The frame bag attaches to the horizontal bar with simple velcro straps.
It optimizes the weight of the load in the center of the bike while keeping precious objects close at hand (telephone, GPS, wallet, etc.). Two parameters must be taken into account: the width of the bag which must not come to rub on your knees and interfere with your pedaling movements (pedaling duck is not recommended!!) and the size of the bag which does not should not prevent the installation of water cans.
For more advice we refer you to our guide: Choosing your Ortlieb bike bags