Its a small plastic item that rivets onto the underide of the bottom bracket. It allows the gear cables to pass under the BB and run smoothly in the channels carved into it.
I need a new one but have no idea how to search for one!
Just be aware that these can be frame specific in some cases. The one in the CRC link above is the generic one for use on bikes with "standard" sized BB shells (like steel frame for example), and is intended to work with Shimano front derailleurs, because that's how it routes the cable. The Campagnolo ones are *slightly* different shape channels, although pretty marginal.
If you have a carbon frame with a massive BB shell, you may find these don't really fit as the curve is too tight in the plastic to seat properly, in which case you need to get one from your frame manufacturer. In particular, the carbon frame ones are sometimes shaped to route the front derailleur cable through a (small) hole in the BB shell to get a nice straight pull line on the mech. That means they are a different shape again.
Open One+BMC TE29Seven 622SLOn One Scandal Cervelo RS
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If you have a carbon frame with a massive BB shell, you may find these don't really fit as the curve is too tight in the plastic to seat properly, in which case you need to get one from your frame manufacturer. In particular, the carbon frame ones are sometimes shaped to route the front derailleur cable through a (small) hole in the BB shell to get a nice straight pull line on the mech. That means they are a different shape again.