GRX/105 Cassette

Hi Guys, has anyone had any experience using a road cassette with Shimano GRX? I bought a 10 speed 105 cassette for my Kickr to use with my my new gravel bike (running 10 speed GRX) but I’m having a fair few hiccups with indexing. I’ve tried both the 1mm and 1.8mm spacers but the 105 cassette is not running as smooth as I would like or expect. Even fine tuning the indexing is still not 100%. Any thoughts? Would a MTB cassette be more suitable on the Kickr with the GRX derailleur? Granted I haven’t tried the GRX cassette on the Kickr but that’s my next port of call.

All Shimano 10 speed cassettes have the same spacing between cogs, so there’s no real difference between a 105 cassette and a whatever you have on the rear wheel. Trying the original cassette is not a bad idea, but you may need to experiment with some smaller spacers to get the cassette positioned in exactly the same place on the Kickr as it is on the rear wheel, so you can seamlessly switch between indoor and outdoor use without a lot of adjustment. If you have trouble reaching the large cog, that would indicate that the spacer is too small. If you overshoot the large cog, the spacer may be too big. Usually you can eyeball the alignment of the top derailleur pulley under the cog and see if it’s positioned too far in or out.

Another factor is the gearing - if the large cog on the original cassette is much larger than the large cog on the 105 cassette, your chain may be too long for the trainer. This can cause poor shifting as well, especially in the smaller cogs where the chain is very slack. In that case, having matching gearing on both cassettes will help. If the large cog on the 105 cassette is a couple teeth smaller than your original cassette, that’s usually OK, but if there’s a big difference it can cause shifting problems.


Hi Paul, thanks for your input. So the cassette on the wheel is an 11-36 (CSHG50) and the 105 cassette I bought for the Kickr is an 11-28 (CS5700) Do you think perhaps the difference in gearing is too great for me to expect the indexing to be transferable between bike and Kickr? I’m finding it very difficult to get in to the 11 on the Kickr, and when I index the gears to allow it to drop in to the 11, it’s reluctant to come back out of it. Perhaps the chain is too long on the Kickr with the 11-28 cassette. My thought buying it was that I could treat the setup more like a “road bike” for Zwift than the gravel bike that it is.

Echoing what Paul wrote and adding…

AFAIK, there’s no such thing as a “GRX cassette” - they’re all road cassettes. (GRX400 is Tiagra, GRX600 105, GRX800 Ultegra) “Gravel” bikes (I have one) are road bikes with clearance for wider tires, and a groupset with ergonomics more suitable to rough surfaces. (clutched derailleur, ergonomic brifters, different offset and gearing of front crank)

To troubleshoot indexing on a trainer: get a digital caliper with a depth gauge and use it to measure the distance between the contact point of your dropout, and the lockring of your cassette. (on the bike) This is your reference measurement.

Then measure the distance between the outside flat of the drive-side axle adaptor on your trainer and the cassette lockring as installed on the trainer. You need this to match the corresponding distance on your wheel.

In other words: the smallest cog on your cassette needs to have the same offset from your bike frame on both wheel and trainer. If those match (and you’re using the same cassette) your rear derailleur won’t need indexing adjustments because it’s attached to the frame with the same offset regardless of what it’s installed on.

To get mine matched (11-speed 105), I needed to add a 1.85mm spacer.

I would also strongly suggest getting a matching cassette, tooth-wise. Mismatching just introduces another variable to the system. If you want to test function before you commit to that, remove the cassette on the bike and install it on the trainer.

Oh, one more thing: wear is a factor. A worn chain will skip like mad on a fresh cassette.

1 Like

Thanks CJ, apologies I was being lazy in my earlier description. The chain set is GRX 400 so Tiagra level. Everything you mention makes perfect sense. After going down a worm hole this afternoon I’m convinced it’s down to the level of mismatch between the two cassettes I have. I’ll most likely pick up a more suitable 11-34/36 cassette going forward. It’s really taking the fun out of the indoor/outdoor setup when I have to keep making significant adjustments. Chain is still in good nick though, there’s probably only 600-700km outdoors on it.

I hear you. I spent a LOT of time trying to fine tune my trainer and bike. At one point, I needed to turn the rear derailleur barrel-adjuster precisely “one turn” to re-index between trainer and outdoors, and I called that “success.” It wasn’t until I was staring at a parts bin and doing some routine maintenance about 6 months later I had the idea to break out the calipers and measure.

In retrospect, it seems so simple - make sure the small cog is the same distance from your frame on both trainer and wheel - but it just took time to put two and two together.

It doesn’t help that trainer manufacturers do a poor job explaining and just throw a pile of spacers at the user.

Once you get it dialled in, the frustration disappears and you know the bike will just work, on or off the trainer, with next to zero adjustment.

1 Like

Have you tried adjusting the B Screw when you change cassettes? The mech has to be the right distance away from the cogs.

1 Like

Thanks for your input on this. I apologize ahead of time for the lengthy reply. I could use some help from you as well as you gave the best answer I can find on the topic so far. I have an 11 speed 28T shimano cassette on my bike. On the hub (new cassette included), the middle 5 or 6 cogs are pretty smooth, but any cog higher or lower is not indexed correctly I’m assuming (very loud chain noise comparatively)? I am pretty new to this stuff but that leads me to believe I don’t have an alignment issue, but more so the spacers within the included cassette are not identical to the cassette on my bike. As well as some variance in the teeth as you stated. Would you agree? I plan to buy an identical cassette to try this theory. Otherwise, I’ll be trying the method you stated above. I did find this info from lightbicycle .com on freehub and cassette compatibility:

Shimano HG 11/12 Speed Road Freehub

Since 11 speed mountain cassettes fit on 8-10 speed freehubs, this HG road freehub is intended for road 11 speed AND 12 speed cassettes. You can, however, run an 8, 9 or 11 speed mountain cassette on these with a 1.85mm spacer. If you want to run your Shimano 10 speed cassette, you can add a 1mm and 1.85mm spacer. A handful of hub manufacturers used the “road” 11 speed spacing and included a spacer as they assumed Shimano would be releasing 12 speed mountain cassettes and would require this wider freehub. However, when Shimano released their 12 speed it ended up using a completely different design (MicroSpline, explained later).

I found it interesting that this was the exact size spacer you needed. Could be coincidence. I wonder if some zwift hubs are being sent out with mountain cassettes, or if shimano in particular is really particular about needing a road free hub with a road cassette and the zwift hubs are not exactly identical? Idk like I said I’m still learning, and would appreciate any feedback you could offer.

Hi Drew, for what it’s worth this is the solution I came to with all of this info. I had a shimano 10 speed cassette 11-34 on my bike, but stupidly bought an 11-28 cassette for my Wahoo and had the trouble stated above. What I subsequently found out was that you can probably get away with coming down a cassette size (i.e. an 11-32) and not have many issues indexing from Wahoo to outdoor setup but anything more than that will result in a sub optimal chain length/cassette combo. I bought an identical 11-34 shimano cassette for my wahoo and shifting issue were eliminated. Granted the new cassette and chain did still sound slightly cranky at the start (just a new cassette meshing with a slightly worn chain approx 600-700km) but that eventually settled. For what it’s worth I have the 1.85mm spacer on my wheel (DT Swiss 350 hub) and the 1mm spacer on my wahoo and no issues indexing going from indoor to outdoor. I hope this helps. Derek

1 Like

I really appreciate your reply Derek. The zwift hub came with an 11-28t cassette which is what I have on my bike. However, when comparing the two just by eyesight the shimano 105 on my bike definitely has a different shape and angle to the teeth compared to the generic cassette on the hub. Especially in the lower cogs where I have the most issues. With amazons easy return policy I’m going to follow your lead and order the exact same one on my bike and see if that fixes the issue.