Why do i need a chain tensioner?
The derailleur gear system requires a different length chain depending on the gear selected. This is due to the different sizes of the sprockets in use. When smaller sprockets are in use, there is an section of chain too much that needs to be tensioned, the rear derailleur does this by also including the chain tensioner function.
With the Rohloff SPEEDHUB 500/14, only one sprocket combination is ever in use (e.g. 46/16 ).
It is only possible to shorten a bicycle chain in steps of one link at a time (1 link = 1 male and 1 female link). You will find that with most frames, the chain will still be a few mm too long. It is these few mm that leave the chain slack and can allow it the to spring off of the sprockets in some circumstances which of course would lead to a spontaneous loss of drive causing accidents and injury to the cyclist.
Therefore, on hardtail frames without an eccentric Bottom bracket which use vertical dropout slots a chain tensioner will always be needed. On such bicycles it is not possible to pull the wheel backwards in order to tension the chain.
Is it possible to use the Rohloff Chain tensioner also for other gearhubs or singleespeed ?
In principle yes, however the measurement from the dropout outside face to the sprocket center must be no less than 24mm!