Bmx vert riders were using their moto 9 mx helmet in the noughties but needed something lighter the full 9 was born and put bell firmly back on the full face map for downhill mountain biking. Full face trail tuned ventilation make bell the best mountain biking helmets on the marke. New Fox Proframe Full Face Helmet Has 24 Vents For comparison the average weight of the half shell helmets in our mountain bike helmet test was around 125 ounces.