Poor Man's Ground Effects -- I need some input

[Jack Olsen]

The thread on ground effects, and Yellowbird RS's link to this document has got me thinking.

As some of you know, Tyson and I put a sheet of ABS plastic on the underside of my car running from the front AC condenser to the rear swaybar. When I was talking to Tyson at the GAF, he mentioned continuing the flat surface back in the way the under-the-engine piece did on 964's and 993's.

A stock 964 undertray won't fit to my 964 3.6, because I run a different tranny and exhaust. But it's a pretty straight shot, back there, and I could easily run a 3x5 sheet of aluminum from the rear swaybar back to my funny rear curved license panel, adding vertical guides on the rear as it curves up.

Obviously, I'd need to add some holes to allow air that's passing over the cylinders to continue out from the engine. But what else would I need to consider on this? I worry about trapping to much air around the headers and exhaust, although I guess they lose most of their heat out the tailpipe rather than through radiating.

Removeable side skirts might make sense, if they can be done with aluminum as well. And I'm probably going to try a bolt-on extension for the front valence to bring the splitter lower.

Is this all worth trying? For the record: I've got no access to a wind tunnel. This is strictly at the DIY level.

Here's a look at the back of my car. If you look closely, you can see the flat black panel ahead of the rear axle line.

[wevoid]

Jack,

I want to be first to say that ideas like this are traditionally run in the "Dream Tunnel", the quasi experts place of choice for ponitifications about aerodynamics.

I reckon you try it, back to back, on the track and see if it's better. In my experience, in both the wind tunnel and the dream tunnel, virtually all air management under the car will be an improvement.

Regards

Hayden

[Mike Feinstein]

OK...just one admittedly smart-assed question.......why? While all the 964 and 993 guys are ditching their engine under-trays, you're thinking of adding one. Do a search on the 993/964 board on Rennlist for a myriad of reasons not to do it including:

-No perceptible difference in aerodynamics (my experience also with my old 964).
-Potential for excessive heat leading to premature exhaust valve roasting.
-More weight.
-Less flat 6 music.
-and on, and on, and on...

Honestly, without the benefit of a wind tunnel, I'm guessing there's as good of a chance that you will increase lift rather than reduce it. Those engineers at Ferrari, Porsche, etc., spend a lot of time in the tunnel making the tiniest tweaks and adjustments to get it to work...and they understand this stuff! Just slapping some sheeting on the underbelly and hoping for the best doesn't seem in keeping with the planning and thought processes that went into BB2 to begin with.

OK...there you go. One party-pooper has spoken.

[Randy Webb]

Yes, I think you can tell if a large effect is present via track testing. You know how to do wool strips?
You'll be doing things the way the pros did them before big wind tunnels were available - inclunding PAG with the 356 and maybe the early 901s. Instead of having friends standing on freeway overpasses, you can use video cameras mounted on the car.

You could even do some smoke injection studies while on a track (I'd tell them about it first...). You'll need a way to generate smoke - TiO2 is the most common smoke used. I've set off smoke grenade fireworks in a lab before, but we needn't get into that...

I suggest using NACA duct shapes for air inlets - they will work on the bottom or sides -- no need for them to be on ext. surfaces.

[Jack Olsen]

I want to be first to say that ideas like this are traditionally run in the "Dream Tunnel", the quasi experts place of choice for ponitifications about aerodynamics.

The 'dream tunnel' is all they'll give me the keys for.

I wish there were a way to measure it more precisely than lap times, which -- even on the same day -- can involve so many variables.

[JP911]

It would probably also be a good idea to test (in addition to aerodynamic changes) the differences in engine temps (cylinder head temp and oil) between both setups on a track.

[Jack Olsen]

Originally posted by Mike Feinstein
Just slapping some sheeting on the underbelly and hoping for the best doesn't seem in keeping with the planning and thought processes that went into BB2 to begin with.

Except that the forward sheet of plastic lowered my lap times at Willow Springs by about a second and a half. That's a huge improvement (especially considering the $29 price tag).

[Jack Olsen]

Originally posted by Randy Webb
I suggest using NACA duct shapes for air inlets - they will work on the bottom or sides -- no need for them to be on ext. surfaces.

From where, and directed toward where?

[Mike Feinstein]

Originally posted by JackOlsen
Except that the forward sheet of plastic lowered my lap times at Willow Springs by about a second and a half. That's a huge improvement (especially considering the $29 price tag).

Certainly can't argue with that. Are you confident that the plastic sheet is responsible? Maybe you're just getting better!

Seriously, be careful to allow plenty of airflow underneath the engine to keep things from getting too hot. Another way to measure the effect might be to do some accurately timed 40-100 mph runs with and without (maybe with a G-tech)...in a safe environment...of course.

[bell]

if it ain't broke............
jack what about making the current undertray more efficiant, lexus has been using thousands of dimples (like a reversed golf ball) to stabilize turbulence, maybe improve on what you already know works.

[MotoSook]

An easy to fab potential solution is to dam the front and sides of the car. By making the air flow around the car instead of under it, you will create lower pressure under the car adding to anti-lift. The next step would be to allow some air under the car, but manage it. That would be harder on a homebrewed test mule.

Another potential advantage of the above, might be less eddies and turbulance that will create drag, but then eddies and turbulence may have some benefit too...

The dams will increase your frontal area slightly but if you allow for some streamlining the positives may out weigh the negatives.

Just my time in the Dream Tunnel , but I have operated a wind tunnel with smoke and drag measuring devices...

[Tyson Schmidt]

Jack, I guess I didn't make it very clear when we talked. My plan was to put the stock 964 undertray back on, then connect the underbody tray to the engine undertray with a custom piece, or modify a 964 piece.

It won't be a difficult task. You just need to bring me the car some night.



Oh, and about the 964 undertray issues regarding heat. The real culprit there is city driving. They are fine at speed where there are NACA ducts for the trans, and carefully managed airflow openings so that the engine cools just fine at high speed. It's around town stop-and-go driving that kills the valve guides.

y guess is that Jack would simply remove it for the hotter short-tracks, and put it on at tracks like Willow Springs and Fontana.

[Oldporsche]

Just looking at the silhouette of the 911, one finds the airfoil shape. What I believe is needed is something to either reverse the airfoil shape or create a partial vacuum under the car.

The partial vacuum could be helped by a very deep air dam and side skirts to keep the air from entering beneath the car as it moves forward.

Well this advice is what it's worth. Cheap.

David Duffield

[RaceProEngineer]

Bravo, Jack!

PCA class rules do not allow us this kind of aero enhancement, but WE WISH THEY DID!!

Unfortunately, I can not imagine whether or not your front configuration blocks all air possible from under the front splitter, or encourages some flow in the middle.

Either way, to be most effective, it appears from your photo there is considerable "daylight" coming in from each side of the bottom, between the front and rear wheels. I would attempt to limit that exchange of air from beneath the sides of the car.

Your sheet aluminum idea is a good one for the diffusser. Perhaps attach it to continue your front understay. As it "progresses" rearward, only the center section continues to (or slightly past) your rear "bumper". On each side, the aluminum stops just forward of the header, leaving heads and headers to breathe.

For maximum effectiveness, the center section which continues rearward should be bounded by short "walls" which extend as low as possible to the track surface. Once the center section is rearward of the motor, it should gently angle up from 8 to 12 degrees, and terminate a couple inches behind the "bumper". The "walls", however, should grow longer, to continue parallel to the track surface.

With the car's side rails properly blocked, the net effect will be a single flat column of "dead" air under the front of the car, spreading into three "tunnels" as it evacuates from the rear. The tunnel on each side will unfortunately have lots of turbulance due to the wheel rotation and the header shape. (But not nearly as turbulant as UNMANAGED air under our cars!)

Jack, I am certain that you already know this, but for those others who may want to try, a couple of caveats are in order:

* When you add downforce, you usually sacrifice top speed. For this configuration to work properly, the car must have plenty of power available.

* Someone suggested using high-speed acceleration runs. These will only work to evaluate DRAG REDUCTION measures. In fact, if your DOWNFORCE efforts are successful, high-speed (straight line) runs will be less dramatic!

* A flat bottom or diffusser configuration is much more sensitive to ride height and rake. Once the diffuser is dialed in, one must be VERY CAREFUL when increasing ride height, like for rain, or for bumpy tracks. The front end can become very "light"!

Keep us posted on your findings.
Ed LoPresti
RacePro Engineering

[Randy Webb]

Jack - you were taling about heat issues - you would use NACA ducts to provide more air if you run into that. They might also help in getting air out from under the vehicle.

How far do you want to go with this intellectually speaking? I think a lot of the air flow mgmt issues are susceptible to "conceptual analysis" and some crude measurements. You might for example, want to read Steve Vogel's book which is very accessible (little math). I posted the cite a week or 2 ago if you wnat to search.

You could make some simple Bourdon tube manometers and use those to measure the pressure in various spots. There are lots of things an inventive mind can do for cheap in this area. ... You could even write a book about your experiences...

[petevb]

What are you trying to do? Reduce drag, or increase downforce?

I’m going to guess that the scale of lap time reduction you noted by adding the ABS was not due primarily to reducing drag or increasing downforce, but instead was due to changing the location of the center of pressure. Moving the center of pressure forwards of backwards is going to affect understeer or oversteer at high speeds.

I’ve only played with practical aero a little bit, but I did quickly realize that moving the center of pressure has dramatic effects- move it significantly forwards of the CG and the car will be nearly un-drivable at speed (as me how I know).

I think if you’re going to try and find a bunch more speed from the underbody you won’t find it by reducing drag- you’ll need to create downforce. When you do, I’d try to balance the underbody downforce independent of the upper body downforce, ie wings and splitter.

What you don’t want to have is the underbody downforce moving the total center of pressure around dramatically as it’s affected by the ride height (bumps, speed). You basically want both the center of pressure from the wings and the underbody a little ways behind the CG.

So in my “dream tunnel” I’d give the car some rake, but keep most of the floor flat or even give it a small reverse rake. This flat floor would end foot or so behind the center of gravity, were it would connect to a diffuser (aluminum or whatever) angled up below the motor at the steepest rake allowable given all the stuff in the way. I’d keep the side skirts small- you don’t want them getting too close to the ground when the ride height changes, as this will make the car all the more sensitive to height changes (though it would increase downforce if you got it right). Basically the center of pressure of this setup is going to be just in front of where the diffuser starts. Since the car is sensitive to this I think I’d start out by trying to make this location (ie the length of the diffuser) adjustable so that I could tune the downforce front to rear. Like I said I think this will make more of a difference that the total downforce. In theory (and no I have not tried this) this setup could give you both reduced drag and a decent amount of correctly placed downforce at speed. Making it all work wouldn’t be too easy, though...

I would not try to make too much downforce. If you did the rest of your suspension setup (springs, etc) would need to change to match.

[Zeke]

I love these kinds of posts. I've been thinking about this since the last one a couple of weeks ago.

I was trying ot install a new muffler with R type wide spread outlets Fri. night before the GAF. (It didn't fit). So, the car sits with only the HE's looking out from under there.

Now, I'm thinking, w/o the HE's and no licence panel, THERE'S A HUGE FREAKING TUNNEL (well, two, one on each side) UNDER THERE!!

It didn't help one bit to see some Carol Shelby recent Cobra stuff on TV the other day with the little cosmetic diffuser on the back.

But think about it. You just have to completely rework the standard notion of exhaust routing and ESPECIALLY the muffler.

As a little side note, I ran a piece around behind my seat on my kart that was connected to and on plane with my side pods. I did it just because I do silly things when I have time if I think it looks cool.

Well, I got about 4 laps into practice when my backside got awfully warm. I pulled in, drilled out some rivets, popped that thing off and all was well again. This dodad wasn't more than 4 " of extension on an aerodynamic pig. It just exacerbated the already and only calm place on the whole rig.

EDIT: I guess I ought to note that on a kart, the expansion chamber swings right behind your head and across the back at the rear.

[Randy Webb]

Don't worry, Jack is going to remove the heat exchangers right after he tosses out the A/C...

[Wil Ferch]

OK....sanity check.....

For grounds effect to work ( think about this)....you need to get a Bernouli effect by changing the cross-section of the tunnel as the air flows underneath the car. For *that* to work ( and as found out by the pioneering race cars), you need a combination of :
- stable platform....I'm talking serious spring rates that don't allow any roll and pitch.
- side skirts and undertray venturi tunnels that hug the ground closely. If you got 3-4" ground clearance...I can't see how all this can work.

The successful racing and formula cars that use this principle ended up having essenttially "no" ground clearance....and spring rates measured in the thousands pounds per inch.....

How do we make this system work for a more "normal" car....is the core question....IMHO......

Wil

[Mark sP]

Wouldn't the panel under the car need to be shaped like an inverted wing, for it to actually suck the car down. Allowing air both under and over it but at different speeds of flow?