Clutch derailleurs are designed to prevent the chain from falling off the chainring and cassette on rough terrain. They can also significantly quieten a bike and reduce chain slap.
When they were first released, Shimano was quick to explain why its clutch derailleurs were a revolution, and SRAM quickly made its own version.
But now, Shimano’s latest and most expensive MTB Di2 derailleurs do not even have a clutch, relying on a stronger dual spring instead.
- Read more: Shimano XTR Di2 M9200 review: Wireless at last, but sorry Shimano, it lacks the wow factor
Latest Di2 does away with the clutch
Shimano's clutch uses a steel band wrapped around a one-way bearing. The steel band is adjustable and adds friction to the derailleur when the cage is moved forwards, but not when it moves backwards. It can be disengaged with a lever on the front of the derailleur to ease removal and refitting of the rear wheel.
Shimano’s latest Di2 derailleurs use a revised system to tighten the chain. Instead of a clutch, they employ a dual-spring design, where the first spring is low-tension and always engaged. The second spring cannot be disengaged via a lever (as with Shimano’s clutch system), and unlike the clutches, the twin-spring system is non-adjustable.
Shimano was quiet about the reasons for getting rid of the clutch on its latest Di2 derailleurs, only offering that they enabled the derailleur to be lower-profile by way of explanation.
Instead, the latest Di2 (XTR, XT and Deore) designs all use a dual-spring stabiliser instead of a clutch, with clutches still employed on Shimano’s cable-actuated derailleurs.
SRAM’s Eagle derailleurs, including their top-spec wireless AXS models, all still use a non-adjustable clutch.
Coming in clutch
Chris Porter is one of the most outspoken critics in the bike industry, as a former journalist turned suspension-tuning guru. Porter told us he discovered how clutch derailleurs hamper suspension in dramatic fashion.
“It was so intrusive to suspension movement that when I put the bike upside down and then removed the rear damper, the rear wheel stayed put,” he says.
The reason a clutch in a derailleur can have a detrimental effect on the suspension is due to its primary purpose – to prevent chain movement. In a suspension system where there is chain growth through the travel (present in most full-suspension frame designs), Porter explains, “you will feel the clutch as a resistance to initial movement of the rear suspension at any point in the travel”.
Porter suggests a test you can do on your own to see the effect of a clutch on suspension. “Get a friend to move the jockey wheels out of the way to give some chain slack and you can literally feel the difference as you push up and down on the saddle,” he says.
“I always removed the clutch assemblies from my Shimano derailleurs as even when switched off, they would rust and stick.”
Porter goes on to say it’s “good to see the big S have ditched the clutch”.
Is the new Shimano system with a spring an improvement?
Shimano’s new dual-spring arrangement is designed to replace the clutch. It works in a profoundly different way.
Porter explains: “A spring has no resistance to speed of movement, so transfers less energy through the system when asked to move either a small amount or move quickly.
“A clutch has a fixed amount of initial resistance to movement, no matter how small or large, or fast or slow the input. Therefore, it must transfer a fixed amount of energy through the system until it reaches the point where the clutch pops open.”
Porther explains why the resistance offered by the clutch can affect a bike’s rear suspension. “Every single movement of the suspension must reach that threshold in compression direction before the suspension moves, not so with a spring,” he says.
So, even a strong spring in a derailleur will have less effect on a bicycle’s rear suspension than a clutch.
Can a clutch and a strong spring be combined?
Porter hints at his dream derailleur setup when he says: “Now can someone please just put a 9- or 10-speed derailleur system in a front triangle with a transfer drive to the rear wheel!”
Perhaps bikes with the Lal Bikes Supre will serve as an interim step towards the drivetrain evolution Porter’s waiting for. This system relocates the rear derailleur body inside the rear triangle, and uses a spring and damper in conjunction to tension and stabilise the chain.
In the meantime, bicycle gearboxes remain the only solution for riders who want to do away with the weighty cassette or internally geared hub but still run gears – unless you want to go back to the two-speed Truvativ HammerSchmidt and a single rear-cog setup.
