Shimano didn’t reinvent the wheel — or derailleur — in developing the Ultegra RX rear derailleur. Instead, the company combined its proven R8000 series rear derailleur with the Shadow Plus clutch system it has used on its mountain bike groups since 2012.
That's not to say it's exactly the same as the company's clutch-equipped Di2 mountain bike rear derailleurs, which have been used for several seasons in lieu of purpose-built derailleurs for gravel and cyclocross use.
According to Shimano’s Hiroshi Matsumoto, who oversaw the development of the Ultegra RX rear derailleur, clutch tension on the new derailleur is lower than the company’s Shadow Plus mountain bike derailleurs.
“We found that the experiences were very different on gravel and on the mountain bike. On the mountain bike, the derailleur deals with large impacts, where on gravel, there is a constant level of vibration that affects shifting,” Matsumoto said.
For those who need a quick primer on Shimano’s terminology, “Shadow Plus” refers to the use of a one-way clutch built into the rear derailleur. Unlike SRAM, Shimano offers an on/off switch to engage or disengage the clutch mechanism. When the lever is flipped to the ‘on’ position, a cam tightens a steel friction band around the derailleur’s main pivot. This keeps the chain taut by reducing unwanted movement of the derailleur cage, which improves shifting.
Shimano Ultegra RX Di2 rear derailleur gravel tested
I tested the Ultegra RX Di2 rear derailleur over three days and 140 miles on the gravel roads of Minnesota. My time riding this new derailleur included racing an Ultegra Di2-equipped Allied Alfa All-Road in the Almanzo 100, one of the largest and longest-running gravel events in the United States.
This new derailleur was mounted to a Dura-Ace Di2 group with a 50/34t crankset and an 11-32t cassette. The Ultegra RX rear derailleur has a maximum chainring capacity of 16t, which is large enough for all major chainring combinations such as the aforementioned 50/34t as well as the 48/32t combination that's increasingly common on gravel bikes.
During endurance gravel events such as this, little nuisances can start to wear riders down. A squeaking cleat, a creaking seatpost or a clicking chain can all take their toll on a rider’s psyche as a long day in the saddle drags on.
As expected, chain retention was excellent. What was surprising was how the Shadow Plus system kept the drivetrain virtually silent. Shimano’s Di2 groups have been praised for their shift accuracy; Ultegra RX raises the bar in shifting over rough terrain.
Gravel, potholes and braking bumps can delay or degrade shifting with traditional drivetrains. With the Shadow Plus system engaged, rough and choppy roads became a non-issue. Shifting was flawless.
BikeRadar has discussed the possibility of a small but measurable amount of drag from the increased chain tension that comes along with using the Shadow Plus System for road applications. After testing, it seems equally plausible that the decrease in vibrations through the drivetrain and increased shift accuracy that comes from a stabilized chain could offset such losses, at least on gravel and cobbled roads.
While wide-range 1x groups have gained popularity for gravel and cyclocross, the large steps between gearing can be off-putting to some riders, including this tester. This brings us to what may be the biggest competitive advantage of Shimano’s Shadow Plus technology. Unlike SRAM’s Force, Rival and Apex 1x groups, Shimano gives riders the freedom to choose one chainring for simplicity or two for smaller steps between gears over a wide range.
Shimano Ultegra RX Di2 rear derailleur overall impression
This clutch-equipped rear derailleur is an overdue addition to Shimano’s component line. That said, it’s clear the company is acutely aware of the changing nature of the road bike and is taking steps to meet the needs of drop bar cyclists who don’t want to be confined to pavement.
The Ultegra RX Di2 rear derailleur combines proven tech from the trail with the refined feel of Di2 shifting at a price point that, while expensive, is approachable for riders looking to upgrade their Ultegra or Dura-Ace Di2 groups for improved performance on roads less traveled.