First spotted at the Tour of Flanders, Shimano has officially released a clutch-equipped electric road derailleur, the Ultegra Di2 RX805.
The idea behind the clutch, a rear derailleur feature that is already prominent in off-road groupsets, is to reduce chain slap and chain suck, while improving chain retention and shift accuracy when riding over uneven surfaces. Called 'Shadow RD+', the one-way clutch also offers an on/off switch to engage or disengage the mechanism.
When the lever is in the 'on' position, a cam tightens a steel friction band around the rear derailleur's main pivot. This keeps the chain taut by reducing unwanted movement of the cage when riding over rough terrain.
The use of the on/off switch is very useful on roads bikes so that the clutch can be employed when it's needed - and turned off when it isn't, since the clutch does add a small but measurable amount of friction.
Unlike SRAM’s clutch-equipped Force 1 and Rival 1 road rear derailleurs, which are designed exclusively for use with 1x drivetrains, the Ultegra Di2 RX rear derailleur can be used with two chainrings or a narrower-range 1x system.
This medium-cage road rear derailleur is compatible with all of Shimano’s 11-speed road drivetrains. It can be used with 11-28t cassettes up to the company’s wide-range 11-34t model.
When it comes to front chainring range, the maximum difference between the large and small chainrings is 16t. This means the Ultegra RX805 will work with most common chainring combinations, including 53/39, 52/36, 46/36, as well as the 48/32t sub-compact system that is becoming increasingly common on gravel bikes.