Bill Z. wrote:
> Frank Krygowski <[email protected]> writes:
>
> [regarding Bill Zaumen's claim that _his_ helmet is more streamlined than a bare head:]
>>
>>I know of no ordinary, off-the-shelf helmet that's been shown to have
>>a "more aerodynamic shape" than a V1 Pro, and I'm sure I've got more
>>experience measuring aerodynamic drag than you have.
>
>
> THen you haven't looked very hard. A V1 Pro was first sold in 1983.
> See <http://www.bellbikehelmets.com/main/about/timeline.html>, which
> BTW has a picture of it. Modern helmets have an assymetric design,
> which fills in the area behind the head.
If you think the shape of a typical bike helmet allows the airflow to
smoothly converge behind the helmet, you must know very, very little
about practical aerodynamics.
Look again at time trial head fairings or time trial helmets. Those
shapes are quite extreme - quite long and gently tapered. Why? Because
that's what it takes to get the airflow to follow the helmet contours to
a reasonable degree. If the rear of the helmet (or other object) tapers
too quickly, the boundary layer separates completely and heavy
turbulence results.
Modern bike helmets are even worse in this regard. The surface is
nowhere near smooth, due to the presence of the vents necessary for
cooling. The air gets stirred up, to the point it wouldn't follow even
a gentle taper.
FWIW, I've never seen mention of wind tunnel work aiming to streamline a
conventional helmet. I assume this is because the designers know such a
thing is practically impossible.
But if you want to understand the principles involved, examine the
fairings used on Human Powered Vehicles - i.e. streamlined bicycles.
Successful ones are very smooth, long and tapered, and as small as
possible in frontal area. You won't find one that's shaped like a
typical bike helmet, which is very rough, _not_ gently tapered, and
larger than a bare head.
>>You could prove me wrong, of course. Just tell us the make and model
>>of your helmet, the one you're making these claims for. And point us
>>to the drag measurements that you're using to make your conclusion.
>>
>>If you won't, it makes it clear that you're just trying to avoid
>>proving yourself a liar.
>>
>>Unsuccessfully, of course!
>
>
> Typical of Krygowski's dishonesty - the particular helmet I have is
> a standard design with nothing particularly unique about it, so it
> is not relevant to the discussion.
If you say _your_ helmet causes less drag than a bare head, I think it's
relevant to ask what helmet you're talking about!
--
Frank Krygowski [To reply, remove rodent and vegetable dot com.
Substitute cc dot ysu dot
edu]