What exactly is the "clutch" doing on a Shimano M8100 rear derailleur?

Question

My new Shimano 12s XT 8100 rear derailleur has the switch with on and off positions. From that I found on the web, it is a "clutch" and I must turn it off when removing the wheel. I can shift the gears with the switch in both positions without noticing much difference. Web says the clutch makes the gear shifting better, still.

I was not able to find technical explanation, how does this this clutch actually work and how it influences the gear shifting. A clutch like in a car would disengage the engine (me?) from the wheels. It does not look doing anything the like. What it technically does?

Answer 1

I assume you're talking about Shimano's 12s XT rear derailleur. For disambiguation, 8100 could also refer to the 12s Ultegra groupset. However, the Ultegra group doesn't have a clutch, and I believe Shimano doesn't intend to put clutches on its road groups (in contrast, SRAM has clutches, all of Shimano's gravel groups have a clutch).

When you ride over rough terrain, the chain bounces a lot. This increases the odds of the chain falling off the front chainring. Adding a clutch to the rear derailleur means that the derailleur cage can't swing forward - this is why you need to deactivate the clutch to remove your rear wheel. This decreases the amount by which the chain can move when you're going over bumps. In turn, this decreases the odds of the chain falling off the front - particularly helpful for 1x groups.

Hence, the clutch does nothing to affect the shifting. It most likely does not increase drivetrain friction. It improves chain retention and decreases chain slap in rough terrain.

Answer 2

I'll try to answer the question of exactly how the clutch works. In essence, the Shimano clutch design uses a one-way bearing that allows the cage to freely rotate clockwise, but adds friction if rotated counterclockwise. When activated, it increases the torque needed to move the cage by around 3-7Nm.

(Picture from BikeRadar)

The clutch lever (orange) rotates a cam (oddly shaped metal piece on the right side), much like what flipping the lever on a QR skewer does. This makes the tensioning band tighten around the central clutch unit (cylinder-like thing with the octagonal metal piece on it). The cam presses the tensioning band against the tension adjusting screw (uppermost piece), which by screwing it in and out allows you to change the clutch stiffness.

I'm not 100% sure what happens inside the clutch unit, but I believe it's a roller bearing cam system. There are long, cylindrical needle bearings inside which are constrained by specially-designed slots. They can freely rotate when the unit is spun one way, but they jam and lock the unit when spun the other way. I believe that in the jammed direction, the outside of the clutch unit rotates in the tensioning band. The compressive force creates friction here.

This explanation is supported by how Shimano instructs you to lubricate the outside of the clutch unit, but to leave the inside dry. These types of roller bearing clutch don't work very well (if at all) when lubricated.

Here, you can see the axle (with the 4mm hex fitting) around which the clutch unit rotates, and then the clutch unit separate from the tensioning band (Images from NSMB):

Here, you can see the inside of the clutch unit. Note the skinny little needle bearings (MTB Direct Australia):

Note that the clutch does not add extra tension to the chain, as some people may claim. It is a passive unit which simply prevents rotation--it doesn't actively apply a spring force or anything. As for shifting feel, I do notice that shifting feels slightly heavier with the clutch on, but there's no obvious reason for why this is. Perhaps some amount of cage rotation is needed when moving the derailleur side to side?

Answer 3

On top of Weiwen Ng answer, the clutch also has another benefit. Large cassettes have a lot of inertia, when you stop pedaling (at high speeds), it keeps rotating. If the derailleur doesn't have a clutch (or at least a strong enough spring), the cassette will keep rotating and driving the chain along with it (then decreasing the tension on the upper part of the chain). It will do so as long as the derailleur can "fold" (with a clutch it will stop very soon). This phenomenon can contribute to the chain falling off the front on terrains that seem smooth.