aero for the Civic. Using used splitters from Trans Am cars, a material called "Polymetal". http://escondidosignco.com/materials/

The sign material supply house around here sells Alumalite.

I used it for the splitter and end plates for the rear wing. Easy to cut and shape. I found it to be surprisingly strong and tough stuff. Probably doesnt like hot exhausts though. I havent tried it under the whole car.

iceracer wrote: You have basically created an air foil, so like others have said you need some flaps.

I already had an airfoil. I just made it an airfoil with less lift and less drag.

All that stuff that used to be dumping out under the car and getting caught by the roughness is analogous to a really huge, really inefficient Gurney flap on the bottom of my car. I've effectively removed the Gurney flap on the front but left it on the rear.

wclark wrote:

DukeOfUndersteer wrote: I've been designing my own aero for the Civic. Using used splitters from Trans Am cars, a material called "Polymetal". http://escondidosignco.com/materials/

The sign material supply house around here sells Alumalite. I used it for the splitter and end plates for the rear wing. Easy to cut and shape. I found it to be surprisingly strong and tough stuff. Probably doesnt like hot exhausts though. I havent tried it under the whole car.

I might be able to get some of this stuff locally - interesting.

Thanks everybody.

So I've done a little more testing on dry roads, fiddled with the front end aero and the car feels great now. A lot more planted and I just destroyed my previous record for fuel efficiency.

I'm stumped, but this stuff sure is cool!

Glad to see you have worked out a few bugs.
PM me if you need some advice from a guy that spends a bit of time doing nothing but aero.

Steve Stafford.

I plan to. Thanks for the email.

wclark wrote:

DukeOfUndersteer wrote: I've been designing my own aero for the Civic. Using used splitters from Trans Am cars, a material called "Polymetal". http://escondidosignco.com/materials/

The sign material supply house around here sells Alumalite. I used it for the splitter and end plates for the rear wing. Easy to cut and shape. I found it to be surprisingly strong and tough stuff. Probably doesnt like hot exhausts though. I havent tried it under the whole car.

I dig this car, wclark. Would like to see a spec page or build thread.

Hasbro wrote:

wclark wrote:

DukeOfUndersteer wrote: I've been designing my own aero for the Civic. Using used splitters from Trans Am cars, a material called "Polymetal". http://escondidosignco.com/materials/

The sign material supply house around here sells Alumalite. I used it for the splitter and end plates for the rear wing. Easy to cut and shape. I found it to be surprisingly strong and tough stuff. Probably doesnt like hot exhausts though. I havent tried it under the whole car.

I dig this car, wclark. Would like to see a spec page or build thread.

+1 -- Im trying to get motivated to start back on my gti.

In reply to mrwillie:

You should hear this car as it passes you going up hill at Philo. Damn nice sounds.

(Check point 6 here!)

Just to add another data point, E46s have pretty nice undertrays that smooth out the front of the car back past the firewall: