Flat Bottom Girls - Learn Me Theory vs Practice

So as some of you know I am in the middle of a body swap with Uncle Ben. 

Long term, I'd like to have a flat bottom on the car and I'm sure what I do now will effect how easy that job will be in the future. 

I've had little luck researching this on my own, as technical data isn't easy to find. 

What considerations need to be made if I want to add a diffuser later? 

It looks like some kind of side skirt is needed to funnel/guide air under the car, but how deep? 

Are skirts necessary, period, or does that only apply to when using a diffuser? 

Can a flat bottom make a car worse if done wrong? Any examples, I can learn from?

Thanks all!

First step to making a good flat bottom, make room for one. You want to keep the exhaust tucked up close to the underbody, or even higher - a lot of production-based race cars with flat bottoms basically run the exhaust through the cabin and then build a heat shield around it. Make sure you have plenty of vertical space in the rear for a diffuser. On a production car, the biggest obstacle might be a rear subframe.

Next, make sure there's room for hot air to escape the engine bay. On production cars, the hot air is mostly pushed out under the car, this is where you'll be blocking so you need a ventilated hood, ideally with a big extractor scoop to get air out of the radiator stack right away.

I don't think you can make things worse with a flat bottom, other than perhaps making one that falls off while the car is driving. A common mistake is having too small a gap at the nose of the car, this is what causes porpoising which is sometimes solved by having the flat bottom rise toward the front of the car to a kicked-up section.

Crackers said:

Can a flat bottom make a car worse if done wrong? Any examples, I can learn from?

Thanks all!

Here's a well made simple one-piece flat bottom for a production car that doesn't get in the way of jacking points.Don't know what all the little triangular cutouts are though, they're facing the wrong way to be NACA ducts:

This is relevant to my interest. 

Any links to the hood stuff?

This is relevant to my interests.

I don't like the idea of coroplast under the car by the hot exhaust and I'm not sure it would be stiff enough to work optimally. Plywood works well as a splitter material, but I don't know how I feel about a wood undertray either. Any ideas on what thickness of aluminum to use for an undertray? I've seen some diffusers that are 0.090 inch thickness - would that be appropriate for an undertray as well?

Dusterbd13 said:

This is relevant to my interest. 

Any links to the hood stuff?

It's pretty straightforward, we've all seen vented hoods, these give hot underhood air another exit:

The Aston-Martin Vulcan has a good example an extractor hood, one or two big scoops that fall right behind the radiator stack to vent the hot air from the stack over the top of the car. The kick-up on top of the hood before the scoop helps pull more air through it:

Ideally there should be ducting behind the radiator that meets the scoops.

TVR thinks two huge scoops are twice as nice:

Ideally if you're running a flat bottom, you should have both the extractor scoop for cooling airflow through the heat exchangers, and the vents to allow some airflow through the rest of the engine bay, because there are other hot parts that could use cooling airflow like the exhaust and any turbos:

I have to say that if you want a flat bottom, then the chassis swap is probably not the way you want to go.

A square tube frame would allow for optimized integration of your components within the confines of the flat floor. Needing to hang stuff under the car that will then be covered with a layer of paneling, does not come across as an elegant solution to anything.

From a different and perhaps subjective  point of view the whole point of a chassis swap is the Trompe-l'œil performed by having one body-style on view and a ringer chassis under the classic clothes.

I love how the bottom view of the Molvo looks so right when I know that it is in fact so very wrong!

Pete

collinskl1 said:

This is relevant to my interests.

I don't like the idea of coroplast under the car by the hot exhaust and I'm not sure it would be stiff enough to work optimally. Plywood works well as a splitter material, but I don't know how I feel about a wood undertray either. Any ideas on what thickness of aluminum to use for an undertray? I've seen some diffusers that are 0.090 inch thickness - would that be appropriate for an undertray as well?

The gauge of aluminum would depend on the spacing and type of structure holding it. I've built kit cars like the Factory Five GTM which has very thin aluminum riveted to the steel square tube frame with no wide spans compared to the sheets of thicker aluminum I made the under tray for my car of which is riveted and bolted to aluminum box tube and right angle aluminum structure with wider spans. For mine I also used coroplast where there's no exhaust heat.

In reply to NOHOME :

Yeah, you're probably right. 

That would be cleaner and more elegant, and while I would love to build a tube frame (it was even my first idea after finding all the rot) I don't have the facilities (or patience, if I'm honest) to do such exacting work to my satisfaction. 

With the swap, I don't have to square/level two separate subframes on both axes or worry about their heights in the chassis relative to each other. 

All that said, if/when I get this car developed to that point, I'll probably fabricate a whole new floor as part of the flat bottom process. 

You know there are so many places this tread title could have gone. I had no clue what to expect knowing this group. 

GameboyRMH said:

Here's a well made simple one-piece flat bottom for a production car that doesn't get in the way of jacking points.Don't know what all the little triangular cutouts are though, they're facing the wrong way to be NACA ducts:

maybe they're trying to use them as extractors?  or "minimal flow turning exits"?

How effective do we think it would be to have a splitter at the front that was possibly a little lower than the rest of the flat bottomed car, with a gap for air to escape the engine bay and perhaps a gap all of the way along for the exhaust?

I also have trouble understanding how one would easily rivet, zeus fasten, etc. enogh to keep the flat bottom reasonably attached and rigid.  Thoughts as to how this would be best accomplished?

In the first picture GameboyRMH posted, I'm guessing the cutouts are for a bit of potential heat to escape?  I notice the panels go  goes up in the middle towards the side of the car.  Would there be any ramifications of this?

How does one ensure the diff doesn't overheat?

I can’t add any useful info but I’m interested in this as well.  I have a theory that many old cars underneath are basically a series of air brakes, and a smooth flowing under tray, even if not designed for downforce, might provide a significant increase in fuel efficiency/top speed.

getting heat out the hood can be important, since that's effectively turning the flow and creating downforce.  If you're trying to push it out the bottom, it's going to do the opposite.  If you're trying to get the splitter/undertray to suck it out and into the diffuser, that could work... but you're introducing turbulent slow air, instead of uniform fast air.

One of the other options is to get excess engine heat out the sides... which most TimeAttack cars seem to be doing... benefit being lower drag because you're not demanding lift from that air.  And the exit flow can help evacuate brake heat too.

re:how to attach
you'd probably want to mount "L" brackets to the top side of the panel.  That way they stiffen the panel, and also give the upward pointing flange to mount to the car.

re:"In the first picture GameboyRMH posted, I'm guessing the cutouts are for a bit of potential heat to escape?  I notice the panels go  goes up in the middle towards the side of the car.  Would there be any ramifications of this?"
Yeah, definitely there'd be ramifications... although I'm having trouble permutating through them in my head.... CFD would be better for that .  I'd guess that it'd siphon flow from the undertray and into a vortex created by outside air curling around the skirt.  Hard to guess if that'd be good/bad.  Could add energized flow downstream, could cause less flow to make it to the diffuser.

In reply to Rufledt :

This applies to most cars, really. 

If you've ever spent time on the eco-modder forums undertrays are evidently one of the biggest returns per dollar. 

I'd really like to build a 70mpg CRX someday. (Actually there are lots of things I'd like to do with a CRX or 7) 

So I have this idea of using the tunnel itself as vent that ultimately would exit out the back between the body and the diffuser. 

I wonder if the low pressure in that area would actually help suck the air through, and with enough flow might even help reduce drag. 

That's for people smarter than me to ponder though. 

Snrub said:

How effective do we think it would be to have a splitter at the front that was possibly a little lower than the rest of the flat bottomed car, with a gap for air to escape the engine bay and perhaps a gap all of the way along for the exhaust?

 

I also have trouble understanding how one would easily rivet, zeus fasten, etc. enogh to keep the flat bottom reasonably attached and rigid.  Thoughts as to how this would be best accomplished?

 

In the first picture GameboyRMH posted, I'm guessing the cutouts are for a bit of potential heat to escape?  I notice the panels go  goes up in the middle towards the side of the car.  Would there be any ramifications of this?

 

How does one ensure the diff doesn't overheat?

Don't make the front end lower than the rest of the car. Nothing should be lower than the middle section. If you made the front end lower, your center of downforce would be shifted far forward, and you'd probably get horrendous porpoising. The front of the flat bottom should be level with or higher than the middle.

For fastening, you can make brackets to tie into factory bumper or suspension mounting points at the front and rear. The middle is the harder part, you'd be lucky if you didn't have to do some fabrication work on the chassis to get enough attachment points there. You could install rivnuts to the chassis or maybe drill laterally through the seams near the jacking points and make brackets to meet them.

I thought those cutouts might be for cooling airflow to the exhaust, that makes sense for an X-pipe exhauist if you look at their positioning, it's just that the reverse-NACA shape is strange. Some quick searching suggests that a reverse NACA duct is a bad idea as they tend to expel air at a 90deg angle to the surface they're mounted to!

The middle section curving up to the sides is a packaging compromise and isn't aerodynamically ideal, ideally you'd want it to be perfectly flat.

To make sure the diff doesn't overheat, an onboard diff cooler getting air from a duct on the body of the car would be ideal, or you could just make a cutout for it and see what happens, at the cost of reduced underbody downforce.

Crackers said:

So I have this idea of using the tunnel itself as vent that ultimately would exit out the back between the body and the diffuser. 

I wonder if the low pressure in that area would actually help suck the air through, and with enough flow might even help reduce drag. 

That's for people smarter than me to ponder though. 

An outlet between the body and diffuser would work well, a lot of supercars and race cars have vents there for this reason. Don't put any vents in or near the front of the diffuser though.

Alls I know is they make the rocking world go 'round.

Flat floors, phfft, trivial, just rivet it on

Interesting stuff.

Another question - Is a diffuser more or less required for this type of setup and would there be much benefit without a diffuser?

GameboyGMH - you mention having a lower splitter than the rest of the underbody would potentially cause porpoising; Would this also be somewhat true when using a splitter without the flat underbody?

Snrub said:

Interesting stuff.

 

Another question - Is a diffuser more or less required for this type of setup and would there be much benefit without a diffuser?

 

GameboyGMH - you mention having a lower splitter than the rest of the underbody would potentially cause porpoising; Would this also be somewhat true when using a splitter without the flat underbody?

You won't get much downforce from a flat underbody without a diffuser, so it doesn't make much sense to have a flat bottom without one. All you'd get is drag reduction, and again not as much as with a diffuser. You can actually get a fair bit of downforce from a diffuser with no real flat bottom in front of it - you just need a small flat section at the leading edge of the diffuser since this is where most of the downforce is formed.

I think it would be difficult to get porpoising from a splitter without a flat underbody. Porpoising is a cycle of your underbody downforce spiking (and falling off) due to the amount of air let in at the front being cut down drastically. If your underbody downforce is negligible, like on a stock production car, nothing you do to affect airflow at the front edge is going to have much effect.

AngryCorvair said:

Alls I know is they make the racing world go 'round.

Fixed it for you

what about ' drumming ' of all that unsupported metal?  ad a few strips of ' peal and stick ' to cancel the ' drumming '?