Shadow Plus was first introduced in 2012 with the Shimano XTR rear derailleur. It prevents chain slap and keeps the drivetrain quiet. It also reduces the risk of a chain jumping off.
This result is achieved via a sliding clutch in the lower joint of the derailleur arm. If it is activated via the Shadow+ lever, the pressure on the pivot joint increases.
This increases the rotational resistance in the joint and thus counteracts the movement of the arm, which would otherwise rotate forwards more easily against the direction of travel in the event of vibrations. An explanatory video is available here.
A widespread opinion is that Shadow + increases the chain tension. As explained above, this is not the case.
Recommendation: Shimano recommends activating this for rear derailleurs with Shimano+. This means that the chain hits the chainstay less on rough roads or potholes. Shadow+ should be temporarily deactivated when removing the rear wheel
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