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...
'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
> 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