The ortlieb mountain x 31l backpack crossing the alps by mountain bike is a unique experience and if your plans include the well designed backpack mountainx 31 your trip is guaranteed to be a success the mountainx 31 is made of durable pu coated nylon waterproof roll top closure and a waterproof zip in the centre the removable inner compartment sheet divides the interior in two sections eg for clean and dirty equipment the padded back carrying system with ventilation channel