This explains:
https://garagistic.zendesk.com/hc/en...inum-Bushings-
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This material is excellent for track-oriented or dedicated track vehicles as it does still offer some vibration absorption over solid aluminum while still being extremely rigid. Unlike polyurethane, Delrin does not have the ability to deform slightly and return to its' original shape. If your vehicle is likely to see impacts frequently (drifters) this material may not be the best for you.
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