Review: Shimano GRX RX400 rear mech

Shimano’s GRX RX400 rear mech
Chris Juden reviews Shimano’s big 36T-sprocket-capacity mech for 10-speed road bikes

I’ve used a 9-speed MTB mech (XT Shadow) with 10-speed Ultegra STI to shift an 11-36 cassette on my road bike for some time. It’s always worked – even better since I fitted 10-speed XT pulleys.

But Shimano now makes mountain-sized road mechs for gravel biking. According to the USA website, GRX is compatible with your existing parts, provided the speeds correspond. And in particular: “Shimano GRX 400 components work great with the 10-speed Tiagra road groupset”.

It turns out that’s not the whole story: GRX is compatible only if your existing parts didn’t exist before 2016, the year in which Shimano tweaked its one remaining 10-speed road group to shift like 11-speed. (I missed the news of that change – if there was any. There should’ve been; it was our last chance to buy compatible spares of good quality.)

The GRX RX400 rear mech does not work at all with 10-speed Ultegra because it needs more cable pulled per click. Not a lot more (like SRAM and Shimano MTB 10-speed) but just a little – like Campag.

Since I had 10-speed Ergopower on my tourer (shifting a Shimergo-modified XT Shadow mech), plus a spare 10-speed chain and 11-36 cassette, I swapped those and the GRX RX400 mech onto that bike. And there it did “work great”!

So Shimano has accidentally made a mech with near enough the right shift-ratio to marry a Campagnolo 10-speed shifter with any 10-speed MTB cassette. Better still: it has the cable adjuster now missing from MTB mechs.

Why don’t I try the other GRX components? Money. Hydraulic levers are nice but would require new brakes.

And I don’t like the double chainset, which accepts only slightly smaller rings of 30 and 46T and has a 50mm chainline: too far out for smooth running in big-and-big – or even next-to-big! My truncated triple, on the other hand, boasts a 24T inner and aligns its 42T middle, that is outer, ring with the exact centre of the cassette.


If you want lower gears on your Campag 10-speed bike, or with new Tiagra, Shimano’s RD-RX400 is a neat solution. Fit XT sealed pulleys when the originals wear out.

Other options

Shimano 9-Speed MTB ‘Shadow’ Mech £58.99

For example: Deore RD-M592. Works with pre-2016 10-spd road STI – even better with 10-spd pulleys.

Sunrace SP570 Hanger Extender £9.99

Cheap but may shift vaguely at small end of cassette.

First published in Cycle magazine, August/September 2020 issue. All information correct at time of publishing.

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Tech spec

Price: £61.99.
Max sprocket size: 32-36.
Capacity: 41T.
Weight: 303g.
Available from: SJS Cycles.

Pros & cons

+ Works with 10-speed Campag shifters
– Doesn’t with older 10-speed Shimano road shifters