Originally Posted by
UncleCoconut
So does he have a front derailleur for changing the crankset gears or does he take links or add links out of or to the chain depending on his activity?
If he needs two different gears for different activities (i.e. polo and street riding), then it's pretty straight forward to combine two chainrings with two cogs sized so the chain length remains the same. Loosen the rear wheel a bit, manually shift the chain over to the desired combination and retension.