Originally Posted by
rogerstg
Just the opposite. It increases ventilation by creating low pressure areas at the back of the helmet.
Bingo!
Originally Posted by
Shimagnolo
Every object moving through a fluid, (air is a fluid), has a low pressure area behind it.
But every low pressure system isn't created equally. If the point is to draw cool air through the helmet and spill hot air out the back, the fins aid in that air flow. The Bell Biker, Bell Metro and, especially, the Bailen (hottest helmet every) didn't have all that great an air flow. I loved my Bell Metro for winter riding but couldn't stand it when the temperatures got up to around 50. On the other hand, the more modern well vented helmets are crap for winter riding...too much air flow.
Originally Posted by
Shimagnolo
"The fashion among helmet designers since 1998 has favored squared-off edges of the foam remaining around the vents, and the addition of sharp lines in the exterior plastic just for style. The elongated "aero" shape dates from that era as well. This is not an optimal design for crashing. We believe that the ideal surface for striking a road resembles a bowling ball: hard, smooth and round. Round shells reduce to a minimum any tendency for a helmet to "stick" to the surface when you hit, with the possibility of increasing impact intensity, contributing to rotational brain injury or jerking the rider's neck. They also eliminate the aero tail that can snag, or in a backward impact can shove the helmet aside as you hit, exposing your bare head. This is such a problem with some models that lab technicians have to use copious amounts of duct tape to keep some helmets on the headform in their test drops, even after they have pulled the straps extra-tight."
Ref:
http://www.helmets.org/ventsqua.htm
Different problem. If it cooks your brain and you won't wear it, is a round helmet without vents or with reduced air flow any better?