cocoabeachcrab 

i've noticed this function on new MTB rear derailleurs, (or should i say new to me)... what is this feature and how does it work?

gomango

I have one on my Yeti mtb.

Helps keep the chain on (tension) when the trail gets super bumpy.

https://flowmountainbike.com/tests/s...ar-derailleur/

non-fixie 

Thank you, guys. Both for posing the question and providing the answer. I was wondering about that.

nlerner

Yeah, essential for 1x drivetrains.

HTupolev

With a typical unclutched rear derailleur, the pulley cage uses a simple sprung pivot to tension the chain. This is fine for smooth roads and stuff, but when you're riding on chunky surfaces or doing gnarly stuff, the chain can get bouncy. It is somewhat possible to counteract this by using a stronger spring in the derailleur's cage pivot, but this causes the lower run of the chain to be under more tension all the time, which has some undesirable effects: the derailleur won't shift as readily and lightly, and it increases drivetrain friction.

A clutched derailleur addresses this by modifying the mechanism used for the pivot. The spring still exists, but the pivot also includes a friction plate, similar to the mechanism used in a friction shifter. When the chain tries to bounce and force the derailleur's cage to swing forward and release chain, the friction plate resists that motion. But, there's an obvious problem with this: we don't want the friction plate to impede the derailleur's ability to wrap chain back up. So the friction plate is attached to a one-way clutch that causes it to "disengage" when the cage is wrapping up chain (rotating clockwise when viewed from the drive side). So the "clutched" derailleur behaves responsively like a traditional derailleur when wrapping up chain, but adds resistance against motions that let chain out chain, and it does all this without necessarily requiring constantly higher chain tension than a traditional derailleur.

There are other ways to address the problem. For example, the new SRAM AXS rear derailleurs use a fluid damper on the pivot. This damper is not directional like a clutched friction plate: instead, it works by impeding fast motion in general. The fluid damper applies very little resistance to slow movements, like what the cage experiences from gear shifting. But when there are fast jerky movements, the fluid applies a LOT of resistance. It's like aerodynamic drag: it's not linearly proportional with your cycling speed, it ramps up with increasing steepness as you go faster.

cocoabeachcrab 

thank you! great explanation!

CiscoKydd

Is it also true that clutch derailleurs have a minimum of 10 speeds, so a 10-speed cassette is needed?

HTupolev

I don't think Shimano makes any clutched derailleurs for fewer than 10 speeds, but there's no technical reason that it can't be done. It looks like MicroSHIFT makes clutched 8-speed and 9-speed derailleurs for their Acolyte and ADVENT groupsets, and they offer variants for use in both 1x drivetrains and multi-ring drivetrains.

scarlson 

Would one of these be useful for a long-chain (AKA front-crossover AKA front-drive) tandem? Hmmmm.. I may try this.

Also, can't you use a ten-speed Shimano derailleur with a 9-speed Shimano shifter and it will just work? The same way I use an 8-speed cassette and shifter with a 9-speed XTR derailleur? The cable pull's the same, right?

cyccommute 

10 speed road will but I don’t think those are equipped with clutches. 10 speed and above mountain bike won’t work that way.

HTupolev

Not in general, no.

All of Shimano's 8-speed and 9-speed derailleurs use the same actuation ratio, which is to say, the amount of lateral movement that the derailleur does for a given amount of cable pull. That's why you were able to use the "9-speed" derailleur on the 8-speed drivetrain: Shimano 8-speed and Shimano 9-speed rear derailleurs are basically the same thing.

But when Shimano moved to 10-speed, they changed the actuation ratio of their mountain derailleurs. So a 10-speed Shimano MTB derailleur not index properly in an otherwise 8-speed or 9-speed drivetrain.

Where this gets really complicated is with the road parts.
When Shimano first introduced their 10-speed road stuff, they used the same actuation ratio as the 8/9-speed stuff. So the early 10-speed road derailleurs were largely interchangeable with 8/9-speed derailleurs.
When they introduced their 11-speed stuff, they changed the actuation ratio, making the 11-speed road derailleurs incompatible with their sub-11-speed drivetrains.
BUT.
In 2015, Shimano introduced the 10-speed Tiagra 4700 groupset. This groupset used the same cassette specification as previous 10-speed groups, but the rear derailleur uses the 11-speed road actuation ratio, with the shifter pulling however much cable per click to make this combination work. This is how Shimano does 10-speed road in general now; the newer GRX400 gravel groupset works the same way.
One of my friends recently broke the 11-speed R7000 rear derailleur on his gravel bike, and replaced it with a "10-speed" RX400, and it works great in the 11-speed drivetrain.

You're mostly right, but see above. Shimano's RD-RX400 rear derailleur is nominally a 10-speed road derailleur with a clutch, but it won't work in an 8/9-speed drivetrain because it uses the post-2015 Shimano 10-speed road spec.

scarlson 

Urgh, ok. Well, that's just too bad. Thanks for the information! I have never messed with any newer 10-speed stuff, probably because I've never gotten any for free.

pcb 

The other issue to keep in mind is that clutched rders are primarily designed to work with single-chainring/1x drivetrains, not considered compatible with or recommended for 2x/3x chainring setups. That's just what I've read, I've never tried using a clutch rder on anything other than a 1x crankset.

I don't want to put Prof. HTupolev on the spot, but maybe he can explain why that is. Or if what I've read isn't 100% correct, why that might be.

HTupolev

The main reason that a lot of clutched RDs are designed around 1x drivetrains is that many of the use cases where clutched derailleurs are especially important - such as mountain biking - are situations where a lot of people find 1x attractive anyway.
Clutched derailleurs for multi-ring systems can work fine, and are often available in catalogs alongside 1x counterparts. For example, in Shimano's GRX800 gravel groupset, the RD-RX810 is intended for 2x systems, while the RD-RX812 is intended for 1x systems.

There are a few reasons that single-ring versus multi-ring variants exist.

One is the gearing arrangements they tend to be used with. 1x drivetrains often use very wide cassettes to achieve adequate gearing range, so the derailleurs need to be geometrically capable of clearing very large cogs. Multi-ring drivetrains tend to be paired with smaller cassettes, but the front shifting means that they sometimes need to deal with huge amounts of chain wrap, and in this case they need very long cages.

But a big part of it is that, since "1x" rear derailleurs don't need to tolerate front shifts, they don't really need to leverage a slanged parallelogram. Instead, they can use a large offset between the cage pivot and the jockey wheel, and allow the relationship between the jockey wheel and the cogs to be controlled by the rotation of the cage as it wraps and unwraps chain. One benefit of this is that the jockey wheel's trajectory across the gearing range can just naturally adjust itself for different cassettes.
But avoiding a slanted parallelogram has mechanical benefits as well. Take a look at a SRAM 1x derailleur, like this. They use a dropped parallelogram with no slant at all. The benefit of this is that, when the chain tries to bounce, the jolts are hitting the derailleur at an angle where they won't really be actual to actuate the parallelogram. And this is actually a bigger benefit with clutched derailleurs than with unclutched ones. In an unclutched system, because the derailleur's cage is so willing to bounce in response to a jolt from the chain, the rest of the derailleur mostly avoids sharp stresses while the bounce gradually dissipates. But on a clutched system, where the cage isn't very willing to budge when the chain yanks on the derailleur, other parts of the derailleur are more likely to move. And a slanted parallelogram is at an angle where a chain jolt can cause it to budge. Sharply budging the parallelogram can have weird consequences, like putting sharp stresses on the high-limit screw or the shift cable, and it's just generally a sloppier behavior than if it doesn't budge.
This isn't to say that all 1x derailleurs eliminate the slanted parallelogram, but a lot of them do. All SRAM 1x derailleurs do, and so does the new Campagnolo Ekar.

non-fixie 

This has become a very informative thread indeed. Thank you!