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Gravel is my current bike, Sora levers, rest is MTB -- 3x9 with front 20 (not a typo) / 32 / 44.

And I am thinking about changing rear to 10-speed, using cassette 12-32. Because of the positive reviews I have in mind Tiagra ST-4700 set, but the specs for rear derailleur https://bike.shimano.com/en-EU/product/component/tiagra-4700/RD-4700-GS.html says:

  • Total capacity: 41T -- while I need 44
  • Max. front difference: 20T (Front triple) -- while I need 24

It is said Shimano specs are written as guaranteed to work perfectly, and exceeding those should be possible. So the question is -- am I going too far or not?

Note1: safer path would be to stick to my current 9-speed derailleur (Deore XT RD-M772 Shadow SGS) which has excellent specs as far the limits are concerned, but if I am not mistaken, the best I can add is 105 lever (10-speed road level is compatible with 9-speed MTB, another info from the net). Because of the positive reviews of Tiagra ST-4700, how Shimano significantly improved shifting, and all, I am tempted to go with this direction first (i.e. Tiagra).

Note2: yes, I am aware Tiagra ST-4700 has new pull ratio, thus both lever+derailleur is needed.

2 Answers 2

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Capacity in term of design for the derailleurs works very different way. You can somehow 'mitigate' a wrong capacity for the rear derailleur, but not for the front one.

For the rear one, exceeding (reasonably) means that you can have some chain tension issues in the extreme conditions: too much tension and you can damage the mechanism or break a chain, or not enough tension and you'll have too much slack — that will depends on the chain length. You will actually only exceed the specifications with the extreme combinations: big sprocket/big chainring, and small/small. So if you are aware of these limits, you can avoid being in these combinations and stay within the design specs of the derailleur. Maybe you'll have to avoid the 2nd biggest sprocket with the big chainring, but it's somehow in your control.

For the front one: capacity refers to the geometrical characteristics of the cage and the parallelogram (size and position of the grooves) — capacity is important, but also the max chainring size. So if you exceed the specs, it won't be dangerous for the derailleur, but the shifting won't be optimal, or it will rub, or may not work at all, because the cage will be too big/small or the grooves won't be at the right position, or the parallelogram won't be aligned properly.

So it's not really possible to say 'it will work', because there are only two of knowing with certainty: trying or working at Shimano. And it also depends on your tolerance to 'not perfect' operation.

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    Thank you very much, taking your answer into consideration I decided to order 105 brifter. I need to buy cassette anyway, so 105 looks like cheaper, and safer choice for now. Feb 4 at 20:12
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I've edited to the title to be just about total capacity, because that appears to be what the question is about.

Total capacity represents the rear derailleur's ability to keep the chain slack (prevent drooping) down at the small/small gear combination even when the chain is properly sized long enough to accomodate the large/large combination.

The definition of total capacity is universally accepted in the design sense, but the implications of it are not, especially if you look across history. There's a lot of liability-consciousness built into the current manufacturer-endorsed premise that everyone must always have a bike set up to make it impossible to shift into a combination outside the RD total capacity, especially on the slack chain end.

Part of the question is how important is it to never find oneself in a small chainring/small-end-of-the-cluster gear combination where the chain goes slack. To answer that, one could point to the various early derailleurs where riding with the chain slack was an inevitability, and also to the period in the history of cycling where riding with the chain under spring tension was considered a potential major efficiency loss. Looking at those examples as evidence that it's a good idea to just not care about having the chain droop is bad. Untensioned chains can derail and jam, and it's not totally predictable when and why. Mostly it happens in aggressive riding situations, but exceptions are possible. However, it does bear pointing out that for the most part it does not happen in low-speed or low-intensity moments.

It is true and has always been true that some very high-mileage, prolific cyclists are animals on the bike and never develop a great intrinsic of what the components are doing and how to treat them. But, that is not most of us. Most regular users of derailleur bicycles always have been able to get familiar enough with how the systems work that staying out of small/small combinations is intuitive and easy if we want to stay out of them.

Cyclists in the latter camp, those with good control and intrinsic feel for the gear they're in, can basically cheat total capacity on the slack chain end as much as they want. You can look at a list of your cog sizes and make the decision for yourself where in the range you know you're not going to find it useful to shift further into the small/small combinations for your riding style. You can also have your own sense of what the situation is likely to be if you do shift into a slack chain combination by accident. There's a big difference in this regard between a road cyclist with a triple who is only really using the small chainring as a bailout gear and a mountain biker who will need to stay in it throughout technical climbing sections where they're throwing the rear end of the bike around.

The chain should literally always be set up long enough to accomodate large/large (with max chain growth if applicable) on everyone's bike without destroying anything. There's just no good reason to cheat it in that direction. The one exception is if you break a chain while out on a ride and then excise the damaged part to finish it out and get home. In that case the answer for most cyclists is just be careful to stay out of the gear combinations where the rear derailleur is over-stretched. If it's a child or inexperienced cyclist that got the broken chain, using the limit screws to lock those gear combinations out completely is reasonable.

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  • Thank you, +1. I reverted the title back though, since I wrote explicitly about two issues, not the one. Feb 4 at 19:25

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