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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?

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  • Good question. I think the clutch is simply adding friction, in a way that prevents fast movements of the derailleur assembly (but still permits slow movements, which is all that's needed for shifting) – but I'm not sure it's actually quite so simple. At any rate it's clear that a derailleur clutch is in no way analogous to the clutch in automobiles and motorcycles. Feb 5 at 20:47
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    I edited your question's title, feel free to revert or change it if you disagree with my edits.
    – MaplePanda
    Feb 6 at 0:24

3 Answers 3

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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.

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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.

enter image description here

(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):

enter image description here

enter image description here

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

enter image description here

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?

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  • I know this style of mechanism generally as a "sprag clutch" in machinery, whereas in motorbikes its called a "slipper clutch" but serves a similar purpose.
    – Criggie
    Feb 6 at 0:33
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    @Criggie I tend to use "sprag clutch" to describe the type of one-way mechanisms with oddly-shaped (i.e. non-cylindrical) moving bits. Perhaps "one-way bearing" or something better describes the mechanism involved here.
    – MaplePanda
    Feb 6 at 0:38
  • Many questions: The friction (under counterclockwise rotation) arises from metal-against-metal; is that right? This would mean that the lifetime of the clutch is considerable—unlike traditional car clutches, which have a metal-against-polymer design IIUC. Also, it seems that the octagonal teeth, when rotated CCW, expand a rubber-like (black) ring, which in turn presses against a metal band, expanding it ever so slightly to introduce friction between it and the outer, adjustable, band. Is that roughly how you also see it?
    – Sam
    Feb 6 at 1:10
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    Actually, I don't believe I feel a difference in shifting with clutch on or off. That is to say, if there's a difference in shift quality, it's small.
    – Weiwen Ng
    Feb 6 at 13:32
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    There is definitely a difference in shifting with the clutch on vs off, but only in one direction of shifting. When you shift a derailleur, the cage has to rotate to accomodate the change in chain length around the cassette gear and keep tension on the chain. Therefore, when the cage rotates in the direction that the clutch mechanism is intended to resist (counter-clockwise), which occurs when shifting up the cassette into an easier gear, the sprag clutch/bearing has to slip inside the tension band. This difference is much more noticeable in older, more worn derailleurs
    – jezza_99
    Feb 6 at 23:10
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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.

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  • This is a good additional point, but like Weiwen Ng's post, it does not answer the question. Feb 5 at 20:44
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    There were two questions: "how does it work (mechanically/technically)?" and "how does it influence gear shifting?"
    – Useless
    Feb 6 at 11:02
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    This may be a critical point, because if you’re wondering whether your three-year old clutch requires service, it would make the following a reasonable test. Hoist on a stand. Shift to smallest cog. Crank the pedals as fast as you can. Stop spinning suddenly. If the chain either derails or slaps the chainstay, the clutch is due for a cleanup. For increased rigour, crank backwards as soon as you cease cranking forward.
    – Sam
    Feb 6 at 16:13
  • @Sam The easier method is to rotate the derailleur cage in the clutched direction by hand, and feel for roughness. Alternatively, you can just perform the 5 minute clean and regrade procedure every year.
    – MaplePanda
    Feb 6 at 20:13
  • @MaplePanda Here youtu.be/8TqdoAAn-go?t=1 he shows us how it sounds when it needs maintenance. On normal (I assume) Shimano and SRAM RDs with clutches, I get a different feel. On Shimano it's snug at first, then it's smooth, but resistive. On SRAM the initial snugness isn't there, but it's resistive throughout.
    – Sam
    Feb 6 at 22:22

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