That plastic disc will stop the chain from eating into your spokes if it goes too far over, but it isn't going to stop a rear derailleur that is hit on a rock. The plastic disc is rendered useless by a well-tuned derailleur stopping the chain from coming over and does nothing in the way of a bent hanger.
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"Mrs. Claus and I watched the Collective Anthology last night and I woke up this morning to a note saying she left me for some guy named Steve Peat."
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