Shimano HG MTB and HG Road freehub compatibility
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This may seem as an old story – MTB and Road freehub compatibility. Indeed, if not for relatively recent Gravel category, why anybody would bother to ask? Although, swapping to Road cassette on a mountain hub was commonplace for particular cross-country scenarios. Until 11 speed cassettes made it to the both segments: MTB and Road, such a substitute used to be no-brainer. Then it started to get complex. And managed to become outright confusing with 12 speed transmissions becoming widely available. Specifically for Gravel as well. Here it is – Shimano’s new GRX 12 speed. The promise is that is should not be complicated, but often it is.
This confusion relates to both Shimano and SRAM. That is HyperGlide (HG) for both MTB and Road, or XD and XDR from SRAM’s technology arsenal.
Counterintuitive at first, but 11 speed MTB cassette requires less freehub’s space than 11 speed Road. And that is because of its wider gears range. Resulting in larger sprockets. Indeed, 11 speed MTB cassette fits onto 10 speed HG freehub. The largest one or several sprockets do not consume any space on the freehub. They are fixed together, forming a cluster. Up to the sprocket which is actually mounted on the freehub. These attachment sprockets are in a way “hoovers” over the spokes.
Mounting Shimano 11 speed MTB cassette on 11 speed HG Road freehub requires special spacer. Spacer’s width is 1.85 mm. It is the difference between length of 11 speed Road compatible freehub and 11 speed MTB. Otherwise, both freehubs are identical. It is almost impossible to tell the difference without being able to closely compare the two. And problematic to spot looking at the picture of these parts alone. The difference of 1.85 mm applies to Shimano HG Road vs HG MTB and SRAM XD vs XDR.
With 12 speed cassettes there are few more caveats. Even within Shimano and SRAM products range. And there is a host of other manufacturers, which cassettes we do not cover here. These cassettes compatible with classic HG, XD, XDR and Micro Spline drivers. Micro Spline made it possible for Shimano to offer 10T sprocket with their cassettes. Whether Shimano or SRAM, if cassette reads 11T – it is for HG compatible freehub. In SRAM’s technology space called PowerGlide or PG. PG is technically identical to HG with fitting cassette onto the freehub.
However, there is still a difference between Shimano 12 speed MTB and Road cassettes. The latter are backward compatible with HG 11 speed Road freehub. For MTB 12 speed cassettes require Micro Spline freehub body. And these cassettes feature 10T smallest sprocket vs 11T for their Road counterparts.
Oftentimes lower-level models tend to be HG (PG) compatible. For example, SRAM’s 12 speed PG-1230 fits HG MTB freehub.
Whilst higher level with either HG 11 speed Road, XD or Micro Spline.
It used to be enough to identify the groupset you are after to be clear which freehub to get. It is no longer the case with Shimano’s latest GRX 12 speed components. Depending on whether you are on 1×12 or 2×12 options, now called personalities, you either going to need Micro Spline for 1×12, or HG 11 speed Road freehub for 2×12. That’s right, the 2×12 option comes with Road 12 speed cassette, thus requires HG 11 Road freehub. If you followed the story of 10T vs 11T smallest sprocket above, it should be clear why this is the case with the latest GRX update.
1.85 mm is a big matter when it comes to how it impacts other hub’s part. The Drive-Side (DS) flange is positioned farther inward toward the centerline of the hub. This requires a balanced placement of the Non-Drive-Side (NDS) flange. Otherwise, there will be significant difference in spokes tension on the NDS. Followed by the related placement of the hub’s shell bearings. And finally, hub’s axle modification to support such an arrangement of the bearings. Above is the side-by-side comparison of Gravel and XCR hubs.
In Trailmech line-up only Gravel hub is compatible with both HG Road and XDR freehubs. Neither Enduro nor XCR hubs fit 11 speed Road cassettes. At the same time, mounting MTB cassette on the Gravel hub is possible. And only requires the 1.85 mm spacer that comes with it.
Fast forward the technicalities – Shimano HG Road freehub is compatible with all HG designed cassettes, including those for MTB except for 12 speed versions. If, however, the freehub’s designation reads just HG than 11 speed Road cassettes are excluded, but not MTB.