Is it normal to need to re-tension drive belt (Gates CDN) after few days?

Question

I'm brand new to the world of belt-drive bikes so trying to understand what's normal and googling isn't helping with this specific question.

One of the selling points of belt drives is low maintenance and less stretch than a chain. However, after just a couple of days and only 10km my belt is feeling pretty loose - not skipped yet but just feels/looks too loose, and is looser than bike was out of the box.

My question then is does this indicate an issue with the belt / build (having left eccentric BB pinch bolts loose for example) or is it just one of those things that has to be done once/early after bedding in then much more rarely after that?

This question points to someone who had a very similar experience to mine - on a very similar model of bike (mines a Cube Travel Pro 2022, XL frame). Is it normal/expected?

=== EDIT: having done some more investigation (and gotten the Gates belt tension app working vaguely consistently) - I now think the tension has not changed since I first got the bike, but instead there is a 'tight spot' as the pedal crank is turned. In ~30-degrees of pedal position, belt tension is about twice what it is anywhere else (~40Hz in the tight spot - which is within accpetable range, VS ~20Hz everywhere else which is too loose). This same issue is discussed in this question. I suspect it just so happened when I first felt the belt when I first got the bike it was in the tighter spot.

I might need to modify title of this question - or ask another, but this raises two things:

1. Is this level of tension variation with crank rotation normal/acceptable? (It rides/sounds all good for now). If not I will follow some of the answers to question linked above.

2. If it is acceptable (or I can't fix it) - I guess my best bet is to have the tension in the loose majority of crank positions close to the bottom of the recommended range (the tight spot will then be slightly above the max recommended value).

Answer 1

The belt itself is supposed to be very dimensionally stable, which more or less bears out in reality.

Eccentric BBs are fine in terms of being a sound design and way of tensioning the chain, but issues with OEM assembly in this era are very common with them and a good thing to check sooner than later on bikes of this sort. You want the eccentric's contact surfaces and all the fastening hardware thoroughly greased (or anti-seize), and then the fasteners torqued properly (usually a pretty high number, but it depends on the type). Inadequate prep and/or torque of the bolts are one cause I've seen of the issues you describe. You could just pull the bolts and grease them, but if that wasn't done at the factory then it usually means the eccentric wasn't either, and it will start creaking eventually. You also want to make sure the rear axle nut threads are greased and the nuts torqued properly.

Edit in response to edit: Alfine hubs don't like being run above a certain tension level that allows things to bind. It can damage them. It's hard to say without seeing the bike whether the situation could be improved, but a mechanic willing to spend the time can sometimes finesse these problems away by shifting the mounting of the front sprocket at the tight spot to equalize tension. It takes some greater effort to do this with belt drive, but is often possible. If you want to attempt DIYing it, the basic steps are pull the crank, bolt something to the ST water bottle to use as an indicator, find the crank orientation where the mounting surfaces run closest to perfect, possibly make them more perfect yet by bending the spider tabs, then find the orientation of the sprocket that's closest to perfect true, then if needed bonk it into running at closer to perfect concentricity by making the (greased) chainring bolts semi-loose and bonking it into position (with the belt off and using something like a big rubber mallet).

Doing all of the above (pull cranks, pull eccentric, grease the eccentric and hardware, and put it all together for optimal concentricity) is about a half hour to hour long process for a shop, and it's much preferred to do it when things are clean and new.

Despite not being what you probably wanted to deal with on a new bike, it would be better to get this sorted out early. The hub will suffer from running at too much tension and the belt system will suffer from too low.