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.

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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.

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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.

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

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