Full Under Floor/Diffuser
Joined: Nov 2007
Posts: 1,683
Likes: 25
From: SoFlo
To test the undertray/flat-bottom's affect on temps you need engine, transmission, and diff temp sensors and log before and after installing the flat bottom.
Smoothing out the bottom of very turbulant street cars does improve the air flow and effectiveness of the rear diffuser. You don't need "2,000lb" springs to have the bottom of the car level under cornering. Ferrari Challenge F430s have a very effective rear diffuser and flat-bottom. The handling is greatly changed when the diffuser is torn off, and those cars don't have even over 1000lb springs.
Many Grand Am GT and Daytona Prototypes also don't run that high of spring rates while having flat-bottoms. Yes having the car 2-2.5" off the ground is much more ideal, but cleaning up the air-flow of the bottom of a car dosn't degrade or hurt the performance of the car.
Where's S2KRetard? i'm sure he can comment.
Smoothing out the bottom of very turbulant street cars does improve the air flow and effectiveness of the rear diffuser. You don't need "2,000lb" springs to have the bottom of the car level under cornering. Ferrari Challenge F430s have a very effective rear diffuser and flat-bottom. The handling is greatly changed when the diffuser is torn off, and those cars don't have even over 1000lb springs.
Many Grand Am GT and Daytona Prototypes also don't run that high of spring rates while having flat-bottoms. Yes having the car 2-2.5" off the ground is much more ideal, but cleaning up the air-flow of the bottom of a car dosn't degrade or hurt the performance of the car.
Where's S2KRetard? i'm sure he can comment.
Registered User
Joined: Jul 2004
Posts: 5,645
Likes: 8
From: Los Angeles, CA
JackOlsen - I'm all ears, and am learning a lot reading your posts. Thank you for your responses.
I'm still unsure just how the springs can effect the efficiency of the underbody aero, but I won't question your word since you have experience in it. If anything I'll be reading up on it to learn a bit more.
I'm still unsure just how the springs can effect the efficiency of the underbody aero, but I won't question your word since you have experience in it. If anything I'll be reading up on it to learn a bit more.
Registered User
Joined: Sep 2004
Posts: 531
Likes: 0
1. if you have enough downforce and not enough spring you can potentially bottom out your shocks and have a fully rigid suspension.
2. from my understanding, in order to get the full potential out of an underbody aero system, you need to have sideskirts that are as close to the ground as possible to seal the underside of the car to the ground as best as possible. this keeps air from entering or exiting through the sides and ruining some of the effect. once you enter a turn and weight transfers from side to side, the gap between the sideskirts and the ground changes (gets smaller on one side and larger on the other) allowing air to come in from the sides, killing part of the effect of the system. the best way to keep this from happening is to have ridiculous spring rates to minimize body roll and keep the sideskirts in their optimum position at all times. that's just the side skirts though... i know the distance between the ground and the undertray also can make a huge difference (and thus can vary depending on spring rate) but that's something that you'd need specific numbers to figure out.
everything jack says makes a huge amount of sense to me, but as billj747 said, there are production cars like the ferrari with proven undertrays that DO work without crazy sideskirts and spring rates so there has to be some kind of in between. i think jack is explaining to us how to get the maximum benefits out of underbody aero, something that (as he states) really isn't practical for our cars. even though the ferrari creates downforce from it's underbody, from what i remember it's only a few hundred pounds of downforce. weren't the badass lemans cars back in the 90s making thousands of pounds of downforce with mainly tunnels and underbody stuff?
hopefully jack or mike or krazik will correct any incorrect statements i've made.
2. from my understanding, in order to get the full potential out of an underbody aero system, you need to have sideskirts that are as close to the ground as possible to seal the underside of the car to the ground as best as possible. this keeps air from entering or exiting through the sides and ruining some of the effect. once you enter a turn and weight transfers from side to side, the gap between the sideskirts and the ground changes (gets smaller on one side and larger on the other) allowing air to come in from the sides, killing part of the effect of the system. the best way to keep this from happening is to have ridiculous spring rates to minimize body roll and keep the sideskirts in their optimum position at all times. that's just the side skirts though... i know the distance between the ground and the undertray also can make a huge difference (and thus can vary depending on spring rate) but that's something that you'd need specific numbers to figure out.
everything jack says makes a huge amount of sense to me, but as billj747 said, there are production cars like the ferrari with proven undertrays that DO work without crazy sideskirts and spring rates so there has to be some kind of in between. i think jack is explaining to us how to get the maximum benefits out of underbody aero, something that (as he states) really isn't practical for our cars. even though the ferrari creates downforce from it's underbody, from what i remember it's only a few hundred pounds of downforce. weren't the badass lemans cars back in the 90s making thousands of pounds of downforce with mainly tunnels and underbody stuff?
hopefully jack or mike or krazik will correct any incorrect statements i've made.
Registered User
Joined: Apr 2003
Posts: 22,888
Likes: 3
From: Covington WA, USA
What Jack was referring to is that it's a matter of consistancy. If you are trying to get a lot of downforce from the underbody of the car, if makes a big difference what your ride height is. So one way or another, you want the ride height very low and you want it to remain constant. Most production cars (even those set up as race cars) can't acheive the necessary dynamic ride height setup to accomplish this.
On the other hand, a well-designed flat bottom that is not making a lot of down force is likely to be a drag advantage. And you are right that there is a distinction to be made between something like an Elise that uses a flat tray as part of a plan to be about neutral in life versus one of the famous "tunnel cars" that was set up to get massive amounts of downforce.
What's certainly true is that most aero elements depend very heavily on the overall integration of the car's aero environment. As Jack noted, the popularity of rear wings has a lot to do with the fact that they are one of the few aero elements you can treat as being mostly independent of the rest of the car. They tend to work as bolt-on add-ons in a way that hardly any other aero devices do.
On the other hand, a well-designed flat bottom that is not making a lot of down force is likely to be a drag advantage. And you are right that there is a distinction to be made between something like an Elise that uses a flat tray as part of a plan to be about neutral in life versus one of the famous "tunnel cars" that was set up to get massive amounts of downforce.
What's certainly true is that most aero elements depend very heavily on the overall integration of the car's aero environment. As Jack noted, the popularity of rear wings has a lot to do with the fact that they are one of the few aero elements you can treat as being mostly independent of the rest of the car. They tend to work as bolt-on add-ons in a way that hardly any other aero devices do.
Registered User
Joined: Dec 2005
Posts: 79
Likes: 1
From: Toronto
I saw a video about the NSX-R detailing the aero improvements made vs. the regular NSX. One of the additions was a flat underbody tray that supposedly increased downforce at speed and resulted in a quantifiable improvement in lap times, especially sector times on high speed (4th. gear) corners. I think the NSX-R would have a stiff suspension, but certainly streetable and nowhere approaching that of a full blown race car. Just a thought...
Registered User
Joined: Sep 2004
Posts: 249
Likes: 0
From: Sunny Los Angeles
We're talking about a lot of different things at the same time here.
I guess you can think of a continuum that goes from the kind of wildly-effective 'it could drive upside down' aero applications that you see on F1 cars and some prototypes all the way to the useless carbon-fiber pieces that manufacturers or owners might throw on because they look cool. Somewhere in between those two extremes, you'll find a tipping point between aero that does nothing useful for the car, and a package that actually generates meaningful and useful downforce.
It's important to understand that just about all performance cars generate lift once you get them up to speed. My old-body-style 911 generates about 300 pounds of lift at 100 mph. This is because the shape of the car makes it function like a wing. I don't know much about the S2000 chassis and its aerodynamic characteristics, but I'd bet that it generates lift when speeds go north of 100, increasing dramatically as you approach the car's maximum speed. The online sales literature for the aerodynamic kit on the S2000 CR model boasts about reducing the car's lift by 70-80 percent over the stock model. But I can't find any data on how much lift the stock model actually generates at different speeds.
On my 911, there are some things I can do to reduce the amount of lift the car generates. You can divide these things into two categories. One would be changes that actually reduce the car body's tendency to generate lift -- I have a spoiler in the back (called a ducktail) that interrupts the airflow that adheres to the fastback shape of a 911 coupe. I addressed the turbulence underneath the car caused by air buffeting around the uneven surfaces with a simple sheet of ABS plastic (this was possible because the rear-engine layout of a 911 means no hot exhaust pipe running the length of the car -- I don't know if it would work on a front-engine car).
But even with both of these mods -- and even if you added a hypothetical set of other modifications that would correct every part of the body that's contributing to high speed lift, the best I could possibly do would be to reduce the lift at a given speed to zero. Neither a spoiler or a flat bottom is going to actually generate net downforce (and by 'net downforce' I mean a situation where there's more pushing down on the car than there is pushing up on it) because the spoiler and the flat belly pan aren't actually functional aerodynamic devices (at least as I'm defining them here) -- they're just fixes to the problem of a production car's tendency to generate lift.
Both my spoiler and my ABS underbelly sheet on the bottom of my car fit somewhere on that continuum. So does my wing -- but a wing is different. A wing isn't merely acting to interfere with something I don't want -- lift -- it's actually generating negative lift. A big enough wing at a high enough speed will actually be able to cancel out all of my car's lift and generate actual net downforce. The same can be true for a splitter, especially for a splitter that drops down low enough to create a low pressure area underneath the car.
And of course, this is true of a lot of the aero devices on F1 cars and prototypes -- including the uberbody treatments. Underbody aero can generate tremendous amounts of downforce -- with less of a drag penalty than a wing. Properly done, underbody aero is huge win-win. (Push things too far, though and you can end up flipping up and through the air -- we've all seen those videos.)
The diffuser and underbody channels I threw together also fit on that continuum -- and like I said, in my straight-line testing they worked. With side-skirts coming down to within an inch or so of the pavement, my crude design probably rivaled the more sophisticated kits you see on expensive performance cars.
(Wait a minute. I could set a marketing team loose with the numbers and my lap times and probably sell a few kits.)
But like I said before, the underbody kit wasn't effective in a way that actually made my car corner better. Downforce when you're going in a straight line is not faster. Having that downforce suddenly decrease or disappear when the car leans forward (under braking) or to the side (while cornering) is also not going to make you faster. Underbody aero is hugely sensitive to position and proximity to the pavement. McBeath's book will show you the data on this.
And as a point of reference, I run 600-pound rear springs on a 2400-pound car. My body roll was way too much for the underbody stuff to function in a consistent and predictable way when I tested it.
The channels and diffusers you see on high-dollar production cars probably all generate some amount of straight-line downforce. More importantly though, they're a marketing gimmick that's effective with typical buyers of those cars. I'm not an aerodynamicist (not by a long shot), but I'd be very surprised if any carbon fiber piece 5 inches or so above the ground on a street-sprung car does much effective work at all when the belly of the car is no longer parallel to the pavement. And by meaningful work I mean actually allowing the car to go faster through a corner. I don't mean generating numbers that show there's some effect.
I couldn't list the number of ways a Ferrari 430 GT model differs from the 430 you can buy in a showroom. And I don't know the changes an ALMS team like White Lightning would make to the springs and suspension before they consider a 430 GT ready to race. But take a look at these four pictures and consider the difference between the underbody stuff that makes a difference, and the stuff a Ferrari owner could actually get over his curb and into his driveway. In the last picture, where the chassis is unsetlled (probably a low-speed, very tight corner), look at how the suspension holds the lifted wheel up -- not a whole lot of compliance there.
I guess you can think of a continuum that goes from the kind of wildly-effective 'it could drive upside down' aero applications that you see on F1 cars and some prototypes all the way to the useless carbon-fiber pieces that manufacturers or owners might throw on because they look cool. Somewhere in between those two extremes, you'll find a tipping point between aero that does nothing useful for the car, and a package that actually generates meaningful and useful downforce.
It's important to understand that just about all performance cars generate lift once you get them up to speed. My old-body-style 911 generates about 300 pounds of lift at 100 mph. This is because the shape of the car makes it function like a wing. I don't know much about the S2000 chassis and its aerodynamic characteristics, but I'd bet that it generates lift when speeds go north of 100, increasing dramatically as you approach the car's maximum speed. The online sales literature for the aerodynamic kit on the S2000 CR model boasts about reducing the car's lift by 70-80 percent over the stock model. But I can't find any data on how much lift the stock model actually generates at different speeds.
On my 911, there are some things I can do to reduce the amount of lift the car generates. You can divide these things into two categories. One would be changes that actually reduce the car body's tendency to generate lift -- I have a spoiler in the back (called a ducktail) that interrupts the airflow that adheres to the fastback shape of a 911 coupe. I addressed the turbulence underneath the car caused by air buffeting around the uneven surfaces with a simple sheet of ABS plastic (this was possible because the rear-engine layout of a 911 means no hot exhaust pipe running the length of the car -- I don't know if it would work on a front-engine car).
But even with both of these mods -- and even if you added a hypothetical set of other modifications that would correct every part of the body that's contributing to high speed lift, the best I could possibly do would be to reduce the lift at a given speed to zero. Neither a spoiler or a flat bottom is going to actually generate net downforce (and by 'net downforce' I mean a situation where there's more pushing down on the car than there is pushing up on it) because the spoiler and the flat belly pan aren't actually functional aerodynamic devices (at least as I'm defining them here) -- they're just fixes to the problem of a production car's tendency to generate lift.
Both my spoiler and my ABS underbelly sheet on the bottom of my car fit somewhere on that continuum. So does my wing -- but a wing is different. A wing isn't merely acting to interfere with something I don't want -- lift -- it's actually generating negative lift. A big enough wing at a high enough speed will actually be able to cancel out all of my car's lift and generate actual net downforce. The same can be true for a splitter, especially for a splitter that drops down low enough to create a low pressure area underneath the car.
And of course, this is true of a lot of the aero devices on F1 cars and prototypes -- including the uberbody treatments. Underbody aero can generate tremendous amounts of downforce -- with less of a drag penalty than a wing. Properly done, underbody aero is huge win-win. (Push things too far, though and you can end up flipping up and through the air -- we've all seen those videos.)
The diffuser and underbody channels I threw together also fit on that continuum -- and like I said, in my straight-line testing they worked. With side-skirts coming down to within an inch or so of the pavement, my crude design probably rivaled the more sophisticated kits you see on expensive performance cars.
(Wait a minute. I could set a marketing team loose with the numbers and my lap times and probably sell a few kits.)
But like I said before, the underbody kit wasn't effective in a way that actually made my car corner better. Downforce when you're going in a straight line is not faster. Having that downforce suddenly decrease or disappear when the car leans forward (under braking) or to the side (while cornering) is also not going to make you faster. Underbody aero is hugely sensitive to position and proximity to the pavement. McBeath's book will show you the data on this.
And as a point of reference, I run 600-pound rear springs on a 2400-pound car. My body roll was way too much for the underbody stuff to function in a consistent and predictable way when I tested it.
The channels and diffusers you see on high-dollar production cars probably all generate some amount of straight-line downforce. More importantly though, they're a marketing gimmick that's effective with typical buyers of those cars. I'm not an aerodynamicist (not by a long shot), but I'd be very surprised if any carbon fiber piece 5 inches or so above the ground on a street-sprung car does much effective work at all when the belly of the car is no longer parallel to the pavement. And by meaningful work I mean actually allowing the car to go faster through a corner. I don't mean generating numbers that show there's some effect.
I couldn't list the number of ways a Ferrari 430 GT model differs from the 430 you can buy in a showroom. And I don't know the changes an ALMS team like White Lightning would make to the springs and suspension before they consider a 430 GT ready to race. But take a look at these four pictures and consider the difference between the underbody stuff that makes a difference, and the stuff a Ferrari owner could actually get over his curb and into his driveway. In the last picture, where the chassis is unsetlled (probably a low-speed, very tight corner), look at how the suspension holds the lifted wheel up -- not a whole lot of compliance there.
