Shifting opinions
Clutched rear derailleurs are becoming more prevalent – but what do they do, and do you actually need one?
The great sport of cycling has a history of adopting technology from one genre and applying it to another, and in recent years the trend in road cycling has been to look to mountain bikes for developmental inspiration.
Rim brakes are being superceded by discs, suspension systems are being introduced to a small degree and now we’re seeing clutches on rear derailleurs make the leap too.
‘A clutch is a device that damps a rear derailleur’s cage motion and thus checks the chain’s motion,’ says Anthony Medaglia, Sram’s chief system engineer for road drivetrains. ‘It essentially creates friction to absorb the energy of the rotating cage, but only in one direction. This can be achieved via a mechanical device, commonly referred to as a clutch but more appropriately described as a brake, and other types of energy- absorbing mechanisms.’
Shimano’s Ben Hillsdon adds, ‘By restricting the forward movement of the cage, clutched derailleurs reduce the chain slap or chain bounce that can occur when riding over rough ground.
This ensures stable performance for a single chainring setup and smooth and silent riding over rough terrain.’ In other words, the risk of dropping your chain or mis-shifting over bumpy ground is minimised by using a clutched mech.
The clutches found on road and gravel rear derailleurs are similar in function to their mountain bike equivalents but differ in aspects such as the amount of tension they add, which can be lower as less extreme impacts are likely to be experienced.
Each of the three main groupset manufacturers has opted to deploy a different type of clutch mechanism. Campagnolo’s Ekar and Shimano’s GRX clutches add resistance mechanically but where Ekar’s is set permanently in place, GRX’S can be turned on and off via a discrete lever. ‘The lever compresses the O- rings within the derailleur pivot points to add more resistance to the pivot movement,’ says Hillsdon. ‘We’ve found the lever mechanism to be maintenance-friendly and easy to adjust. Next to that you can turn it off for easy removal of the wheel or when riding on smooth surfaces.’
Meanwhile, the clutch mechanism on Sram’s latest AXS road derailleurs is different again.
‘Our Orbit fluid damper mechanism channels a viscous fluid through an orifice, creating friction that adds additional reaction force to the motion of the cage,’ says Medaglia.
It doesn’t impinge on low-speed movements such as changing gears but resists high-speed movements of the cage caused by jarring impacts.
To clutch or not to clutch?
The purported benefits of a clutch make their inclusion on road derailleurs as standard seem like a no-brainer, yet only Sram is championing their ubiquitous use.
‘The rougher the surface, the more difference it makes. More than twothirds of US roads are unpaved, and our paved infrastructure is deteriorating, which I’m sure you have to deal with in the UK too,’ says Medaglia. ‘Many riders are choosing to ride more challenging terrain on their road bikes too, and managing the chain’s motion improves ride quality for nearly all road riders.’
Shimano and Campagnolo are more reserved in their opinions, suggesting clutches aren’t all that necessary for the comparative smoothness of road riding and may actually come with performance drawbacks.
‘Road riding is still predominantly
2x, and in that case a front derailleur helps protect against dropped chains,’ says Giacome Sartore, Campagnolo’s groupset product manager. ‘Our Ekar rear derailleur has a clutch, but gravel and road riding have different needs. Without one our road derailleurs can be lighter.’
There is roughly a 40g penalty to Shimano’s clutch-equipped GRX Di2 design compared to its Ultegra Di2 equivalent, plus Hillsdon suggests there is additional friction during shifting. While overall drivetrain efficiency isn’t affected, that extra shift resistance may drain battery life or stiffen the feel of mechanical shifting.
Technological development is never something at risk of stagnating in the bike industry though, so are we likely to see more clutch-equipped derailleurs in future if the weight can be reduced and extra resistance minimised?
Campagnolo remains reluctant to include them in its road groupsets, at least for now. Sram however has already shown it is possible to out-design some of a clutch’s drawbacks, so Medaglia is more optimistic.
‘Better chain management makes a better ride experience. It’s quiet, shifts better and reduces the risk of chain drop. It will likely become a basic feature that everybody expects, especially as we see the use of road drivetrains in gravel applications. Overall it improves the experience for the user under the wide range of conditions that today’s riders put their bikes through.’
‘Many riders are choosing to ride more challenging terrain on their road bikes, and managing the chain’s motion improves ride quality for nearly all road riders’