I know this topic was covered a bit, but, anyway, I'm looking for some info.
Undertrays, are they legal? Dimentions? Can they deviate from horizontal like the 'tail' on the Swifts do, as long as they are under an inch? How far can they extend from the radiators, outward? Are they worth it?
I've checked the GCR and can't get a clear picture.
I need the same information as I am building one too. I got the undertray mounted on my car now with Dzus fasteners but I haven't put any support rods on it yet until I am postive on the width and length.
According to the rules it seems as long as the undertray is not wider the the maximum allowed body (i think 37.5 from memory) and the curvature is under 1" total then we should be alright. I am covering up the entire bottom of my FF from the width of the sidepods and from the rollbar back to the end of the cover on the transmission. The panel is about 24" x 48". Then I have been doing measurements for support rods to keep the thing at the specific height I need and within curvature specs.
Chris
Marshall Aiken
www.aikenracing.com
Hey, interesting that we are working on the same thing, and, it seems nobody wants to touch this topic with a 20ft. pole.
Ah, the black arts.
Do you mean that the undertray can have curvature upward, as in, deviating from horizontal on the trailing edge like a diffuser?
I'm just extending my sidepod floors back to the axles. Just don't want to have to saw anyting off in tech. In CENDIV, they'll make me!
Thanks, Andy!
The rule now reads "No diffuser undertrays", yet the still have the 1" rule, and have yet to define just what constitutes a diffuser undertray.
To be safe, since the interpretation may be different from one techie to another, I'd keep it flat to the axle centerline, and cut it off there.
I built one years ago for the Dulon and we ran it flat to the centerline and then 'upped it' to the 1" max and attached it to the rear cover of the gearbox. I think it ran a few inches behind the tail of the gearbox too.
Results? Well, nothing really. Except that it took longer to do any work on the car and was a pain to remove.
I finally figured that in reality the CF didn't need any more grip, the loss of speed was perhaps greater than the gain in grip. Can't prove it tho.
Gentleman!
Thank you! I'm going to run'em to the centerlines of the axles, and probably have little or no 'up.' They'll probably serve as mudflaps more than anything.
This was exactly the input I needed. Thank you! I guess, in theory, the larger the floor the lower the pressure, and, the greater the suction. Air, as a fluid, get's stretched when forced to increse velocity within a confine... pressure drops... blah-blah, I'll never notice a difference. Oh well. Just feeling like a woodworking project.
Pray for Mojo.
Andy
Is there any problem with running it past axle center line? I don't see anything in the rules about that and I agree that probably staying completely flat is the best way to not make anyone mad. The 1" curve without being a true tunnel would probably have amount as much downforce effect as eating at MacDonalds.
I am just wondering how far back I can run the undertray.
Does anyone have any idea?
Chris
Marshall Aiken
www.aikenracing.com
In the protest on Brewer's car at the Runoffs a few years ago (that prompted the change in the rules to say "No Diffusers"), the only way that they could find anything wrong with what he had (and they REALLY wanted to find something wrong!)was to say that, since it was in an undefined area (bodywork restrictions behind the axle centerline are not defined in any way except to say that it cannot increase in width), that it ran into the statement of "If it doesn't say that you can't do it, DON'T."
Even with that, they didn't find what he had done illegal, but, since they absolutely didn't want to see diffusers like what are on FC's (which, by the way, are built to the FF specs!), they directed the rule change be done for the following year.
So, the short of it is that since no one knows exactly what the rule means (especially the tech guys and stewards), to be safe, cut it off at the axle centerline and keep it flat.
Todd: A properly done undertray will decrease overall drag, not increase it. In fact, almost ANY undertray will decrease the drag.
Alright.... I will cut it off at the axle centerline... Just going beyond the axles would be much easier for mounting purposes - At least it would of worked out easy for me. I was just hoping I could get the air to flow really nice below the car and get it beyond the back of the transmission. Seems like the decreased drag would be a benefit but I don't want to take a chance with gray rules like that.
Since my car won't be doing the runoffs are the officials going to be that anal in just regional racing?
Chris
Marshall Aiken
www.aikenracing.com
I guess I don't understand why the long, flat tail with the <1" rise is legal and accepted, while the undertray issue is contested and an actual issue?!
The use of any downforce producing devices is prohibited, yet, some are allowed.
NOWHERE in the rules does it say that downforce producing devices are illegal. What it DOES say is that wings and airfoil devices which create aerodynamic downforce are prohibited. The Swift tail, which definetly produces downforce, was ruled legal 18 years ago as it is neither a wing nor an airfoil, as both such devices by definition need to have a leading edge. You could graft a Swift tail onto any car out there and be perfectly legal.
Your mistake in thinking that way is common amoung those who don't know the real history of the rules, so don't feel bad! [img]tongue.gif[/img]
Ahhh. Interesting. Thank you for really clarifying this for me. Things that non-directly produce downforce: legal. Airfoils, wings, etc.: illegal. So, the undertray, if it is flat and fore of the axles, is legal.
Yeah, I don't know the history and evolution of the rules. When the Swift came out I think I was 8 years old. I would think, though, that the Swift tail would produce more drag than it helps, but, their records indicate something is done right.
"So, the undertray, if it is flat and fore of the axles, is legal."
Yep. The question that no one can answer, though, is at what point can shaping the undertray to anything but dead flat make it illegal!
Allright! Now this is what I'm talkin' 'bout!
Mister Smith, you are obfuscatory, and, my new hero!
I believe horizontal is defined for all non-bodywork as being on the same plane as the lowest point of the (for lack of a better term) fuselage/empinage-cart-before-the-horse. They expressly prohibit us from deviating from that plane in all instances except when it pertains to bodywork, then, there is a 2.54cm/1" allowance, which, gets into an entirely different realm of surface area and the parasitic and static drag arguement vs. the 'Gurney flap' camp, or, both, combined.
Being 1:38AM on a working day, I'm gonna throw in the towel and query this: The greater the surface area, setarus parabus, the faster the fluid (air) will have to go to wanna equal the pressure that all its friends are attaining elswhere, pressure drops, suctions occur, again, all things held at a constant. What are the numbers people are running in terms of surface area, and, how far wide of the rad.'s and fore/aft of the axles would the typical SCCA National Tech Inspector limit me?
Like Todd at TCE says, many other things come first, basically and more effectively than the bean counting I'm asking about, but, could there be a grey area where the black magic-ians are working? Mister Pritchard seems to swear by undertrays. Humm...
Thank you, seriously, everyone, this is really fueling me. (And making my girlfriend mad)
Mr. Art Smith... I'm glad you are on our team.
-Andy
P,PS- Some bretheren of mine have writen a bit on aerodynamices... [url="http://www.tfd.chalmers.se/~bredberg/FA/Reports/grupp9.doc"]http://www.tfd.chalmers.se/~bredberg/FA/Reports/grupp9.doc[/url]
[size="1"][ 03-12-2003, 02:22 AM: Message edited by: A J Graff ][/size]
I started a similar post a while back but gave up on the idea of a "diffuser" for the same reasons that Todd mentioned earlier.
BUT, for those of you that want to have a floor as long as you want, I never found a rule that limited the length of the frame! So, maybe an extension of the chassis back past the gearbox is legal and we know that a floor pan is legal.
[img]tongue.gif[/img] You might get so much suction behind the rear axle that you lift the front end on high speed straightaways. :eek:
Have fun fabricating - in the meantime, I'll be out racin'...
the thread to date seems to have focused on "Bodywork" implementations of ".........., for the full width of the body between the front and rear axles, the lower surface (surface licked by the airstream) shall not exceed 2.54cm (1 inch) deviation from the horizontal in any longitudinal section through the that surface." my read of the GCR suggests that "bodywork" probably isn't the optimum approach for maximizing the aerodynamic drag reduction benefits given the "flat bottom like" language above. remembering drag reduction is a delta-P x A dominated function, the choice of an implementation that comes with a fairly explicit restriction on both length and width (L x W = A) may need revisiting.
"D.6. Chassis/Frame" does not have a length or width restriction and therefore may offer more productive trade space for aerodynamic drag reduction. D.6. Chassis/Frame is not without its own challenges in keeping with that fundamental engineering law of "no free lunches, only choices". D.6.d requires a stress-bearing panel between the front bulkhead and the rear roll hoop bulkhead. D.6.d allows stress-bearing panels in three other locations: 1.) front bulkhead; 2.) front roll hoop bulkhead; & 3.) rear roll hoop bulkhead. No other stress-bearing panels are permitted; ie: the floor pan/undertray forward of the front bulkhead and aft of the rear roll hoop bulkhead can not be stress-bearing. thanks goodness for homologation; how many DB-1's have non-stress-bearing floor pans/undertrays aft of the rear rool hoop bulkhead? mine extends to the forward engine mounts..............
I find D.7.e "It is the intent .............." confusing at best. how does one demonstrate compliance to intent? where/how is "horizontal" defined? does the rule as written limit the amount of static pitch; ie: combination of forward and aft ride heights?
extending the frame/floor pan aft is one of the potential choices given no restrictions on frame width and length; probably my last choice. without the benefit of front wings, I can't think of a reason to move the center of pressure aft other than yaw stability. but then FFords don't go fast enough to benefit from "vertical stabilizers" like first used on the Penske Indy cars. left, right, and forward are far more interesting since they could be potentially "trimmed" via a rear spoiler.
what do people think D.7.e Bodywork says given I don't believe intent is objectively verifiable???
It is the intent of these rules to minimize the use of "ground effects" to achieve aerodynamic downforce on the vehicle. Thus, for the full width of the body between the front and rear axles, the lower surface (surfce licked by the airstream) shall not exceed 2.54cm (1 inch) deviation from the horizontal in any longitudinal section through that surface.
where/how is horizontal defined? referenced to local normal?? how measured; how accurately measured? as raced; driver aboard? how does the 1 inch limit on floor pan curvature relate to this 1 inch deviation from the horizontal, if at all? surely no one wants the adjustable ride height insanity that gripped F1...............
why think about potentially more intersting frames; most people seem to forget that Lotus 79's were very fast in a straight line inspite of having more frontal area than all of its comptemporaries.... cars with well managed air flow around them go facter at all levels
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