Different vibrating tone on a belt drive with a Shimano Alfine hub

Question

I have recently bought a new Cube EXC Travel bicycle with a Gates carbon drive. This bicycle is equipped with a Shimano Alfie 8 speed internally geared hub. The bicycle is new and it seems that the supplier did properly check and adjust all parts of the bicycle before shipping it.
After a month of everyday riding (about 500km), I have noticed that I am sensing a different vibration while pedaling on the 4 gear (the middle gear). For some weird reason, my legs "vibrate" from the pedals on the bike in a different tone on the middle gear. Once I go up to the 5th gear, the vibrations turn smoother and that is why it is so noticeable to me. These vibrations are practically not felt when going down to the 3rd gear as well. The general feeling is that the belt "grinds" more when on the 4th gear than any other gear.
These vibrations do not seem to compromise my speed or acceleration when on the 4th gear. The yellow markings on the internal hub form an almost straight line while on the 4th gear. I suspected that belt tension might be the cause but I made sure that the belt is at about ~37Hz (going by the Gates application).
Should I worry about it? Is it common? Is it an indicator that something is wrong or that some parts should be inspected in a deeper manner by a professional? Or maybe it is just standard behavior? (unfortunately there aren't many belt experts in my area)
I believe I experienced minor skipping for maybe 3 times in 500km, I suspect this is not connected.

Peter Duniho · Accepted Answer

I ride a road bike with a 2010 Alfine 8 speed hub. The hub is very sensitive to shift cable adjustments. You wrote:

The yellow markings on the internal hub form an almost straight line while on the 4th gear

The yellow marks shouldn't be "almost" aligned. They should be exactly aligned.
Friction in the cable housing can make it tricky to get this right. You have to come at it from the tension side (i.e. when shifting into 4th), because the cable sometimes gets hung up a little in the housing.
There is a transition shifting between 4th and 5th that separates the top and bottom halves of the gearing range. The shifting is a bit more sensitive in those gears to misalignment and when the hub is on the verge of shifting but not quite there, it can make noise. I have experienced symptoms similar to what you describe when the cable isn't adjusted correctly.
That said, if you experience any skipping with a belt drive, the tension is definitely too low. My experience has been that I get unwanted sounds at higher tensions, not lower, so I doubt that's the cause of what you're asking about. But since the belt should probably be tensioned higher anyway, it's worth a shot to see if that fixes the sound.
Do take care to not tension the belt more than needed. Excessively high tension puts unnecessary stress on drivetrain.

David D · Answer

Try more tension on the belt. It will help with the skipping problem and a tighter belt will change how the belt vibrates.

For internal gear hubs Gates recommends 35 – 50 Hz
Montague Bikes - Adjusting Belt Tension with an Eccentric Bottom Bracket

Based on this article you are adjusting the belt just 2 Hz more than the minimum.
This gives you some wiggle room to experiment with. Try 40 or 45 and test for changes.

I think, perhaps, the #1 problem with the Alfine 11 is belt drive. Lots of Alfine 11 bikes come stock with Gates. Not a single belt bike I have ever worked on came in with the belt tension high enough.
Sheldon Brown - Additional comments by Aaron Goss (toward the bottom of the page)
Note: the Alfine 11 is similar to the 8

Different vibrating tone on a belt drive with a Shimano Alfine hub

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