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