I have just bought my Cube Travel Exc with Thun Zumba Bottom Bracket and Alfine 8 speed. I noticed that just after a few days of ride the belt has slacked and measured at under 30 Hz. I have viewed video on how to tighten the belt tension by adjusting the bottom bracket in other bikes but somehow I am getting mixed message the Zumba Bracket is not "eccentric" and therefore the belt tension cannot be adjusted, as the entire system is fitted to last. Does anyone have any idea what to do with slack belt on a Cube Travel Exc? I have added a short video of the belt drive set up for this bike
You've got an eccentric bottom bracket. Underneath the bottom bracket shell you'll find two binder bolts. Loosen those and the eccentric can rotate to adjust the belt tension. To rotate it, poke an allen wrench into the fitting for it you'll find on the left side of the eccentric that will look like a blind hole. It usually takes some force but should move fairly readily.