under wing question

nothere · May 31, 2007, 07:32 AM

maybe wrong forum?,

I am building a transition from the spare wheel well to the bottom of the bumper of my modified rear VIII bumper. Its cut up in a fashion similar to the look of the nine bumper. Just cut the cover with a dremel.

My question is, should I put a little gurney flap on the trailing edge?

next question, should I put a wing below it? there is likely room for smallish wing with a chord of 4 or 5 inches that wouldn't look absurd. with a slot of 3 or 4 inches.

if it goes well I will put side panels on the transition piece back towards the suspension/diff.

seems I've seen oem stuff try to emulate that idea, in their cosmetic way.
http://picasaweb.google.com/bruryan/Set1 (pic of bumper)

Sap12687 · May 31, 2007, 07:41 AM

I say line it with a custom diffuser.

KevinD · May 31, 2007, 08:15 AM

the goal of the rear diffuser is to create a low pressure zone with low drag. gurney flaps will cause drag, and most likely create a higher pressure zone under the car. so no, i would not put a gurney flap under the car. leave it as smooth as possible.

likewise for the wing, it will cause drag, and because it is already a low pressure zone, i don't think the effect will be as great as putting a wing in a high pressure area (i.e. on the top of the car).

if your looking for more downforce, lower the car and put a nice smooth diffuser in the rear. the lwoer you get it the more downforce you have.

nothere · May 31, 2007, 05:33 PM

the effect of this part will be to mimic a diffuser, it would use the spare wheel pan as part of its section.

my thinking was, what air came out the back would want to follow the line of the diffuser and at the bumper it would curl up in bad turbulence. thus the gurney or spoiler. but as you say there isn't supposed to be much air under the car.

redvolution · Jun 1, 2007, 09:23 AM

i had a diffuser fabricated for me that involved cutting the stock 8 bumper. search the motorsports section.

nothere · Jun 1, 2007, 04:52 PM

have you any new impressions on how it performs?

volks-traitor · Jun 27, 2007, 01:03 AM

i'm looking into doing the same thing with a pan from the suspension arms to my IX bumper. are you bringing the leading edge up towards the body? i can't tell if it matters. aerodymnamics are new to me.

nothere · Jun 27, 2007, 06:58 AM

I have it installed now. mine just runs from the spare wheel well to the bumper. Its about 12 inches long and fits tight to the spare well. my reason for it being short was so it made the biggest difference in elevation and the wheel pan is mostly flat. As a result it has about a 30 degree slope while if it ran to the suspension part like the commercial piece it would have been nearly flat. The
VIII bumper is cut up to the maximum point (about an inch above the bottom of the license plate) which allowed me to use its lower mounting points to secure the wing.

thats more than you wanted to know, but , in your application you would want to keep as much air from above the panel as possible. if that poses a problem I'd try some dense foam as a filler. since the front edge is semi obstructed by the suspension it is not too important, a blunt front edge is OK.

did I notice a difference with it installed, nothing I can point to, but it is mentally soothing and I have too much elbow grease in it to take it off.

volks-traitor · Jun 27, 2007, 06:32 PM

have you attached vortex generators to it or is it simply a plate? do you have any pics?
i think i'll plan on following the profile of the bumper which allows for about 10 degrees. from the little bit of reading i've done so far, 10 to 15 degrees is good for a diffuser attack angle. it would terminate at about one inch below the suspension pivot, but now that i think about it, i could just make a bend before that and level out with the pivot. the diffuser would have one bend level withthe suspension and floor pan and one that returns upward to keep stay air out.

i think your right, a blunt edge would be good,
and a little elbow grease never hurt anybody!

DaveK · Sep 29, 2007, 03:17 AM

Any pictures?

Thanks,

Dave

nothere · Sep 29, 2007, 12:22 PM

http://picasaweb.google.com/bruryan/...92084501011586
http://picasaweb.google.com/bruryan/...91878342581362

voidhawk · Sep 29, 2007, 11:46 PM

Nice job, you're on the right track. Rear underbody work can give you a reduction in drag and/or downforce.

To get good downforce it needs to be longer though, and you need something from keeping the air squishing out the side (side skirts or vertical fins in the diffuser). Without the side pieces any low pressure zone you'd create under the car would simply suck in air from the sides.

For drag reduction, all you need is to direct air into the low pressure zone behind the car - you want the bottom edge of the diffuser as high as possible to make the "trailing edge" as thin as possible.

nothere · Sep 30, 2007, 09:30 AM

hi,
I was thinking of channeling, for the time being thats left to JZ Watts. (is she/he still around?)

I've ordered some linear sensors so I can log suspension travel. Hope to do, before and after, with and without, a number of sub frame aero assemblies.