How do I calibrate my auto tension belt? I have a Trek District IGH3 with a Mitsubishi belt drive that has an auto-tensioner. Last winter, the belt started slipping over the sprocket. It happened frequently enough that it ended up wearing the teeth of the sprocket right down, making the bike all but unusable until I got a replacement installed. Fast forward a year and I’ve got the same slipping starting.

The timing is coincidental, I believe. The weather isn’t all that different to account for a sudden change. What’s more likely is that when I recently replaced a flat tire, I didn’t set it all back together properly.

I have checked the adjustment of the wheel itself, the alignment of the gear cable, and tried to determine if anything was looser than normal. I also followed the directions in the IGH3 maintenance manual, but even tightening it as far as it’ll go didn’t seem to be enough to stop the occasional slip. Is there something I’m missing here?

Does anyone have any advice on how to properly tune an auto-tension belt?
I suspect the tensioner is working correctly, but the sprocket is starting to wear and is not holding the belt teeth properly.

You say that when the belt was slipping previously, it wore the sprocket away. I would not expect a reinforced rubber belt to severely damage a metal sprocket so presumably it is of a softer material such as aluminum or a composite plastic that is subject to wear even when the belt is not slipping.

Inspect the sprocket and see if there are wear marks visible on the teeth.

It’s also possible is that the belt itself is worn as well.