The Underbody (flat belly pan is suboptimal - explanation & solutions offered)

[TheEnemy]

I think what he means by airfoil shaped is that it slants up away from the ground as it goes towards the back.

[Xist]

Well, if he unsubscribed from his own thread then I will go ahead and say that he first called flat belly pans "sub-optimal" in the thread name, but then said they made things worse, but everybody here says to do ABA testing anyway, and everybody who has done so has seen an improvement. Can we make them better?

Undoubtedly.

However, wouldn't making an airfoil shape increase frontal area?

[UltArc]

Originally Posted by Xist

Well, if he unsubscribed from his own thread then I will go ahead and say that he first called flat belly pans "sub-optimal" in the thread name, but then said they made things worse, but everybody here says to do ABA testing anyway, and everybody who has done so has seen an improvement. Can we make them better?

Undoubtedly.

However, wouldn't making an airfoil shape increase frontal area?

Well actually, it was changed by a mod because his original title was pretty rude. He is saying that a flat belly pan make no difference from leaving the car plain, without one. That s where the big hoo ha has come from, as many people were upset about that claim, since people here with credibility have done meticulous testing and have yielded results. So he hasn't posted or been on since the 19th, at the latest.

There was a communication of peoples frustration of word choices and miscommunication through text. It appears that upset him, and he has only returned once since.

[freebeard]

I hope it wasn't something that I said.

And yet the thread lives on. I guess people care about the underbody. Anybody have a guess what a 'diffuser' looks like?

I always thought old VW buses came with a bellypan, at least the ones with double doors or a sunroof opening, to stiffen the body. But I went looking for an illustration and I was surprised by how little it covers:


There is also a removable pan that covers the front between the rails. I'm going to compare and contrast with this:


Which picture may be the reason the O.P. started the thread in the first place. It is pretty.

[jime57]

Originally Posted by freebeard

Anybody have a guess what a 'diffuser' looks like?

According to an earlier post by aerohead, a diffuser is an upward slanted surface which allows some recovery of pressure at the rear. This minimizes wake behind the vehicle. As the OP stated, such a section might take a curved shape, like the rear of an inverse airfoil.

Hucho provides two examples of flat single plane diffusers in figure 4.48 of his "Aerodynamics of Road Vehicles." In example one, where the diffuser length/car length= .326 (i.e. a rather long diffuser) the diffuser works best at about 2.6-2.8 degrees upsweep and provided a Cd reduction of something more than .025. The second example, the ratio is .178(i.e. a rather short diffuser), the diffuser worked best at about 4 degrees upsweep and provided a Cd improvement of about .020. These are small improvements, but significant when added to other aero improvements.

In practice, one would fabricate these diffusers with flat sections of metal or coroplast. Look earlier in this thread and you will see examples on my Insight. If you study the pictures you will see that there is a clear angular break between the forward underbelly and the rear diffuser section. The diffuser is at about 3 degrees, not perfect, but close.

Later: Sorry, check post 17 in this thread:
http://ecomodder.com/forum/showthrea...s-22691-2.html

[UltArc]

VWVortex.com - Complete front to back undertray diffuser?

Checkout post 22. I thought a diffuser straightened out the air, with those fins. But I see it also includes an elevation change. I can't wait to get under my car and make one.

[2000neon]

It may be worth noting that he (the OP) is in school, and this can be an extremely busy time. It may not be that he has unsubscribed, but simply too busy to be back on here.

[mcrews]

Originally Posted by freebeard

Which picture may be the reason the O.P. started the thread in the first place. It is pretty.

That is the picture that inspired my belly pan 3 yrs ago (see link in sig)

I would argue that Audi put the recessed points on there to give more rigidity to the mounting point. not for some 'perfect' flow attachment pattern.

Let's look at this logically.
1. Audi (and all of the top line mfgrs) are making HUGE strides at (for lack of a better term....) ECOMODDING.
2. whether it's on/off engines from BMW 3 series or regen brakes from Buick, companies are pushing the envelope to gain 1/10 more mpg.

So you have to ask, why are the recesses ONLY at the mounting points??
And not evenly spaced across the surface??

[UltArc]

Originally Posted by mcrews

That is the picture that inspired my belly pan 3 yrs ago (see link in sig)

I would argue that Audi put the recessed points on there to give more rigidity to the mounting point. not for some 'perfect' flow attachment pattern.

Let's look at this logically.
1. Audi (and all of the top line mfgrs) are making HUGE strides at (for lack of a better term....) ECOMODDING.
2. whether it's on/off engines from BMW 3 series or regen brakes from Buick, companies are pushing the envelope to gain 1/10 more mpg.

So you have to ask, why are the recesses ONLY at the mounting points??
And not evenly spaced across the surface??

Good point. Plus, if they hung down, they would catch junk.

[redyaris]

Originally Posted by MTrenk

There's a reason Formula 1 requires a flat section under the car: it lowers top speeds because of the huge amount of drag it creates. In no way shape or form would the F1 engineers choose to put that flat surface there on purpose.

The flat underbody did not reduce top speed what it did was reduce cornering speed due to the reduction in downforce.

The flat bottom did not increase drag. In fact the drag due to the flat bottom was redused, compared to the ground effect type underbody, due to the loss of downforce.

Mostly within the rules the F1 engineers do everything on purpose.

from
technical F1 dictionary
After ground effect was banned and flat bottom rule introduced 1983, speed of the car and downforce produced by flat floor went up every year more and more. Flat bottom rule was revised again after Senna death 1994, and from that year the floor has to have a step along its length. That’s why we see the stepped shape of the car in frontal profile, with the reference plane sitting lowest in the middle of the car. The reference plane, which is the part all heights of the car are related to (datum for all other measurements), is defined as the lowest part of the floor of the car (excluding the plank) and runs along the centre of the car and must meet a minimum width and length. The reference plane starts on splitter (T-Tray or Bib) 330mm behind the front wheel centre line and go all the way to the rear wheel centre line, and must be between 300mm and 500mm wide and symmetrical about car centre line. Reference plane must have a 50mm radius (+/-2mm) on each front corner when viewed from directly beneath the car. At it sides it leads through a radiused edges and transition plane (or step) to the step plane.