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I think I am okay here but would be really good to triple check!

My current cassette: Shimano 105 5800, 11-speed, 11-28t

My front chain ring: 52-36t

My rear/front derailleur: Shimano Ultegra 6800, 11-speed

I would like to change my cassette to Shimano 105 R7000 11 Speed Cassette (11-34t). I've had a look around and don't see any problems with this switch. I will likely need a new chain but will this be okay with my rear derailleur and is there anything else I need to consider as this is my first cassette change.

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  • Edit: I am less sure now as it seems like the max number of teeth is 32....
    – James O'dare
    Commented Sep 25, 2023 at 13:58
  • See productinfo.shimano.com/#/com?acid=C-454&cid=C-453 - looks like only the newer-gen RD-R7000/8000-GS derailleurs can take 34t cassettes (with spacer!) - that's at least what Shimano says in their compatibility chart.
    – DoNuT
    Commented Sep 25, 2023 at 14:23

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The Ultegra 6800 rear derailleur comes in two versions with a different capacity for each. The version you have will dictate what cassette range you can run. The SS version has a shorter derailleur cage and the GS version has a longer cage.

If you have the shorter cage SS version rear derailleur (which is compatible with the 11-28 cassette), the 11-28 cassette is toward the upper limit of its range and it will not handle the capacity needed for an 11-34 cassette.

If you have the longer cage GS version rear derailleur (compatible with the 11-32 cassette), then you have the possibility of making the 11-34 cassette work. Even though Shimano says it won't work, there tends to be a little leeway in the Shimano world. No guarantees, but the rules may be able to be bent just a little.

For help in identifying your version of RD-6800, the center-to-center distance between the pulleys on the short cage SS version is 2 5/16 inches (59mm) - (direct measurement on my RD-6800-SS, which is running an 11-28 as well). If this is the same measurement on your RD-6800, then as stated above, this will not have the capacity for an 11-34 (or 11-32 for that matter). You would need to upgrade your rear derailleur to a GS version, and you might as well go with the RD-8000-GS or RD-7000-GS since they should be compatible with your current shifters as well.

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  • I am currently running an Ultegra RD-8000 with the GS cage and an 11-36 SRAM 11-speed cassette without any issue at all. The RR-8000 GS is only rated by Shimano for an 11-34, but the slightly larger cassette works just like it was in spec. Although the 8000 series is the next generation from the 6800, still a case in point.
    – Ted Hohl
    Commented Sep 25, 2023 at 15:45
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    urgh sadly mine is SS! Might need a new one then....
    – James O'dare
    Commented Sep 25, 2023 at 16:23
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    @JamesO'dare, sorry you don't get to use your current RD, but the newer ones are pretty nice. Also note that the 11-34 cassette, although 11-speed, has a body that is narrower. The cassette comes with a 1.85mm spacer that goes on the 11-speed freehub first, followed by the cassette. There isn't enough space here to explain the reason, but it does provide an 11-speed option on older 8/9/10 speed wheels (but only if you want 11-34 gearing). If used on an 8/9/10 freehub, one does not use the spacer (narrower freehub than an 11 speed unit). Just FYI.
    – Ted Hohl
    Commented Sep 25, 2023 at 17:57
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    6800 GS RD + 11-34 cassette worked on my bike.
    – Weiwen Ng
    Commented Sep 25, 2023 at 19:16
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    I'm running 11-30 on my RD-6800-SS with zero problems (the space looks like it could go to 32, but I'm not going to stretch my luck). It's definitely not within Shimano's spec, but whether it works or not for you depends on the hanger geometry. Shimano is very conservative with their compatibility.
    – throx
    Commented Sep 26, 2023 at 6:18
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Based on the Shimano specs, which are very conservative, the RD-6800 with a GS cage (medium length), has a max tooth count low sprocket of 32t. The short, SS, cage version has a max low sprocket of 28t. If you are running a GS cage you'll certainly be fine up to a 34t low sprocket. The issue is (mostly) one of chain wrap. The derailleur must be able to take up the chain slack, the difference between the chain on big chainring front and big sprocket back vs. Small chainring and small sprocket. Never mind that one shouldn't be using either of those combinations, but that is essentially the deal.

Thus, if the RD-6800 is the GS cage version, you should be ok using an 11-34t cassette. It's a good idea to start the new cassette life paired with a new chain. You'll have to lengthen your current chain anyway, and a new chain will not be stretched and you avoid inducing abnormal wear on your new cassette by pairing it with a new chain.

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