It would take a PhD in fluid mechanics to answer all the way but - the air
inside the engine is also hotter than above it which would cause an increase
in pressure below the metal. The rounded shape of the hood is also similar
to a wing, which because the air has to travel faster over the top,
generates lift. Mine's popped up also, but at 30mph it's only makes you
look kind of silly. But that's part of the attraction - being old enough to
look silly in a car and not caring about it!
-----Original Message-----
From: Gary McCormick [mailto:svgkm@halley.ca.essd.northgrum.com]
Sent: January 27, 2000 9:53 AM
To: Todd Osborn
Cc: Roadster List
Subject: Re: double locking hood
It would take a wind tunnel test (simulated or real) to confirm it, but I
would suspect
that the average pressure above the hood is significantly lower than below
it. Right at
the base of the windscreen you'll have higher pressure in the relatively
calm air pocket
that is formed there (that's why the vent air entrances are located in that
area), but
that area is body, not hood. The moving air over the hood is, I'll wager, at
lower
pressure than the relatively still air cell in the engine compartment, and
will cause the
hood to lift if unlatched. I (and, I suspect, many other Roadster owners)
have experienced
that float-y front -end feeling at high speed (90 - 95 mph) - if aero
forces generate
enough lift to make the steering feel light, don't you think that they would
lift the
hood?
Gary McCormick
SRL311-13291
San Jose, CA
----------------------------------------------------------------------------
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Todd Osborn wrote:
> My 68 (single latch) hood used to pop up during high speed corning. I
> suspect the body flexed enough to shake it loose. I found the the
> alignment of the plug (part on the hood) to the socket latch is
> critical. The plug mount on mine was bent (probably by the PO) so the
> hood would close, however the plug did not enter the latch in a straight
> line (common longitudinal axis). I had to re-locate the latch assembly
> to get a good alignment, the hood has not popped since and opens/closes
> much better.
>
> I don't think aero forces would cause the hood to open, the air pressure
> above the back of the hood is high (lots of air backed up against that
> vertical windscreen). The air under the hood should be comparatively low
> since the flow has to thread its way through/around all those high drag
> components (grill, radiator, fan, engine...).
> Todd Osborn
> 2x68 2L
> San Jose, CA
>
> thorn wrote:
> >
> > As Mark Dent mentioned, my '68 didn't have a secondary hood latch
either,
> > it used to pop up all the time while I was driving.
> >
> > Being a young teenage dude, I finally got used to it and continued
driving.
> > It used to lay flat until around 80 or so, then it'd lift up a few
inches.
> > It never lifted more than a foot, even at HI speeds. I guess it became a
> > dumb hat trick for friends, I wouldn't mention it and then could always
get
> > an alarmed reaction from the passenger.
> >
> > I wonder if it got "pulled up" into a low pressure pocket that developed
> > over the car, and if any "push" happened from under hood air?...
> >
> > Thorn Filippelli
> > Santa Rosa, California
> > '70 2000 Solex
> > #13575
> >
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