Bought this for a single cycling trip round the North Coast 500. It rained every day and everything was bone dry at the end of each ride. The inner dry bag has a vent valve to allow air to be squeezed from the bag to minimise the volume once placed inside the outer bag. There’s still room in the outer bag after the dry bag has been inserted. There’s also strapping cords on the outside. The complete unit secures well to the bike. Some reviews suggest a lot of swing from the bag but you would need to be out of the saddle putting in a lot of effort to make it swing so in my opinion it’s not an issue. It’s pretty much in mint condition.