The chain transmits power from the chainring to the rear hub's sprocket. It is subjected to extreme loads and weather conditions. Chains stretch at different rates depending on the usage conditions, but each one must be replaced sooner or later. Timely replacement of the chain is the only way to protect chainrings and sprockets from excessive wear. To maintain the best possible performance of the Shimano HyperDrive System, it is important to use original Shimano HG chains (the latter only for Dura-Ace). The latest chains from Dura-Ace, Ultegra, and 105 feature a unique new asymmetrical design. Therefore, the direction of rotation must be considered when installing. The specially shaped links optimize engagement on the sprocket and lead to even smoother shifting, even under load. To save weight, these chains have cutouts in the inner links and hollow rivets (the latter only for Dura-Ace).