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Shimano GRX RD-RX812 Rear Derailleur


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GRX RD-RX812 Rear Derailleur

If you've dabbled in gravel racing and riding, chances are you already know how important a large gear range and tidy setup is, and if you have yet to experience it first hand, we can assure you, that we covet simple drivetrains with few opportunities for failure, and as many gears as possible to get us up poorly-graded hills that make up the Crusher and Kanza. With that said, it took quite some time for component manufacturers to jump on the gravel bandwagon, but with the Kanza's starting line amplifying from sub-50 cyclists to over 3,000 in just twelve years, its a welcome sight to see Shimano launch its GRX gravel-specific groupset that finds the perfect balance between MTB and Road, with a sleek and streamlined drop-bar setup, and improved modulation and bump compliance to handle off-road conditions. Shimano's GRX RD-RX812 follows in the footsteps of Ultegra RX which was launched last year, which took queues from the Shadow mountain bike derailleur, adding in a clutch mechanism to reduce chain slap and dropped chains for a quieter and more secure ride both on and off-road. When in the on position, the clutch increases the spring force to provide crisp shifting and zero chain slap and is easily flipped off for wheel changes. What we find to be big news is that Shimano crafts the GRX RD-RX810 for both 1x and 2x setups, stepping outside of the 2x only comfort zone that the brand had held to, with this short-cage model being optimized for an 11 - 34t cassette.

Following in the footsteps of the revamped Dura-Ace and Ultegra groups, this GRX derailleur gets its Shadow cage treatment placing it closer to the frame and under the cassette keeping it out of harm's way and less likely to take the brunt of a driveside crash. This also allows the derailleur to move in a much more linear path allowing for shorter throws with less effort. The RX812 model comes in one cage length, long, which accommodates an 11-42t cassette allowing for a wide range gear ratio when paired with a double without the usual huge jumps between the cogs like on a 1x setup, ranging up to an 11-42t cassette.

Sticking to what the brand's customers know well, Shimano continues to use the Dura-Ace, Ultegra, and Tiagra designation with the GRX line, with this particular model falling under the Ultegra umbrella, which means it gets the usual materials and finishes that we expect from a groupset that's just one step from the top. Its weight, smoothness, and durability make it more than capable of racing against the competition on course, or the sun to your campsite for the night on a bikepacking trip.

  • Shimano's off-road ready derailleur for charging down rough gravel roads
  • Shadow clutch mechanism increases spring tension to eliminate chain slap
  • Clutch is easily flipped to make wheel changes and flat fixes a breeze
  • Derailleur cage tucks closer to frame to avoid hits and bumps
  • Durable alloy construction handles mud and off-road adventures
  • Works with current Shimano 11-speed road shifter/brake levers
  • Low-profile Shadow design helps avoid damage from impacts
  • Long cage allows you to run lofty 11 - 42t cassettes
  • Item #SHIU19E

Cage Length
Cassette Range
[maximum] 11 - 42t, [minimum] 11 - 40t
current road dual control lever, GRX levers
Claimed Weight
9.4oz (267g)
Recommended Use
Manufacturer Warranty
2 years

Tech Specs

What do you think about this product?


>Rating: 5

GRX RD 11-42

I've put it through the wringer
True to size
6` 3"
230 lbs

I was looking at using the Tanpan device to get lower gearing on my RLT9 RDO. The excellent mechanic at my LBS recommended the GRX RD 11s 42t capacity and I couldn't be happier. I paired with a Praxis carbon crankset 32/48. I am using an Ultegra FD and the stock Ultegra 6800 shift/brake levers that came with the bike. 11-42 rear XT cassette. While I have mixed 3 different groups everything works perfectly.

>Rating: 4

A Big Step in the RIght Direction

I've used it several times

Gravel has been the fastest growing segment for the past few years and Shimano has finally done something about it. GRX is a much needed drivetrain but doesn't quite go far enough with gearing for us casual riders in the mountains. To start with, performance and looks are on par with Shimano's road groups, so really good. It's a very clean looking groupset. So if you've liked your Ultegra drivetrain but want easier gearing, going to GRX makes good sense. There's some compatibility between the two groups too which is nice. This rear derailleur interestingly enough has only a 5mm shorter cage than an SLX which fits a 46t cog so despite Shimano's stated max cassette size as 42t I plan to try it with an 11-46t eventually just to see what happens. I've had a 105 shifter paired with an SLX rear derailleur and 11-42t rear cassette (using Wolf Tooth's excellent Tanpan adapter) on the bike until now so I already knew what to expect for gearing but GRX really does shift crisper without that adapter and this group feels lighter weight. So with no substantial complaints on the quality of this group my big gripe is the gearing options. Shimano has provided good options for a lot of riders on flatter terrain and for gravel racers but us bikepackers and for those big days in the mountains we need better options. Sram's AXS drivetrain mullet builds are popular for that giant 10-50t range, Shimano should have used their Micro Spline freehub to allow for a 10t cog and made the derailleur for 46t cassettes, then with more chainring options they'd have a perfect, more versatile group.


Is grx compatible with ultegra shifters?

Looking forward to hear how it works with the 46t. I'm running an xt/tanpan with eyes on going to the grx rd but dont want to limit myself to a 42.

Looking forward to hear how it works with the 46t. I'm running an xt/tanpan with eyes on going to the grx rd but dont want to limit myself to a 42

Looking forward to hear how it works with the 46t. I'm running an xt/tanpan with eyes on going to the grx rd but dont want to limit myself to a 42

Daniel, I too am very interested in your experience with 11-46 cassette? I am considering the same thing, just got a 3T Exploro 3 weeks ago now and want a lower gear!

Daniel, I rode my Ridley X Trail Gravel Bike yesterday up some ridiculously steep climb here in LA. I was running a GRX crank so a low gear of 30 x 34. Before that ride, still riding lots of hills and struggling but yesterday was just Brutal! Why has the gravel industry led us to believe that 1:1 is the lowest gearing we'll need? So Force AXS comes out this week with a 31 x 36 or something for a $2800 drivetrain? None of this makes sense when you are riding very very steep terrain as many of us want to do on gravel bikes. I bought this rear derailleur yesterday with a Shimano SLX CS M7000 11 x 42 cassette. I am hoping that I can finally find some thing I can use to spin up the mountains. Already I have to sell my X Trail frame that I bought just 1.5 years ago because the rear clearance only gets me a 36mm tire. The gravel industry is finding their way but unfortunately its us consumers who are the ginea pigs throwing down thousands $ hoping the equipment will work- I have done just that and in the end, hope just to enjoy riding my bike instead of the brutal suffering of riding gear that is ill-equipped and not ready for prime time.


I am considering moving to 1x and would need this to get the gear range I want. However, this bike has an extra set of wheels with an 11-28 road cassette, while the derailleur has a minimum max cog size of 40t. Will it work with the smaller cassette, with the understanding it may not shift as well as something closer to spec? Thanks.


I was wondering if this would work with Shimano 105 11 spd shifters?