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Took apart my Shimano Acera derailleur cage to clean it up a bit and didn't bother to note the shape of the tension pulley which is a bit thicker on one side. It seems like it would be shaped this way for a reason since the guide pulley is symmetrical. Should the thicker side face toward or away from the wheel?

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Edit: Here's a close-up showing an arrow or #1:

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Update:

Per user in comment section, the arrow is indeed an indicator as to which way the pulley should be installed. See below excerpt taken from an unrelated Shimano derailleur model:

https://si.shimano.com/pdfs/dm/DM-TRRD001-00-ENG.pdf

  • Depending on the model, the guide pulley has arrows on it to indicate the direction of rotation. Install the guide pulley so that the arrows are pointing counterclockwise when looking at the outer side of the derailleur.
  • The tension pulley has an arrow on it to indicate the direction of rotation. Install the tension pulley so that the arrow is pointing clockwise when looking at the outer side of the derailleur.
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  • 1
    Generally speaking, the cuppy thing goes on the outside.
    – Daniel R Hicks
    Sep 16, 2019 at 22:44
  • Does the jockey wheel have direction of rotation arrows printed on it? If so that will tell you which way around it should be installed. Note that the top and bottom wheels are different, the top wheel usually has a couple of mm lateral play built in.
    – Argenti Apparatus
    Sep 16, 2019 at 22:58
  • @DanielRHicks By outside, you mean facing away from the wheel?
    – oscilatingcretin
    Sep 17, 2019 at 0:19
  • @ArgentiApparatus Nope, there's nothing on the tension pulley except for the number 1 and also the acronym POM next to a recycle symbol
    – oscilatingcretin
    Sep 17, 2019 at 0:22
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    @Criggie The guide pulley has no arrow (or numeral). My guess is that it doesn't require a particular direction of rotation due to its symmetry. The t-pulley probably needs to rotate in a certain way due to its asymmetrical design, perhaps as a means to somehow balance/offset tension forces (I am just guessing here)
    – oscilatingcretin
    Sep 17, 2019 at 12:35

2 Answers 2

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Often jockey wheels have direction of rotation arrows printed on them. That will tell you which way around it should be installed.

Note that the top and bottom wheels are typically different, the top wheel usually has a couple of mm lateral play built in.

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I checked several from a variety of years. If they weren't symmetrical the thicker side was toward the wheel.

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