The quarter turn mounting unit is retained on the bicycle by 2 rubber O rings . I am not sure if the ones that came with my Edge unit were defective, or they typically deteriorate due to exposure to the elements. The instructions with the mounting unit does state: To avoid damage to the device, use EDPM (Ethylene Polypropylene Diene Monomer) replacement bands only.
I used the Garmin for 6 months before the trip and replaced the O rings, with Garmin supplied rings, after several months due to some cracks appearing in one ring. Before the trip I replaced both rings, again using Garmin products, just for preventive measures. Three weeks into the cross country trip I went to remove my Edge 800 and realized that one of the O rings had broken.
Rather than use more O rings I used a length of nylon string wrapped around the mount and handlebar, in a crisscross pattern, to make my own attachment system. It might seem somewhat crude, but seven months later the quarter turn mounting unit is still firmly attached.
In the future I plan to ‘tie’ all of my Garmin mounts to the handlebars rather than rely on the O rings.