Smoothing out the bottom...

[dd74]

When's the last time you guys had your car on a lift and looked beneath it? The bottom of the floorpan and where the front and rear suspension components are don't necessarily make for an aerodynamically smooth design.

Now, on the newer Porsches, the undercarriage is as smooth as the body surfaces up top. I have to believe on the newer 996s, 997s and 998s, this smoothness aids in aerodynamics, downforce and handling. Or maybe I'm wrong.

Nonetheless, I was thinking if there was a one-piece aluminum section that could be mounted to the bottom of the floorpan and also cover the suspension parts, would lessen drag and increase downforce on our old 911s. The engine portion I could see leaving uncovered for issues of air cooling, or maybe not if cooling isn't an issue.

Anyway, any ideas? Has anyone tried this?

Thanks.

[island911]

Tyson did this on JO's BB . . .but I imagine that you already knew.

It's a good idea. However, the details of the execution are always key. --I was just thinking about this too. (that aero thread)

[dd74]

Yes, I remember this was conducted on Jack's car. My hope is he or Tyson might introduce their knowledge here.

So I wonder how one would go about doing this? Measure out the chassis and its angles, then cut a piece of thin-gauge aluminum to fit?

I'm no engineer. But I have an active imagination if that helps.

[island911]

Well, imagine making a template out of cardboard. . .


We'll want to see pict's on this.

[dd74]

You have to admit, Glenn, it'll be darn cool if it works. Jeremy at TRE has a vague memory of this being done, but nothing substantial. We shall see...

[Jack Olsen]

I just re-did it on my car with the AC condenser removed. I used ABS platic. Apart from putting a length of L-shaped aluminum stock back by the rear swaybar, it's pretty straightforward. A 4x8 sheet will cover you from just behind the front AC condenser to the rear swaybar.

There's a plastic place in the valley called Garavelli's -- something like that. I use .9mm sheet ABS.

[island911]

Somewhere theres a thread on aero & rake angle (Tyson started it.) The short of that story is that it's a complex problem. In general tho' one can gleam a huge amount of info from looking at existing under-side solutions, starting with the 959, 964...

My SC was up on the rack a couple days ago. Staring at the underside, I was thinking about how the newer 911s have so much more put into underside aero. ...not just smoothness, but pressure management. . .. like handling the high pressure infront of the tires. THe geometry of our SC's doesn't really allow for much in the way of a diffuser of the back. Tho' I understatnd that the 964 'sound tray' help with that effect.

[Jack Olsen]

Yeah, the low hanging fruit on our older cars is just to smooth things out to reduce turbulence (and hopefully drag). With the big, non-aerodynamic AC condenser down there, I got immediate lap time improvements with the underbelly piece, although I never got any kind of ride height or drag data from it.

Ironically, any really effective underbody treatment will rely on specific (and low) ride heights front and rear, which will be undone as soon as the suspension starts to work. You'd need to reduce suspension travel to just about nothing (think 2000# springs) to avoid porpoising -- where the ground effects work, and compress the suspension, and then the compression changes the angles and ride height enough so that the effect is changed and downforce is lost, and the car springs back up, and then the cycle starts again.

It would be nice to see the data on the 964, 993, 996 and 997 underbody treatments. I think with any kind of compliant suspension, you can't go for big ground effects results -- you need something that's robust enough to not change dramatically when the four corners of the car move up and down.

[Tyson Schmidt]

Oddly enough, the 964 had the best underbody treatment.

The 993 took a small step back for better engine cooling.

The 996 underbody is actually quite poor compared to the 964 and 993, but the upper body more than made up for it.

My guess is it saved them some R&D money, and also left room for "improvements" on the 997 so that they could keep the same CD or even lower it while adding flares, etc. The 997 has a smoother underbody than the 996. The 996 has no real tray under the engine, and the rest of the pan is very rough. Rougher even than an '89 and older 911.

[island911]

porpoising & suspension-- excellent point. Thanks.

So Tyson, how far down Haydens list is a underbelly aero product? ...and will it be shiney?

[Tyson Schmidt]

Originally posted by island911
porpoising & suspension-- excellent point. Thanks.

So Tyson, how far down Haydens list is a underbelly aero product? ...and will it be shiney?

LOL!

Pretty far down I would think.

I have been working on something that is for the front steering/suspension that I would like to have a smoother belly pan incorporated into. That's the worst spot aero-wise, IMO.

The front is generally where the most significant aero gains are made, and the factory belly pan, and front suspension is far from flat, and there are lots of gaps and resesses.

The second worst is the area aft of the pan where the trans and engine are. That could be done with plastic, but you'd have to consider trans temps.

The 964 vented the heat exchanger outlets into the rear wheel wells, and also incorporated a naca duct right below the trans to aid in cooling. It's easiest to mimic the factory, when you're covering ground that's already been covered.

[dd74]

But this project is doable, right? I can't see what's so terribly difficult about it.

[widebody911]

I did this to my car, using ABS and pop rivets. After a couple of good OTEs, the ABS split and started to come off, so I ripped it out, and I've been too lazy to re-do it.

A lift really makes your life easier for this sort of project.

TAP plastics also has ABS sheets.

[MichiganMat]

Hey, aren't you guys supposed to be over in OT with all the other riff-raff?

[RSBob]

OMG Tabs lives!

I installed a piece of tin from the front splitter back to the floor pan. On early cars with air (removed), there are lots of pockets and cutouts which this nicely covers, as well as suspension pieces. This is rather crude, but still creates a nice smooth surface for not much cabbage.

The car needs to be on a rack to take a decent pic.

[Shuie]

How did Ruf do this on the Yellowbird?

[dd74]

Yeah, I can't see this costing too much "cabbage."

What I am worried about is any additional weight this modification might cause.

Does anyone have a record of their weight gain after doing this, and of course, what type of material and its thickness?

[Bill Verburg]

I would only worry about the bottom of the car after

• car is lowered
• corner balanced
• has an efficient front and rear spoiler
• has the wheels and tires of your choice
• has a reasonable nose down attitude

The whole point is to keep air from passing under the car

[Chuck Moreland]

The bottom of the car really isn't that bad, save a for a few areas. The front area between the valance and tank is the worst imho. The rear sway bar, engine and tranny area is very poor too.

But the tank, steering rack cover panel, and mid floor form a reasonably smooth transition.

I went for the low-hanging fruit, forming a splitter that meets the front of the tank:





This inuitively cleans up airflow on the front end, though I have no testing data to back it up. It is very easy to do, if you are going to make a splitter. It's just AL stock, about .050 thick.

This concept could be extended to partially cover the control arms to really clean up the front.

[dd74]

Yeah - that's what I'm talking about. But I'd truly like to cover everything except around the engine. Thanks, Chuck.