I have used 1/8" thick Lexan/Margard for windshields on a few cars that ran in the 4.80-4.70's area 1/8 mile without any problems. The size/shape of the windshield and how much it's laid back can play a part in the decision. A few things listed below can help the 1/8" material work if it's used as the windshield.
(1) One or two small tube vertical supports to help support the center area of the windshield.
(2) A windshield that's "pushing" less air because it's laid back a good amount, or shorter because of a chopped roof.
(3) A windshield with a decent amount of curve (or like a "wrap-around" windshield) will be a little stronger/stiffer facing the wind than a flat windshield.
The 3/16" stuff can be harder to turn tight bends/corners (as in a wrap-around windshield) but of course will flex less in the wind. As the 3/16 stuff ages and dries out you could see more micro cracks the length of a tight curve/bend area compared to the 1/8" thick stuff. Some fiberglass or carbon fiber bodies have a windshield mounting flange that's recessed for flush mounting 1/8" Lexan, and some are deep enough for 3/16". And depending on how crappy the mold was that's used to make the body, the flange depth could be all over the place. :roll:
1/8" or 3/16" thick just remember don't over tighten the fasteners (#6, #8, or #10 screws) or put them in a bind because Lexan is so easy to bubble/buckle if you tighten screws too tight. They are tight enough if you can still rotate the screw some with light pressure applied to the allen wrench or screwdriver. The drilled holes in the body's mounting flange need to have a loose fit around the screw body to help reduce the chance of binding.
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