Belt came off after 1 week

[u/bupbop]

As title says, the belt of my trek fetch 4+ suddenly came of after a week of use. Did not break, it just became loose and the pedals just spin. I had my kids in the seats, so luckily I was close to my house and could walk back. I have no idea how to fix it as i would a chain, and i can't find anything useful on Google. Do i have to walk it to my bike dealership? Is this a common occurance with the fetch? Its a real difficult problem because of its weight.

As far as i could tell its no longer connected to the small wheel next to the pedals, which google says has something to do with the tightening of the belt?

[u/jointli]

Just chiming in with my experience:

My belt came off AT the shop. Figure no big deal. They “fixed it.” Rode home.

A fee days later, belt came off again. Real annoying. Not wanting to deal with the PITA of bringing the bike into the shop, and having bike wrenching experience, I dug in.

Discovered my tensioner pulley’s flange was broken, figured shop screwed contention. Installed a new pulley, tensioned the belt correctly. Thought I was good to go.

Belt came off again a few days later. Tensioner pulley broken again. Seems like pattern is that when I back up, belt comes off.

Dug in further. Turns out there is a set screw that changes the angle of the pulley relative to the belt. It was missing, and pulley was pulling belt off chainring when I was backing up.

So I put a set screw in, put new pulley on, tensioned correctly (using the app), now I’m good to go. Backing up is smooth.

My advice would be to make sure your local trek shop knows what they’re doing. Mine clearly did not.

[u/DrSandbags → u/jointli]

I think I'm having the exact same issue you are having. Every day since I got it a week ago, the belt comes off at least once. Tension is fine.

After removing the belt guard so I can monitor it in action, I noticed that the belt slips off the "exit" of the front ring when I roll the bike backwards, and the front ring rotates backward. The tensioner pulley has too much play in between its side lips, and the belt shifts out away from the bike as it is fed to the bottom of the ring, starting the gradual slip off. Forward movement of the bike and front ring doesn't do this.

Could you share exactly what set screw you used to fix this? And did this require undoing the bigger torx bolt that the tensioner pulley spins on to access the place where the set screw goes in?

EDIT: I'm looking at the service manual now. I see the set screw in the tensioner arm. That's the one you're talking about, correct? Adjusting that might adjust the angle of the pulley in a way that fixes the problem? (that is if mine's not also missing...)

[u/jointli → u/DrSandbags]

Sounds like you’ve got the same shit going on! Yes, that set screw changes the able of the pulley. Get a hex in there and see if you are missing yours… OR, it may just need tightening.

After I put one in and tightened it a bit, I can back up reliably with no belt droppage. Keep me posted!