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DaewooOfDeath
DaewooOfDeath HalfDork
12/4/11 7:55 p.m.

I don't care about the drivetrain, I don't care about the suspension and I don't care about the underbody. Assume a ride height in the 3 inches neighborhood and a flat bottom/ground effects tunnels.

Which car do you think would offer the best combination of low drag and a large, clean vacuum area in the back?

Daewoo LeMans

Festiva/Kia Pride

unevolved
unevolved Dork
12/4/11 7:57 p.m.

I'm going to say LeMans due to the sloping rear hatch.

Tom Suddard
Tom Suddard SonDork
12/4/11 8:30 p.m.

LeMans. I don't remember what that shape is called, but I do remember reading that is the most aerodynamically efficient in a practical body or something like that.

nderwater
nderwater SuperDork
12/4/11 8:36 p.m.

lol @ the Tico. I wanted to take one home with me when I visited Korea a decade and a half ago. Or a Matiz, which were brand new at the time.

DaveEstey
DaveEstey Dork
12/4/11 8:53 p.m.

In reply to Tom Suddard:

Kamm tail I think.

corytate
corytate HalfDork
12/4/11 9:05 p.m.

rounded-ish nose, sloping windscreen, sloped hatch ending in a vertical with really tiny rear bumper, le mans is the winner.
i saw one of those today actually lol
weird.

corytate
corytate HalfDork
12/4/11 9:06 p.m.

and yeah it is kamm tail or kammback

slantvaliant
slantvaliant Dork
12/4/11 11:04 p.m.

I had a Kammback - like this but orange:

Peter Brock spent time studying the Kamm effect before designing this:

DaewooOfDeath
DaewooOfDeath HalfDork
12/4/11 11:29 p.m.

Cool. I thought so. What do you all think of cutting out that entire black panel in the tail gate and making it a ground effects tunnel connected to right behind the front seats?

JohnyHachi6
JohnyHachi6 Reader
12/5/11 12:55 a.m.

The frontal area will have a significant influence on total drag as well - might give an advantage to the festiva, since it's so small. I don't know how the lemans compares in overall size.

SVreX
SVreX SuperDork
12/5/11 6:10 a.m.
DaewooOfDeath wrote: Cool. I thought so. What do you all think of cutting out that entire black panel in the tail gate and making it a ground effects tunnel connected to right behind the front seats?

I've been playing with similar ideas.

Preventing air from going under the car in the first place will always trump directing it through the car. So, skirts and low chin spoilers to direct the air around the car accomplish more.

I have also decided that this is an area where I need to tread lightly. With tunnel ground effects you run a couple big risks. One is creating lift accidentally instead of downforce. This is an easy mistake without good engineering.

A second is not fully understanding what happens when the air coming over the car combines with the air coming through and under the car at the back bumper. It can create a great deal of turbulence and/or negative pressure.

The third is kind of big. If you build a car that is dependent on the ground effects to maintain traction, what happens if you loose the air? For example, when leaning through a hard turn if the car hits a bump, the potential is there to have a temporary loss of downforce. This equates to a loss of traction, which is a quick introduction to the wall. This was one of the problems encountered with some of the testing with air tunnels done around the time the Chaparreal sucker cars, etc.

Are you gonna have enough power in the LeMans to need the air advantage?

Just a few thoughts. I'll watch with interest to see what you come up with.

mad_machine
mad_machine SuperDork
12/5/11 7:42 a.m.

the le Mans is another "woulda, shoulda, coulda" GM car. Look to the Vauxhall Astra GTE if you want to know what I mean.

pinchvalve
pinchvalve SuperDork
12/5/11 7:49 a.m.

Not everyone understands aerodynamics.

tpwalsh
tpwalsh New Reader
12/5/11 8:16 a.m.

Go look at ecomodder.com they've got a fair amount of grassroots aero work. Second, I'd take the lemans in that group, but I doubt it's still very good, other than it's small. A CRX's sloping hatch is actually much better since it can keep the air moving over the rear hatch "attached" to the boundary layer.

<- no expert but picked up a couple things from ecomodder

unevolved
unevolved Dork
12/5/11 9:53 a.m.
DaewooOfDeath wrote: Cool. I thought so. What do you all think of cutting out that entire black panel in the tail gate and making it a ground effects tunnel connected to right behind the front seats?

Sounds like it may be too sharp of an angle. You really don't want more than a 7-9 degree angle from horizontal on the venturi tunnels or you risk flow separation.

Merc
Merc New Reader
12/5/11 10:49 a.m.
unevolved wrote:
DaewooOfDeath wrote: Cool. I thought so. What do you all think of cutting out that entire black panel in the tail gate and making it a ground effects tunnel connected to right behind the front seats?
Sounds like it may be too sharp of an angle. You really don't want more than a 7-9 degree angle from horizontal on the venturi tunnels or you risk flow separation.

That's correct, it would be better to start with a diffuser at the bottom of the car and have some adjustable straps so you can test the angle that best suits the car. This pic shows the ideal shape that you are trying to acheive with a flat under body system. Left being the front and rear being the back.

http://www.stress.com/images/venturi_stream.gif

The top line being the car, and the bottom being the ground.

Btw, what exactly are you trying to accomplish? Aero for performance or fuel economy? The approach is like apples and oranges to a certain degree.

DWNSHFT
DWNSHFT Reader
12/5/11 11:52 a.m.

Thinking back to the ground-effects era of F1 they had some peculiarities. They ran side skirts that basically rubbed on the ground because they wanted to eliminate all air sneaking under the car from the sides. Also, the drivers were severely punished by the ride, as they were using spring rates over 1,000 pounds/inch to basically eliminate body roll. Body roll would "open up" the side, allowing air to leak into the tunnels and ruin the ground effects. I'm guessing this is also true for squat and dive. I think formula cars without downforce run spring rates under 200 pounds/inch, so 1,000# is killer.

Also, remember that aero in general is very susceptible to changes in rake. Remember the Mercedes and then Porsche prototypes (GT1 class, I think) that took off? A little turbulance and a small bump, maybe, got a little air under the cars and they would imitate a plane, for a little while. Not including the landing.

Finally, add a bunch of downforce and you add a ton of stress on the suspension and chassis. Are they strong enough? Do you know? Do you know how to upgrade their strength?

A little aero is cool. A lot of aero is dangerous.

David

DaewooOfDeath
DaewooOfDeath HalfDork
12/6/11 2:06 a.m.
JohnyHachi6 wrote: The frontal area will have a significant influence on total drag as well - might give an advantage to the festiva, since it's so small. I don't know how the lemans compares in overall size.

The LeMans is a little bit narrower and about the same height. The Scoupe is the biggest, but not by much. The Tico is by far the smallest.

DaewooOfDeath
DaewooOfDeath HalfDork
12/6/11 2:58 a.m.
SVreX wrote:
DaewooOfDeath wrote: Cool. I thought so. What do you all think of cutting out that entire black panel in the tail gate and making it a ground effects tunnel connected to right behind the front seats?
I've been playing with similar ideas. Preventing air from going under the car in the first place will always trump directing it through the car. So, skirts and low chin spoilers to direct the air around the car accomplish more. I have also decided that this is an area where I need to tread lightly. With tunnel ground effects you run a couple big risks. One is creating lift accidentally instead of downforce. This is an easy mistake without good engineering. A second is not fully understanding what happens when the air coming over the car combines with the air coming through and under the car at the back bumper. It can create a great deal of turbulence and/or negative pressure. The third is kind of big. If you build a car that is dependent on the ground effects to maintain traction, what happens if you loose the air? For example, when leaning through a hard turn if the car hits a bump, the potential is there to have a temporary loss of downforce. This equates to a loss of traction, which is a quick introduction to the wall. This was one of the problems encountered with some of the testing with air tunnels done around the time the Chaparreal sucker cars, etc. Are you gonna have enough power in the LeMans to need the air advantage? Just a few thoughts. I'll watch with interest to see what you come up with.

This is something I'm planning for the future, after I'm done with the Nubira.

The plan right now is to use either a Yamaha R1 engine or Rotax 1.5L triple in the nose of the car. Then send the power to the rear, either a four link stick axle (for the Yamaha) or an Alfa Romeo transaxle (for the Rotax).

Either engine will be tiny, which allows me to devote a large portion of the front to aero. Sort of like this.

Splitter, large extractors, basically making the front of the car into a pressurized air box that dumps onto the top of the hood.

Past that, I was playing with the idea of cutting out the entire floor behind the driver's seat (and moving the driver forward, if possible), and turning that into an aero tunnel. In order to maximize surface area, I would try to have the entrance to the tunnel go from just inside the doors on either side to just in front of the rear suspension and subframe.

I would run the leading edge of the tunnel upwards at a 7 degree angle. The trailing edge would likewise move upward at a 7 degree angle. Wherever this intersects with the car's tailgate/trunk, I make a big hole that vents to the atmosphere. In order to keep this exit as clean as possible, I would put a spoiler like this over the top.

I want to put this tunnel exit in a large, clean vacuum to create the maximum "suck" on the tunnel entrance. The thinking goes that this is a relatively low drag way to make downforce and, if I put the tunnel entrance at basically the middle of the car, it won't affect balance much as the speeds increase. This is why I can't do a mid engine.

Do you think there will be a problem with sudden loss of grip if I run this at a (barely) streetable ride height?

DaewooOfDeath
DaewooOfDeath HalfDork
12/6/11 3:26 a.m.
Merc wrote: Btw, what exactly are you trying to accomplish? Aero for performance or fuel economy? The approach is like apples and oranges to a certain degree.

59 seconds at Tsukuba for under 5k.

I'm going to be at about 200 hp for iteration number one, probably in the neighborhood of 1500 lbs. I want downforce but Tsukuba has a lot of tight corners and a big straight, so I can't sell out and go crazy on wings that will give the car some Godawful coefficient of drag. That's why I'm thinking ground effects.

DaewooOfDeath
DaewooOfDeath HalfDork
12/6/11 3:37 a.m.
DWNSHFT wrote: Thinking back to the ground-effects era of F1 they had some peculiarities. They ran side skirts that basically rubbed on the ground because they wanted to eliminate all air sneaking under the car from the sides. Also, the drivers were severely punished by the ride, as they were using spring rates over 1,000 pounds/inch to basically eliminate body roll. Body roll would "open up" the side, allowing air to leak into the tunnels and ruing the ground effects. I'm guessing this is also true for squat and dive. I think formula cars without downforce run spring rates under 200 pounds/inch, so 1,000# is killer.

Do you think this is likely on a car with a 3 inch ride height and no external wings?

Also, remember that aero in general is very susceptible to changes in rake. Remember the Mercedes and then Porsche prototypes (GT1 class, I think) that took off? A little turbulance and a small bump, maybe, got a little air under the cars and they would imitate a plane, for a little while. Not including the landing.

I remember. Scary stuff. Isn't that largely the effect of some extremely low cars that are ultra optimized for super stiff springs?

I'm actually aiming for a more extreme version of what Ferrari did with the F430. While I would like to run rubber skirts and make 4,000 lbs of down force, I don't think it wise to try without a wind tunnel.

edit * Also, wouldn't the tunnel make for a pretty efficient pressure release if the car went seriously nose up?

Finally, add a bunch of downforce and you add a ton of stress on the suspension and chassis. Are they strong enough? Do you know? Do you know how to upgrade their strength? A little aero is cool. A lot of aero is dangerous. David

The plan is to use the donor car for a body shell, the front crossmember, the pedal box, steering, maybe the basic front suspension and brakes, the floor pan and frame rails until the rear edge of the driver's seat.

I was planning to use a large roll cage to connect with the rear edge of the now cut frame rails and mount the rear suspension. Then the rest of the cage will go through the firewall and onto the strut towers. Semi-tube frame, in essence.

SVreX
SVreX SuperDork
12/6/11 4:52 a.m.

Those goals will put you in the 130 mph+ R32 Nissan Skyline GT-R territory.

In a LeMans.

You are the one person here who might get there! This should be good!

DaewooOfDeath
DaewooOfDeath HalfDork
12/6/11 9:34 a.m.

In reply to SVreX: Thanks, although I think you're giving me too much credit. There are an awful lot of people on here, including you, who are more experienced and accomplished than I.

59 seconds is within the reach of 230 hp Civics, provided they are on slicks, are aggressively raceweighted and have good aero. I'm definitely going to be matching them for slicks and raceweighting.

I'm trying to get a University position right now. If I succeed, I know for a fact this university builds racecars because I'm friends with the racecar building professor. If this works out and I get access to a real shop and university level resources, I don't think there's going to be a whole lot of LeMans left over when we finish.

GameboyRMH
GameboyRMH SuperDork
12/6/11 9:43 a.m.

+1 for the Daewoo (never thought I'd recommend a Daewoo for anything but destruction)

DaewooOfDeath
DaewooOfDeath HalfDork
12/6/11 9:56 a.m.
GameboyRMH wrote: +1 for the Daewoo (never thought I'd recommend a Daewoo for anything but destruction)

If I get the LeMans, it will be my second Daewoo track car.

What the hell has gone wrong with my life?

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