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I've just purchased a bike with a 105 group-set, 50 + 34 chainrings and a 11-34 cassette.

I was expecting to be able to replace the cassette with a closer ratio one, with a slightly lower top gear: maybe 13- or 14-. However the shop I purchased it from said the 105 rear mech wouldn't accommodate cassettes with a top ring smaller than 11 or 12.

Is this accurate? Surely if the derailleur can handle the combination of 50T chainring and a 13T or 14T sprocket two or three gears down from top, it can handle it 50T combined with 13T or 14T as top gear as well? Wouldn't a 14T-32T cassette require less forward and backward take-up from the derailleur, compared to the 11-34 cassette that's currently on the bike?

  • Would changing your crankset be an option? – Paul H Sep 10 at 22:31
  • Shimano makes an 11-speed 14-28 Ultegra CS-6800 cassette. A compact crankset with a 28t large cog should be sufficient for most riding. It probably won't be easy to find, and it probably won't be cheap, either. – Andrew Henle Sep 10 at 22:59
  • In my experience shimano specs are extremely conservative. If you think it should work it likely does work, but maybe not as smoothly shimano would like. You should be able to get "junior" racing cogsets with 14t top gear if you look around, they are roughly the same price as regular cogsets, but almost never on sale. – Fred the Magic Wonder Dog Sep 12 at 19:17
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Modern rear derailleurs do in fact have a min and max smallest sprocket specification as well as largest min and max largest sprocket specification. You can find the specs here.

The min and max size of the sprockets is not just affected by the chain slack the derailleur can take up, it's affected by the angle of the line the cage moves along relative to the center plane of the bike to maintain separation from the cassette. This is different for short and long cage versions of derailleurs - a long cage derailleur moves at a different angle than a short cage to track a wider range cassette.

The current RD-7000 SS (short cage) and GS (long cage) min and max sprocket specs are:

                 SS    GS 
Low        Max   30T   34T
sprocket   Min   25T   30T
Top        Max   14T   12T
sprocket   Min   11T   11T

So if you have a short cage version you can use a 14 tooth smallest sprocket.

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    But based on having an 11-34 cassette, he must have the long cage derailleur, so the 11 or 12T top sprocket size quoted by the shop is exactly right. – DavidW Sep 10 at 21:01
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    The existing derailleur could probably be made to work by adjusting the B screw. Shifting might be a little degraded, but I think it would work. The trick will be finding a cassette with a 13-tooth top end. – Adam Rice Sep 10 at 21:11
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    Given its Shimano, their specs are known to be pessimistic at the low end. I'd expect them to work well enough on something just outside the posted specs. – Criggie Sep 10 at 21:28
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    Based on those specs it must be the long cage version. So it sounds like, officially at least, only 11 or 12 would be supported. I've seen Miche Primato cassettes online with a wide choice of smallest rings (right up-to 18T). No idea if they're any good though, or how much using one would compromise the shifting – NotRelevant Sep 10 at 21:51
  • @NotRelevant Miche is a maker of niche cassettes and they are known for their quality prodiucts, but they my be expensive. – Carel Sep 11 at 6:22

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