Saris H3 Direct Drive Indoor Bike Smart Trainer Expired

Time for my real-life experience with the Saris H3 PSA.

You will have no problems using a bike with 8-speed to 11-speed transmission with the H3. Saris says 12-speed isn't compatible. This is partially true.

Some are saying that a 12-speed HG cassette is the same width as a 11-speed HG cassette. This is false. While it is true that the cog spacing is narrower, the whole cassette is also wider... not in the HG interface, but that the largest 12-speed cog actually overhangs the HG interface and is reliant on the the normal dish of a bike wheel for clearance. Therefore that while you will be able to slide a 12-speed HG cassette onto an H3's HG freehub body... it is true that you will not be able to use the largest cog. Your derailleur simply will not have any space to move the chain far enough to complete a normal shift.

You will have a significant chance of running into problems trying to use the SRAM NX cassette with H3. The SRAM NX (PG-1230) cassette leans into the "overhang" toward the wheel that it has some rivets that extend toward the inside that only leaves a small fraction of a millimeter in clearance relative to the trainer body that will go away when you output some real wattage and introduce flex into the bike/trainer interface... to create contact. I have done this. I'm sure there is going to be manufacturing variation in the trainer that would result in one have more-or-less clearance than another... but it is a crapshoot you don't want to take.

If you want to use a 12-speed MTB on the H3, I recommend using the Sunrace CSMZ90 (for SRAM, direct experience) and Sunrace CSMZ913 (for Shimano; projected, no direct experience). They do not have have any rivets that extend beyond the plane of largest cog, unlike the SRAM NX cassette. It is pretty much flush to the plane of the largest cog. I replaced the SRAM NX cassette that I had tried to run with a CSMZ90, and it has been flawless. It did require the use of the Saris-included HG spacer, though.

In terms of gearing, standard MTB gearing will be too low for anything but ERG mode. I mean, it'll work in a non-ERG mode... but you'll know it's not optimal and you'll spend 95% of the time in the smallest. roughest, and most-wear-prone cog. If you want to ride it like a real bike (using the 11-42T part of the cassette), you will need a chainring of ~42T or even slightly bigger if you're stronger. This is basically gravel bike "mullet" gearing.