Join Date: May 2006
Location: Washington, DC
Bikes: 2008 Pedal Force ZX3, 2006 Jamis Eclipse, 1997 Marin Indian Fire Trail
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There's a bit of a difference in design between road and MTB helmets from Giro. The difference has to do with the design of the vents.
On Giro road helmets, the vents are designed to be used at speed: drawing air in, moving it over the head to cool, and allowing for easy exhaust from the back. There are very few squared-off vent openings on Giro road helmets, as they want to allow air to enter from the front of the helmet as the rider moves forward.
On Giro ATB helmets, the vents are optimized for low-speed operation, where a lot of cooling involves radiation of heat without the aid of high-speed forward movement. Thus, the vents tend to be more squared-off. Additionally, there tends to be more foam/shell surface area on ATB helmets (especially ones aimed at the freeride set) to help protect against penetration from sharp objects (e.g. rocks, stumps, etc.).
Where you'll see more similarity is amongst helmets aimed at the ATB XC racing set, who are usually moving at a speed more akin to road riders - thus, the reason that the Bell Sweep XC is essentially the same as the Sweep R road helmet, the only difference being a visor. But for the less-speed-oriented ATB set, you'll tend to see the squared-off vent openings.
There's a similar design philosophy for Bell, MET, Rudy Project and Specialized helmets, too: bigger, more aero-designed vents for road-oriented helmets, more foam area and squared-off vents on ATB helmets.