Even though I am tall enough that my head sticks up above the windscreen, my
hair still blows from back to front, so I think you want your windblocker as
tall as possible -- I don't think it can be "too tall" (within reason --
obviously, six inches over your head might be too tall). The airflow doesn't
come straight back over the top of the windscreen -- it arcs up in a curve,
which you might think of as approximating the profile of the early soft top
(or even higher than that).
I would concur that it needs to be as close as possible to the seat back, or
it will just be introducing a new source of turbulence. I would suggest it
be angled slightly rearwards at the top, in order to fit in more closely to
the rake of the seatback, back of the head, etc. -- leaning forward would
open a considerable gap at cowl height. I also suspect that the tonneau area
should be covered -- the large "hole" would in itself create turbulence, and
allow airflow between the seat backs.
My guess is that some manufacturers use screens with the idea that by
allowing a certain amount of "leak-through", it reduces turbulence from wind
rushing into a low-pressure area. But possibly it is merely because it looks
cool, or doesn't scratch like plexiglass or Lexan.
Just some thoughts...
--
Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the primer red one with chrome wires
on 8/1/05 7:35 AM, Gosling, Richard B at Richard.Gosling@atkinsglobal.com
wrote:
> Bill,
>
> Firstly, us Brits are entirely conversant with imperial units, and you'll
> find that probably most people over 40 (and plenty of us under that age)
> prefer them. Bit of a sore point with some of the population (a "metric
> martyr" grocer was prosecuted for pricing his fruit in lb and not kg!) but
> don't get me going on this issue... (but thanks for translating
> "plexiglass" to "perspex"!!!)
>
> Being an engineer with an extremely vague grasp of aerodynamics, I am not
> that surprised at all that 12" makes a difference to the effectiveness of
> your screen.
>
> As for the height, this is bound to matter. I have no idea what the optimum
> height is not to catch the rearwards flowing breeze, but I guess you want
> the screen to be close to that, and probably at least an inch or two above
> the top of your head. Maybe I should get one of the more fluid dymanics
> conversant engineers in my office to do a computer model of it... Or you
> could just hold your hand at different heights above your head while driving
> at speed, to try and detect where the strong airflow starts. You may get
> some funny looks from other road users!
>
> Richard Gosling
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