Depends on the helmet, I guess. Bucket-style helmets typically don't have visors, simply viewing-slits and some small holes so your head doesn't boil. If you have a crow's beak helmet, though, the upper part of the "beak" is generally hinged... though I imagine it must hurt something fierce if the hinge is weak or the pin is loose and it suddenly slams shut on you... not to mention loud.
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