Mountain bike helmets tend to be slightly larger than road bike helmets as they offer more protection for the user. Full face helmets offer the most protection as they stem from motorcycle design and also protect the jaw but are too restrictive for everyday riding, unless your everyday riding is downhill or in the Alps. Mountain bike cross country helmets usually extend further down the rear than road helmets to protect the back of the head and there aren’t as many vents as cooling is not such an issue. Each manufacturer has their own unique retention system which helps keep the helmet firmly in place no matter what terrain you are riding. What literally sticks out about mountain bike helmets though is the peak on the front, this handy device may be detachable but nothing screams off road more than a peak on your lid, no aero advantages needed here!