Shimano’s new Ultegra RX rear mech - first look

More mountain bike tech comes to the road

Earlier this year Shimano quietly launched Ultegra RX, a clutch-equipped rear mech for road and gravel bikes aimed at reducing unwanted chain movement to minimise noise and dropped chains.

With the rise of gravel, adventure and allroad bikes (we've built some stunners recently) it looks like the ideal addition for keeping the chain on when riding over uneven terrain.

Using Shadow Plus technology borrowed directly from its mountain bike line, available in mechanical and electronic versions, and costing the same as the regular Ultegra mechs, it looks like a no-brainer if you're riding bumpy rides, pave and off-road trails.




Shadow Plus comprises a one-way clutch mechanism integrated into the derailleur. It accounts for a little extra size and also ups the weight a small amount. There's a lever to disengage the clutch for easier wheel removal, but the idea is to leave it engaged all the time during normal riding - so it's ready whenever you encounter some rough roads or trails. It adds about 30-40g compared to a regular Ultegra mech.

With the clutch engaged, the derailleur cage is less able to move forward. The cage moving forward can happen when riding over bumpy roads, cobbles and gravel with a regular rear spring, the vibrations overcoming the regular spring. The taut chain minimises the chance of the chain slapping into the chainstay, which is commonly called chain slap, making for a quieter ride.

The biggest benefit is the reduced risk of the chain jumping off the chainring, an increased likelihood in cyclocross and gravel riding or anytime you're riding over bumpy terrain. It's especially important with 1x drivetrains as you are effectively removing a chain retention device in the front mech and the inner chainring for the dropped chain to fall onto. It's why a clutch-style rear mech is pivotal to SRAM's Force 1x groupset.

At the moment the Ultegra RX is designed for double groupsets and can be used on 11-28t cassettes up to the wide-range 11-34t model, with a 16t maximum difference between the small and large chainring. It's not specifically designed to be used with a 1x drivetrain but there's no reason why it can't be, other than its limitation to be used with a much wider range cassette.

Riding with the new Ultegra RX mech, in either mechanical or electronic form, reveals a quieter ride when cycling over bumpy roads or barrelling along gravel tracks. It's alarming how much the chain traditionally jumps around on cobbles or bumpy tracks - look at any photos from a race like Tour of Flanders or Paris-Roubaix and you'll see the chain really swinging around.

The Ultegra RX rear mech greatly reduces this. It keeps the chain taut all of the time so the drivetrain is always fully engaged, and offers slightly more accurate shifting under load when the pressure is on, as it usually is in a race or when you're going flat out.

You can turn the clutch mechanism off when you don't need it, but it's better to leave it engaged at all times - the shift quality is still smooth and crisp, if anything slightly sharper and less prone to mis-shifts, especially when rattling around a cyclocross course. Shifting is slightly heavier in mechanical guise, but there's no discernible difference with the Di2 mech.

There is going to be the potential for increased drivetrain resistance with a clutch-style rear mech. In real-world riding conditions (normal riding, not racing) you can't detect this, so lab testing would be needed to assess the difference. Likely the benefits outweigh this slight disadvantage.

The benefits for pure road riding are less appreciable, but if your riding regularly takes in really rough roads, cobblestones and you do a lot of gravel and off-road riding, the peace of mind offered by the new Ultegra RX rear mech is certainly a bonus. The fact they are priced the same as regular Ultegra mechs and come with the smallest weight penalty makes choosing one a no-brainer for an allroad and gravel bike build.

If you're tempted by a clutch rear mech, whether for your existing bike or on a new build, get in touch - we can advise on compatibility, parts and also arrange fitting in our workshops.