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Max derailleur capacity vs max low cog capacity?

A derailleur's max capacity is different to max low sprocket capacity but does one effect the other?

I assume they aren't related and that the max low sprocket capacity is the maximum a derailleur can be extended by the b tension screw for the top jockey wheel to clear the biggest rear cassette cog?

Looked up the low sprocket capacity for my rear derailleur and it has two figures, 46t for (1x11) and 42t for (2x11) so am confused why it would be different between a 1x and 2x drivetrain?

https://bike.shimano.com/en-EU/product/component/slx-m7000/RD-M7000-11-GS.html

Can anyone clarify the reason for this please?

Posts

  • JBAJBA Posts: 2,622
    I assume it is because with a 1x system you will usually have a smaller chainring (28T - 36T). However, with a 2x system the large chainring could be up to 46T.
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  • whyamiherewhyamihere Posts: 7,333
    edited 9 September
    There's a few things going on here, such as the total capacity, which is the last line in the spec, and is effectively a function of the cage length because it's determined by how much chain slack can be taken up. This is 41t for the mech you've linked. This is the maximum value for the difference between the big ring and small ring plus the difference in big cassette cog and small cassette cog.

    For a 2x11 with 22t and 32t chainrings, this is:
    (32-22)+(42-11) = (10)+(31) = 41. This is the same as the total capacity, so you can use an 11-42 cassette with a 10t difference up front.

    For a 1x11, there is no difference in chainrings, so you use the 35t difference between the 46t cog and the 11t, which is well within the capacity.

    The other thing which comes in is the geometry of the mech, which largely determines how big the biggest cassette cog can be. The geometry of this mech makes the biggest cog it can handle a 46, so even though an 11-50 would only technically require a 39t capacity which is within the 41t capacity spec, you still can't use one because the mech can't clear the 50t cog. This is how things like Goatlinks work, by changing the geometry a bit to make the mech clear a bigger cog.

    In reality, manufacturers are often a bit conservative on these things, and they can be pushed a bit, especially if you are willing to accept a compromise, such as the chain being a bit loose on the small-small combo on a 2x setup. The Goatlink is also a compromise, because they move the mech lower for the whole range, giving increased range at the cost of worse shifting on the small cogs.
  • whyamiherewhyamihere Posts: 7,333
    An extra point on geometry. The reason Sram Eagle mechs can't be used in a 2x configuration, and the reason there's 1x and 2x specific versions of the Shimano 12 speed mechs is that they both achieve the feat of clearing a 50-51t cog by significantly offsetting the guide jockey wheel from the axis of rotation of the cage. This means that the amount the jockey wheel moves vertically is controlled by the chain tension. This is fine in a 1x configuration, but if it was used in a 2x setup, the guide jockey wheel would have a very different position depending on whether you're in the big ring or the small ring, so the rear shifting would be significantly affected by front shifting.
  • reaperactualreaperactual Posts: 539
    edited 10 September
    JBA said:

    I assume it is because with a 1x system you will usually have a smaller chainring (28T - 36T). However, with a 2x system the large chainring could be up to 46T.

    On my old 3x9 set up when fitting a new chain I would check length was correct with an extreme big/big and small/small test on a works stand.

    The big/big was only 40:36 but with what your saying it makes perfect sense. I imagine no derailleur could handle a big/big (e.g. 46:42!) combo that you describe.

    Shimano state these figures because this extreme combo, although rare they can't rely on end user not to fit something like this, shift into that extreme gear and rip off their derailleur! Thanks for the response JBA. 😎👍
  • reaperactualreaperactual Posts: 539
    edited 10 September

    An extra point on geometry. The reason Sram Eagle mechs can't be used in a 2x configuration, and the reason there's 1x and 2x specific versions of the Shimano 12 speed mechs is that they both achieve the feat of clearing a 50-51t cog by significantly offsetting the guide jockey wheel from the axis of rotation of the cage. This means that the amount the jockey wheel moves vertically is controlled by the chain tension. This is fine in a 1x configuration, but if it was used in a 2x setup, the guide jockey wheel would have a very different position depending on whether you're in the big ring or the small ring, so the rear shifting would be significantly affected by front shifting.

    Late response, took the whole evening to absorb your full explanation and get my head around it but now it makes sense. Wow, complex stuff!

    Set the b tension on my Buddy's new bike with an SLX 12 speed last week so know what you mean about offset jockey wheels.

    Thank you for taking the time to explain whyamihere, my conclusion is I'm glad to leave the multi chainring set ups behind and stick with my simple 1x11 drivetrain which is well within these max parameters of my derailleur, phew!

    Good to know that capacity can be pushed a little if needed but (for me) not at the expense of shift performance when using Goatlinks etc, not a fan of those. Thanks again Pal. 😎👍
  • whyamiherewhyamihere Posts: 7,333

    JBA said:

    I assume it is because with a 1x system you will usually have a smaller chainring (28T - 36T). However, with a 2x system the large chainring could be up to 46T.

    On my old 3x9 set up when fitting a new chain I would check length was correct with an extreme big/big and small/small test on a works stand.

    The big/big was only 40:36 but with what your saying it makes perfect sense. I imagine no derailleur could handle a big/big (e.g. 46:42!) combo that you describe.

    Shimano state these figures because this extreme combo, although rare they can't rely on end user not to fit something like this, shift into that extreme gear and rip off their derailleur! Thanks for the response JBA. 😎👍
    Interestingly, the actual size of the chainrings makes no difference, only the difference between them. As long as the max difference is only 10t, you could use whatever chainrings you want. You could build a road bike with a huge gear range using an 11-42 cassette with 52t and 42t chainrings, and as long as you used a shifter which could operate the right mechs (another minefield of complexity there), it would all work perfectly.
  • reaperactualreaperactual Posts: 539
    edited 10 September
    Yeah, I suppose so, I would only need a longer chain, so a 42/22 up front would definitely cause issues one way or another.

    One chainring is sufficient for me and no plan on more than one in future!

    Only just getting over your last explanation whyamihere, maybe save the next 'minefield of complexity' for another day! 🤣😂 Thanks again for the input Pal.😎👍
  • david37david37 Posts: 300

    An extra point on geometry. The reason Sram Eagle mechs can't be used in a 2x configuration, and the reason there's 1x and 2x specific versions of the Shimano 12 speed mechs is that they both achieve the feat of clearing a 50-51t cog by significantly offsetting the guide jockey wheel from the axis of rotation of the cage. This means that the amount the jockey wheel moves vertically is controlled by the chain tension. This is fine in a 1x configuration, but if it was used in a 2x setup, the guide jockey wheel would have a very different position depending on whether you're in the big ring or the small ring, so the rear shifting would be significantly affected by front shifting.

    Reading these posts has made my head hurt :) But he's right you know :)
  • Absolutely david37! whyamihere knows his stuff. 👌 My head is still spinning from the info, need a rest and to get to some sleep tonight. 😴😂
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