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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

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    There are a few different kinds of eccentric BBs and I suspect what's happening here is that's what you have but it doesn't look like the kind you were looking at. Can you post a picture of the BB area from the left side of the bike?
    – Nathan Knutson
    Commented Sep 18, 2021 at 2:25
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    Does your bike have a belt tensioner elsewhere ? Or does it have track-ends for dropouts ?
    – Criggie ♦
    Commented Sep 18, 2021 at 9:12
  • Thanks for responding. I have posted a short video of the belt drive set up for my bike. you can find the video at : youtube.com/watch?v=P5ZFHpScjDM
    – yew choh Lee
    Commented Sep 19, 2021 at 10:47

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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.

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  • Thank you Nathan. Most helpful ! Reading you explanation I don't suppose there is something inside the blind hole (like an allen key bolt) that needs to be loosened with the allen key before I rotate the eccentric ?
    – yew choh Lee
    Commented Sep 23, 2021 at 13:07
  • No. With this type, all that holds the eccentric from rotating is the binder bolts that are part of the frame. The allen hole is just there to give you a way to rotate the eccentric. Some bikes in history have done a similar thing with other kinds of tools, like holes for a big 2-prong pin spanner, but that's not what you usually see now.
    – Nathan Knutson
    Commented Sep 23, 2021 at 16:57
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    Hi Nathan. I did exactly as you said , loosened the two bolts, turned the BB using a long allen key placed all the way into the blind hole and sure enough after a half rotation, the belt became taut. I maintained the tension and then tightened the two binder bolts. Job done !!. Now the belt hums to a nice frequency within the recommended range. Thank you.
    – yew choh Lee
    Commented Sep 26, 2021 at 18:22
  • I have posted your solution on the youtube to acknowledge your contribution
    – yew choh Lee
    Commented Sep 26, 2021 at 18:30

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