Shimano GRX Di2 12-speed Drivetrain
Stated Weight:
- GRX RX825 group: 2,522 g (full group including chain, cassette, battery, wires, and brake calipers and discs)
- GRX SPD M8100-UG pedals: 342 g
MSRP:
- GRX RX825 group: $2,533.87 USD (full group including chain, cassette, battery, wires, and brake calipers and discs)
- GRX SPD M8100-UG pedals: $138.00 USD
Intro
The battle of the electronic drivetrains continues, and while SRAM remains alone among the two Big S brands in offering a fully wireless option, Shimano is pushing the evolution of their time-tested Di2 drivetrains. Having launched 12-speed options for the mechanical GRX groups just last year, Shimano is now updating their Gravel-specific GRX Di2 drivetrain with a 2 x 12 option — RX825.
The big news is that, as with its latest Dura-Ace, Ultegra, and 105 Di2 Road drivetrains, the new RX825 lineup is now semi-wireless. That means wireless levers for much easier setup, while the rear derailleur and front derailleur still share wired connections to the central battery mounted in the frame.
There’s quite a bit here, so let’s dive into the details of the new RX825 lineup.
GRX ST-RX825 Hydraulic Disc Brake Dual Control Lever
MSRP: $556.99 USD (price includes BR-RX820 brake caliper)
Stated Weight: 830 g / pair (including brake calipers and hoses)
The updated ST-RX825 levers are the heart of Shimano’s new GRX Di2 system. The hoods feature some familiar details from the outgoing 11-speed GRX RX815 Di2 levers, such as a textured rubber pattern on the hoods and anti-slip coating on the levers for a more secure grip in rough terrain. The shift buttons are positioned on the sides of the lever for similar ergonomics to Shimano’s non-Di2 drivetrains, but the levers also get a third Di2 button on the inside of the hood which can be customized to the rider’s function of choice. We’ll talk more about all of the programmable customization options in a bit.
Shimano also says that they’ve updated the lever ergonomics for a Gravel-specific fit. As part of those updates, the ST-RX825 levers supposedly interface better with flared drop bars, which is great to see — as someone who has spent a lot of time on the GRX RX800 mechanical drivetrain but also prefers the fit of flared bars, I definitely saw room for ergonomic improvement there.
Of course, the biggest update here is the wireless communication from the shifters to the derailleurs. While Shimano’s older Di2 systems were a bit fussy to set up due to the required junction boxes hidden in the frame, the new wireless arrangement means that the only thing physically connected to the levers are the brake hoses.
As a final note, Shimano prices the levers with GRX BR-RX820 brake calipers. While precedent would suggest that Shimano will sell the lever sans brake caliper, we don’t have prices for the a-la-carte option just yet.
GRX RX-825 Rear Derailleur
MSRP: $409.99 USD
Stated Weight: 310 g
With the levers now wireless, the RD-RX825 derailleur is now the central communication hub for the drivetrain. With the battery mounted separately in the frame, the rear derailleur serves as the charging port for the system via its wired connection to the battery. A button on the rear derailleur allows riders to check battery level, pair via Bluetooth, or go into trim adjustment mode for aligning both front and rear derailleurs.
Outside of those electronic functions, the RD-RX825 gets a lot of other familiar Shimano features like Shimano’s SHADOW RD+ layout which tucks the derailleur somewhat inboard for a lower profile while also having a clutch for chain stabilization.
From a compatibility standpoint, the new 12-speed derailleur can be used with 11–34T and 11–36T cassettes.
GRX FD-RX825 Front Derailleur
MSRP: $209.99 USD
Stated Weight: 142 g
Like the rear derailleur, the GRX Di2 front derailleur is wired up to the central battery. This does mean a bit of work routing the small wires during setup, but also means that only a single shared battery needs to be charged between both derailleurs.
Shimano’s earliest GRX groups ran a slightly wider +2.5 mm chainline for better tire clearance, and though the last couple of iterations walked that back to conventional Road spacing, the new GRX Di2 12-speed sees a return to the 2.5 mm wider stance. Gravel bikes are seeing wider tires being used for a more comfortable ride in rough terrain, and the added clearance from the chainline helps mitigate any potential clearance issues.
GRX SPD M8100-UG Pedals
MSRP: $138.00 USD
Stated Weight: 342 g
Along with the new RX825 Di2 group, Shimano is also releasing a limited edition pedal in the M8100-UG. The pedals are the same as Shimano’s smaller-bodied Deore XT M8100 pedals, but get special United in Gravel graphics for the Gravel crowd.
Customization
Of course, one of the big selling points of an electronic system is the huge range of customization in shifting options. Using Shimano’s E-TUBE PROJECT Cyclist app, riders can customize shift button arrangements, select different gear shift intervals for each button, program auto-shift modes that synchronize front and rear derailleur shifting, and fine-tune derailleur alignments.
The Bluetooth compatibility of the ST-RX825 lever means that riders can also control accessories such as compatible cycling computers. The shifters are also compatible with accessory switches, which can be added as a satellite switch in a place of the rider’s choice for controlling shifting, computers, or compatible lights.
Shimano has also announced a feature update for Di2 to coincide with the RX825 launch — Front Shift Next. A firmware update allows any 12-speed Di2 button to be programmed to shift the front derailleur to the unused front chainring. This allows simpler flipping between front chainrings while relying on a single button, and frees up another button for other shifting customization options, or controlling a cycling computer or compatible lights.
Compatibility
GRX RX825 is compatible with the 48/31T RX820 or 46/30T RX610 cranksets, and works with the 11–36T HG710 or 11–34 R9200, R8100, or R7100 cassettes. Shimano makes 3 different freehub spline systems across MTB and Road, including Micro Spline, HG spline L2 (12-speed Road), and HG spline L (12/11-speed Road), all of which have varying levels of compatibility based on the cassette type chosen. The chart below helps to explain further, but implies that 11-speed cassettes can be used by installing a 1.85 mm spacer, and of course programming the Di2 system to recognize 11 rather than 12 speeds.
Some Questions / Things We’re Curious About
(1) The semi-wireless nature of the new GRX RX825 group promises easier setup and lots of customization potential, but just how easy is it to bolt up and get going?
(2) Adding a battery and electronics introduces more potential for things to go wrong out on the trail. How much attention needs to be paid to battery life in practice, and are the performance gains worth the added complexity?
Bottom Line (For Now)
With the release of GRX RX825, Shimano is bringing the semi-wireless functionality of their Road groups to the world of Gravel. We’ve appreciated the buttery shifting of a well-tuned Shimano GRX drivetrain, and the added customization options of the Di2 system hold a lot of promise. We have a full GRX RX825 group coming our way, so stay tuned for a Flash Review and Full Review to follow.