Underpanel opinions?

Any opinions on this: http://store.miataparts.org/mibealunpa19.html

I have the original plastic/cardboard/regurgitated-compressed-yakfat
thingy which is pretty well shredded.

I'm sure I can find better uses for $200 such as a couple of gallons of
gasoline but......

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On Fri, 14 Mar 2008 12:06:23 -0700, XS11E wrote:

> Any opinions on this: http://store.miataparts.org/mibealunpa19.html
>
> I have the original plastic/cardboard/regurgitated-compressed-yakfat
> thingy which is pretty well shredded.
>
> I'm sure I can find better uses for $200 such as a couple of gallons of
> gasoline but......

Hum. Seems to be the exact opposite of the idea of spoilers. Spoilers
provide a massive obstruction to apparently achieve the same objectives.
I got "Aerodynamics of road vehicles" from SAE with the idea to look at
the underlying ideas of these things, but never got around to actually
look in it. I guess it is at work if I did not "lend" it to a student,
so I cannot ballpark it right now.

The idea that you are going to draw large volumes of hot air through
those slits with whatever minor pressure reduction you get seems just
weird to me, anyway.

And in my experience, that plastic Mazda cover is pretty tough.

What I would like to see is not "track tested and developed" but
"tested and found effective". Preferably with some hard numbers
to show *how* effective.

Leon
--
Leon Bess Bozo .)

Leon van Dommelen > wrote:

> On Fri, 14 Mar 2008 12:06:23 -0700, XS11E wrote:

> Hum. Seems to be the exact opposite of the idea of spoilers.

I don't believe it's intended as a spoiler, simple a replacement for
the original plastic thingy that disintegrates quickly.

> And in my experience, that plastic Mazda cover is pretty tough.

Not tough enough in my experience, mine is bent, folded, spindled,
stapled and mutilated to borrow a phrase...

My interest in the aluminum one is that it should last and can be bent
back into shape if needed, rather than tearing as the original cover
did. (I don't know how, it was pretty beat up when I got the car)



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Well "X",

I have one on my 2003. I don't know how much it helps on heat of
downforce yet, I have only had it for a month and it is still cold here.
I do know that the sides of the thing are open and water off the road
can spray in and "cool the pan" if it needs it :-). It comes with some
spacers to keep it from rubbing on the pan, but I made some thicker ones
to keep the rattle away. The part extends back past the drain plug and
the hole is large enough to drain the oil with no problem. ( I have a
quick drain on the oil pan )
I have not had the car up to a good speed yet to say if it helps on aero
control like they claim. Summer is too far off to go find a place to try
that. I would guess that the "slots" would do more for cooling the oil
pan, as it sets directly behind them.
I do not have a front spoiler on my car ( air dam )

I guess time will tell on how well it works.

Bruce Bing '03 LS

XS11E wrote:
> Any opinions on this: http://store.miataparts.org/mibealunpa19.html
>
> I have the original plastic/cardboard/regurgitated-compressed-yakfat
> thingy which is pretty well shredded.
>
> I'm sure I can find better uses for $200 such as a couple of gallons of
> gasoline but......
>
Cooling is better with the stock panel in place than with it removed
(some people have tried it for track days and found out the hard way),
so you'd better hope they got the design of the aftermarket one right.

On Fri, 14 Mar 2008 18:04:34 -0700, XS11E wrote:

> Leon van Dommelen > wrote:
>
>> On Fri, 14 Mar 2008 12:06:23 -0700, XS11E wrote:
>
>> Hum. Seems to be the exact opposite of the idea of spoilers.
>
> I don't believe it's intended as a spoiler, simple a replacement for the
> original plastic thingy that disintegrates quickly.

It says "increasing down force by creating higher suction pressures
between the panel and road." And that is also used to support the
claim of increased cooling.

The way that people with no real knowledge of fluid mechanics selling
products often reason is that the incoming air speed in the gap between
car bottom and road is the car velocity, because it is "in front of the
car." Then they know that if you decrease the flow area, you speed up
the flow, and that Bernoulli says that lowers pressure. Unfortunately,
nonsupersonic flows are quite capable of sensing an obstruction ahead
and going around it in a more comfortable way. Having essentially
stagnation pressure below your car is not ideal. (And there are some
other problems with flow area reduction and Bernoulli, such as three-
dimensionality and viscous effects, but anyway.)

>> And in my experience, that plastic Mazda cover is pretty tough.
>
> Not tough enough in my experience, mine is bent, folded, spindled,
> stapled and mutilated to borrow a phrase...
>
> My interest in the aluminum one is that it should last and can be bent
> back into shape if needed, rather than tearing as the original cover
> did. (I don't know how, it was pretty beat up when I got the car)

Driving over big stones on the million dollar highway or over exhaust
parts will do it. It may well be true that the aluminum cover
might not tear and can be bend back into usable shape after an
encounter. But, IIRC, you can get about 4 plastic covers for the
price of this aluminum one. And my plastic cover was never torn so
bad that it really *had* to be replaced. And my best guess at this
time would be that there will be no measurable difference in down force
between the covers.

But I have an open mind, all of this is just guessing. I am waiting
for Bruce to tell us how right I am. (OK, wrong is also theoretically,
possible, but I consider it unlikely.)

Leon
--
Leon Bess Bozo .)

I agree with Leon. The only thing I can see with this is that the sides
are open and that must do something to the air flow, into or out of the
engine area. If you put a "lip" on the rear of the plastic unit, it
would suck air "out" from around the engine, but where would the air
come from to replace it ? ALSO , under my 2003, there is a plastic 1
inch "lip" fwd of the start of the Alum. underpan, that I would guess
that Mazda uses to spoil the air flow under the car. I think I will just
need to wait a while for good weather to see what, if anything this
thing will do. :-)

I must admitttt , I bought it for show as part of my engine bay clean up
:-) If I run over someone, they can look up and say " Gee, look at thet
nice under plate ! " :-)

Bruce Bing '03 LS