Shimano GRX 1x12 with 42/10-51T

Long before the current Shimano GRX Di2 group came onto the market, which is still exclusively available as a double version, Shimano already presented their second mechanical GRX generation: hydraulic, twelve-speed, optionally with single or double crankset, and a variety of cassette and freewheel technologies from Shimano's MTB universe.

The simple version with the 10-51 cassette in combination with the large 42 chainring is aimed at both fast tour riders and adventurers. The crankset with Hollowtech II construction is available in three lengths from 170 to 175 mm. The derailleur used is the model with a long cage (RD-RX820-1) including RD+ technology for chain stabilization.

It is worth mentioning that the derailleur cage is replaceable. So if you want to switch from the wide-ranging 1x12 system with 10-51 teeth to the more closely spaced 10-45 or if the cage is damaged in the field, you don't have to replace the entire derailleur.
In addition, with the MTB cassettes, the Micro Spline hub technology is also making its way into the gravel segment, which makes the 10-tooth sprocket possible in the first place.

Despite the downgrade from the electronic to a mechanical group, switching from Shimano's old GRX 2x11 Di2 to the new GRX 1x12 can achieve a real performance and comfort gain.
Not only can every gravel racer enjoy the tight and precise gradation of the 1x12 setup with a 10-45 cassette, but the new GRX 1x is also compatible with Shimano's 10-51 Z-MicroSpline cassette. Combined with a 40 or 42 chainring, four possible gear gradations are created, which should satisfy almost all riders.

The first rides already hinted at what was later confirmed through sober calculation.

Comparison: GRX 1x12 42T (10-51 MSR) vs GRX 2x11 48/31T (11-34 HG)
GRX 1x12 (smallest gear): 42:51 = 0.8235 (smaller ratio = better)
GRX 2x11 (smallest gear): 31:34 = 0.9118

GRX 1x12 (largest gear): 42:10 = 4.200 (almost as fast as with 2x11)
GRX 2x11 (largest gear): 48:11 = 4.3636

Conclusion: The biggest advantage over the GRX 1x11/2x11 predecessor is the extended cassette range of 10-51 across the 12 gears. Finally, there is no real disadvantage downhill compared to the 2x11 version, and uphill it even offers at least one additional bailout gear, making climbing even more relaxed and enjoyable.

Additionally, chainsucks and chain drops are almost impossible, and the GRX 820 crank is visually and technically identical to its predecessor (810), which is why I could continue to use my old 810 Stages power meter (also 172.5 mm).

Moreover, the mechanical group with the left mechanical GRX lever (ST-RX820-LA) offers the possibility to operate a dropper post without an additional lever. You can read how I managed to combine the Shimano lever with the FSA post in the related thread.

However, there was a small downside. Because for the MicroSpline cassette, a special MSR MicroSpline Road freehub is required, which was initially very difficult to find for my wheels.

With the new GRX group, Shimano once again mixes road and MTB components and additionally complements the ensemble with their Microspline MTB cassettes. To mount a Microspline cassette on the wheel, you need a Microspline freehub - that's clear.
However, current HG-Road freehubs are about 1.1 mm longer compared to MS-MTB freehubs, which means that after the freehub upgrade (switching from HG-Road to MS-MTB), a rear hub designed for 142 mm could be only 140.9 mm wide in total (!).

This is not so serious when installing it on a gravel bike, but the gears will most likely need to be readjusted, as the derailleur hanger will no longer be perfectly aligned and the gears may not shift smoothly or the chain could even fall off the 10th cog. Additionally, swapping the rear wheel with multiple wheelsets becomes a test of patience.

Our tip: When retrofitting your gravel wheelset (based on road), be sure to order a Microspline "Road" freehub and check afterward if the hub has an installation width of 142 mm. If you are riding a gravel wheelset based on MTB, it might be the case that you need a normal (MTB) Microspline freehub, otherwise, you might end up with a width of 143.1 mm.
All those who have bought a complete bike with a 10-51 setup probably don't need to worry anymore, as the manufacturer has hopefully taken care of this.

Since I have been riding a GRX setup with slim Di2 levers for years, I was initially unsure how I would cope with the comparatively bulky mechanical levers and their longer lever travel. However, these concerns quickly dissipated during the ride.
Shimano has ergonomically redesigned the mechanical shift levers to distribute pressure more evenly across the palm and better suit drop handlebars with more than 10° flare. Thanks to their flattened surface, the new shift levers actually feel good in the hand, operation is relatively easy, and the longer lever travel is less intimidating compared to the crisp Di2 buttons than its predecessor.
Even braking with just one finger is easily possible from various hand positions. However, a longer lever travel is required to release the dropper post, which minimizes the risk of accidental activation.

Shifts were precise and extraordinarily smooth in every situation. Additionally, it was really pleasant to never have to worry about batteries or recharging. Thanks to the activated chain stabilizer, there was no chain slap even with the wide-range MTB cassette, not even on the 10-tooth cog.
Another feature that the GRX 820 1x12 has inherited from its MTB relatives is the ability to shift under load, which also worked very well and relatively quietly.

Let's stay on the topic of "noise level". The GRX disc brakes have been adapted to the two-piston flat-mount brakes of the current road groups, and the brake pad clearance has been increased by 10% to avoid annoying pad rubbing. Additionally, the bleed port has been moved to the outside of the caliper to facilitate access, which is sure to please all mechanics. 
Overall, the GRX brakes offer superior braking power and good modulation. The bite point is very early, which I personally find positive because it reduces the chance of pinching your fingers between the lever and the handlebar when wearing gloves.

There is also nothing to complain about in terms of reliability and durability. After more than 2,000 kilometers, the length of the Baranski chain is still well within specification, and there are no unusual signs of wear on the cassette, chainring, jockey wheels, or brake pads to report.

Although the advantages of electronic shifting should be particularly pronounced when graveling, the tested mechanical Shimano GRX 820 1x12 also offered excellent performance in demanding off-road use with fast, precise, and smooth shifting.
Subjectively, the GRX 12-speed derailleur in conjunction with the XT cassette is even one of the quietest drivetrains I have ever ridden. Additionally, I particularly liked the ergonomics and feedback of the new levers – both when shifting and braking, as well as when operating the dropper post.

In the multi-month practical test, there were no problems such as chain slap, chain drops, or chainsucks. In fact, there were no drops or chainsucks at all – zero, nada, nothing. Even the cable tension screw remained untouched after the derailleur installation.

Overall, the Shimano GRX 820 1x12 group sets new standards for mechanical shifting groups and shows that electronic drive systems are more of a luxury than a real necessity.

Finally, let's come to our thesis on why Shimano's GRX 1x12 Di2 is not yet on the market. In short: We believe that this generation will be skipped and the Japanese will instead cause excitement among their fans with a GRX 1x13 Di2.
NoPain will probably be one of the first to rip the existing components off his bike and treat himself to a 1x13 Di2. Until then, it's: ride, ride, ride - let others shift and reload.