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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've never heard of an auto-tensioning belt other than something with a belt tensioner like a derailleur. – Criggie Nov 9 '18 at 3:25
  • @Rider_X good spotting! If that's it then there's definitely a tensioner that is underperforming. – Criggie Nov 9 '18 at 7:42
  • Wasn't there a manual that came with the bike? Or could there be an online manual? – Carel Nov 9 '18 at 7:59
  • @Carel, there is a maintenance manual available online (as mentioned in my third paragraph). – Ian MacDonald Nov 12 '18 at 14:41
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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.

  • The spec page that Rider_X linked in the comments says that the chain ring and rear cog are "composite", i.e., most likely glass fibre reinforced plastic. – David Richerby Nov 9 '18 at 13:14
  • It will certainly start wearing, yes, but it wasn't happening at all before I reset my wheel. I suspect I did something wrong when I put it back and would like to avoid further damage. – Ian MacDonald Nov 9 '18 at 14:11
  • @IanMacDonald. Does the frame have slotted dropouts that allow movement of the wheel fore and aft? (it's hard to see from the Trek archive page) If so, you may not have enough tension on the belt. – Argenti Apparatus Nov 9 '18 at 14:21
  • @IanMacDonald I did some more research and see that the District has removable dropouts (to install the belt) with fore-aft adjustment. Did you adjust those when re-installing the wheel? – Argenti Apparatus Nov 9 '18 at 14:26
  • No, the slotted dropouts are not appropriate for an auto-tensioner. The dropouts on this frame go straight down. I did not adjust the dropouts, and I'm not sure that they are adjustable. I would have to check. – Ian MacDonald Nov 9 '18 at 15:05

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