After unsuccessful try to use SW-Motech Daypack Pro tank bag on my K 1600 GT, I searched for an alternative and found a new series of tank bags from Wunderlich named CLICK BAG. They offer 5 different sizes, 3L, 6L, 13L and finally 16L. CLICK BAG 3L is not extendable, but remaining sizes expand to 8, 16 and 19 liters respectively. 3L bag is the only one that comes with the rain cover while other bags come with internal waterproof bags.
