Chain length sizing for bicycles with derailleurs - article comments

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Chain length sizing for bicycles with derailleurs

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  1. Don
    30/12/2016 at 09:52
    I have found that a very quick and precise way to see whether you can accommodate a cog slightly larger than the current largest cog is to select the current/big combo and then assess the chain slack by seeing how far you can push chain down at about the middle of its upper run. The chain can typically go down by at least a few inches as the derailleur cage gives up the slack. For typical chainstay lengths, a 3 inch drop means you can add one tooth with the current chain. A drop of 4.5 inches allows a two tooth increase. If you follow this procedure, the chain will likely grumble a bit on the new big/big combo. But that can be useful as a warning not to stay in that combo if you inadvertently select it.
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    23/12/2018 at 08:04
    Thanks. Will give this a try. I usually take the lower part of the chain in the big-big combo, holding two links that are not adjacent, pulling them towards each other. If there’s RD cage movement left, it will allow for the links to come closer. Depending on how much “movement” can be done this way, you can tell approximately how much more teeth can be accommodated. Just over 0.5 extra inch movement per extra tooth.
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    richard miller
    18/05/2020 at 00:58
    very informative information, thank you. been working on bikes for many years, and have always been curious about chain length versus derailleur position. since all I work on are mostly discarded or donated bikes I build for poorer folk, I am not always as fastidious as could be. now I can be that much better.
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    28/03/2021 at 08:28
    When you say “and then add one more inch, or one link because of the derailleur”, don’t you mean “and then add one more inch, or one link PAIR because of the derailleur”?
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      28/03/2021 at 09:29
      Well noted. I mean “one pair of inner and outer plates” – one inch in length.
      I have edited the text to make it less ambigous.
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    Raymond P Vaughn
    07/07/2021 at 10:18
    I was thinking about a new ProX o-ring chain for my dirt bike, but would like to know your suggestions on how to clean and re-lubricate the chain so that I don’t harm the o-rings or wash out the internal grease. I don’t mind taking the chain off – Can I swish the chain in a solvent tank? How do you guys do it?
    Thank you,
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      07/07/2021 at 10:43
      Hi Raymond,
      Just to avoid any misunderstanding – are we talking about motorcycles or bicycles?
      If it’s bicycles – what are the riding conditions: sand, dust, mud, snow, or a combination of those?
  1. Michael French
    03/08/2021 at 23:36
    Are chain typically bought new for a bicycle the right size or need to be a shortened?
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    28/07/2022 at 13:17
    Great article (like many in this “wiki”).
    By how many links can you get the chain length “wrong” without running into trouble?
    Some context: I’m considering sliding dropouts to adjust chainstay length on a self made custom frame. How much sliding dropout range is possible without having to change the chain when going from one end of the dropout range to the other?
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      Relja Novović
      28/07/2022 at 16:58
      Hi Nic,
      The minimum length is determined as explained in this article.
      In your case – that would be tested with the wheel furthest out from the cranks (as far as you intend to move it).
      Maximum lenght generally needn’t exceed the determined minimum length – no use having those extra links.
      Nonetheless, you RD chain wrap capacity is the real limiting factor here – the point when using the small-small chainring combo (front-rear) causes the chain to go as shown in the second picture in the 4th chapter in this article.
      This is less problematic than using a chain that is too short – it won’t damage the RD (unless you ride for miles like that).
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    29/07/2022 at 07:47
    Hi Relja,
    that makes sense if I already had the bike and could test it but I still have to build it and pick a range (10mm, 15mm, 20mm etc) for the sliding dropout. Is it possible to say how many links one can be away from the optimal length without having issues (given a MTB long cage derailleur)?
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      Relja Novović
      29/07/2022 at 08:18
      Hi Nic,
      The answer depends on the drivetrain.
      If you have a wide range cassette (11-40 or similar) + a wide range triple cranks, there will be less “room” for the RD to pick up any chain slack even with the optimal chain length.
      In that case, perhaps one extra pair of links (1″) is as far as it will go.
      With double cranks, and a more “conservative” cassette (say 11-32, or even 11-34), the long cage RD should be able to handle 2 or more extra pairs of links (2″, 4 pins/rivets).
      Generally, with a long cage RD, I would expect 20 mm movement to be manageable.
      But, to be certain, check the stated RD chain-wrap capacity, and see your total drivetrain tooth count difference (lareste rear minus smallest rear + largest front minus smallest front).
      If you have room for 4 more teeth in terms of RD chain wrap capacity, 20 mm should be manageable.
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    29/07/2022 at 15:38
    That was an exhaustive answer, thanks a lot. I should have given you my complete specs which are: 11 speed system, 2x GRX front derailleur (30-46), 11-42 MTB cassette, Shimano XT SGS RD, and the chainstay range I’m aiming for is around 440mm if that matters. So that would make a 1 (or even 2?) chain link deviation in either direction from the mid point of the sliding dropout range possible?
    Side note: I have a flat bar so I will use a flat bar road FD shifter and a MTB RD shifter. Just in case you were wondering how I will shift this Frankenstein construct.
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      Relja Novović
      29/07/2022 at 16:39
      Hi Nic,
      I’d start by googling the exact RD model’s stated chain-wrap capacity.
      For example, Shimano XT RD-M8130-SGS has a 39 teeth total capacity.
      Shimano Deore RD-M5120-SGS has 41T.
      Alivio RD-M3100-SGS has 45T.
      And so on and so forth.
      Your current drivetrain has a total of 47 tooth difference.
      That’s huge.
      Based on that, I think you will have to make a compromise:
      – Either go with a chain that is a bit too short and avoid shifting to the big-big combo (to avoid damaging the RD).
      – Or go with a chain that is of an optimal length for your longest (farthest) setup, and avoid using the small-small combo to avoid chain slapping too much and sliding over the jockey wheels as noted in one of my previous replies.
      Alternately, you could use a 12-34 T cassette, or an 11-32 or similar, to reduce the total tooth-count difference and give yourself some room.
      I suppose that moving the rear wheel by 20 mm equals roughly 1″ (1 pair of links) chain length difference, and about 4T capacity difference (in terms of RD chain wrap).
      Guessing, didn’t sit down to calculate diameter differences and similar.
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    31/07/2022 at 19:48
    I was going to use an Shimano XT RD-M8000 11 speed SGS RD. Acc to it has a 47 tooth capacity. That does sound a lot in the context of the other RD you mentioned. Does that then still require the compromise you suggested? I guess not because the capacity matches.
    The formula I used to calculate chain length is (in chain links) 0,157 * “chainstay length” + 1/2 “biggest front ring” + 1/2 * “biggest rear ring” + 2. The result with 445mm chainstays is 116.8 links. I got the formula from Länge der,dieses Ergebnis dann durch 2. This one spits out 116 for the same specifications. If I reduce chainstays length in the above formula to 430 then it’s 114.5 links. The Omni calculator website says 114 links. So the formula seems to work. Then the delta in ideal chain length for a 15mm sliding dropout is a total of 2.3 links and a 1.15 link difference when going from the ideal mid point at 437.5mm chainstay length to either end of the range. I just don’t have a clue whether 1.15 link deviation from the perfect chain length is bad enough to chause problems or might I even be able to afford a 20mm sliding dropout, or a 30mm…the more the marrier:)
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      Relja Novović
      01/08/2022 at 06:03
      Hi Nic,
      Your drivetrain has 47 teeth difference in tooth count. So that RD should be able to take up all the slack if the wheel is not moved back/forth.
      The optimal chain length is basically the shortest length that allows you to shift into the big-big combo without breaking anything.
      If you intend to move the wheel back-forth, you should measure that in the furthest rearwards wheel position.
      Then, when you move the wheel forward, the RD will have more “extra” chain length to keep wrapped (the optimal chain length is effectivelly reduced).
      Moving the wheel forward by 20 mm resulty in about 3 more teeh of the needed RD capacity increase (if my math is remotely correct).
      Since XT RD-M8000 chain wrap capacity is right at the limit with your drivetrain, it may not be able to pick up all the chain slack when you shift into small-small combo.
      And it may not do too well for the smallest 2-3 rear chainrings when you are on the small front chainring.
      In such situations, some riders prefer to cut the chain a bit shorter and avoid shifting into big-big combo.
      That way, the chain will not slap as much, will be held tighter, but if you shift into big-big by mistake, you risk damaging at least the RD (possibly hanger as well).
      Others prefer leaving the chain with the “safe” length, and put up with a bit more chain bounce on bumpy roads, and more noise if they shift into small-small combo.
      Having said all this, the safest method is to just test.
      Measure the needed chain length for the big-big combo (as explained in this article) when the wheel is furtherst to the rear.
      Then install it through the derailleurs and over the chaingins, move the wheel forward, shift into the small-small combo and see if it’s all good, or too long for the RD to handle.
      It might just be OK. And it might not. If it’s too long – it’s your call how you want to deal with it (shortening the chain vs leaving it with the “safe” length – as explained above).
  1. Nic
    02/08/2022 at 21:04
    Ok understood. I’m pretty aware of my gears and so pretty much never shift into small small so going the safe route or installing a 11-40 cassette seems the best option to me. Thanks for taking so much time out of your (days) for answering my question. Hopefully other readers learn something as well.
    And just to make sure: a MTB cassette plus a rear MTB derailleur with a MTB rear derailleur shifter plus a front road derailleur with the appropriate road (flat bar) shifter works fine right? There is no chainline problem there? As far as I understand the chainline only has to match between crankset and FD.
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      Relja Novović
      04/08/2022 at 12:18
      Hi Nic,
      Having your front chainrings too far out or too close inwards can prevent the FD from shifting onto a chainring.
      When that is the case, chainline is usually also incorrect. I.e. when you set the chainrings to match the rear sprockets’ chainline, FD should be fine (unless it’s an exotic frame/setup).
      For more details, see my article about bicycle chainline.
      As far as shifting goes, the setup you noted should be OK (as long as the derailleurs match their shifters and as long as shifters match their chainrings/cassettes).
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    04/08/2022 at 14:34
    You have a new Patreon supporter;)
    I will reply on the chainline entry you linked to above because this is a bit off topic for this page.
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    Mini Velo
    11/08/2022 at 15:43
    Hi, there is not much information on how to size for a 20 inch wheel mini velo.
    For a front chainring 53, rear casette 11-32 setup. Do i add two or four links when measuring the big/big?
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      Relja Novović
      14/08/2022 at 12:40
      If it’s a one-by (1x), I’d size the chain as explained for the one-by drivetrains, i.e. add 2 inches (i.e. 2 pairs of inner+outer links, i.e. 4 chain pins).
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    27/09/2022 at 05:54
    What a great resource! Thank you.
    My question regards swapping front chainrings on a 1 X system. I want to be able to run two different sized front chainrings that I can swap out depending on the terrain.
    I run an eThirteen 9 to 50 cassette and want to be able to swap out a Wolf Tooth Camo 38 front chainring to a Wolf Tooth Camo 28 front chainring.
    Chain length calculators estimate that with my 450cms chainstays, the length of my chain with the 38 front chainring will be 148cms (116 links) but 142cms (111 links) with the 28 front chainring.
    Is this difference going to be OK for the SRAM 1 x X01 Rear Derailleur to handle? Or am I going to experience the chain dropping or rubbing the chainstay, when I switch to the 28 front chainring?
    Many thanks in advance.
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      Relja Novović
      27/09/2022 at 11:34
      Hi Carlos,
      To answer that question, we need two pieces of information:
      1) The total chain wrap capacity needed (tooth count).
      2) The chain wrap capacity of your rear derailleur.
      For the first piece of info, here is the calculation:
      (50-9) + (38-28) = 41 + 10 = 51
      Now, since we’re using a 1x drivetrain, we should also add two more teeth just in case, to avoid any chain slap, and to assure a good chain retention.
      That’s why I’d look for a RD with a 53T chain wrap capacity in order to handle the front chainring swap.
      Even 51 is way beyond the range of most RDs, even the long-cage 1x ones.
      Alternativelly, you could cut the chain a bit shorter.
      That way, you would avoid any chain slap, or chain tropping when using the smaller front chainting (the 28T one).
      However, if you do that, you risk damaging the RD (and/or RD hanger and frame, spokes, wheel…) if you shift on the largest few sprockets while using the larger, 38T, front chainring.
      Likewise, you could cut the chain to the optimal length for the larger chainring (38T), and in that case, the risk is just more probable chain drop if shifting to the smallest few sprockets when using the smaller front chainring (the 28T one).
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    28/09/2022 at 07:57
    Thank you Relja.
    Much appreciated!
    I´ve tried to research the chain wrap capacity of the 1 X SRAM AXS X01 Eagle Rear Derailleur that I have but cannot find a definitive size. SRAM mention that it “It features a shorter cage for 10mm more ground clearance, with more chain wrap on the cassette to sit slightly further forward, and further inboard from mechanical Eagle.” but don’t give a definitive answer. Unhelpfully, they only give a maximum tooth which is 52T.
    As you say the 51T difference would appear to be pretty big and that´s why I asked the question.
    The 28T and 9 to 50 cassette will give me a range of 16.23 to 90.15 gear inches. If I cut the chain “long” i.e. normally, due to chain drop/rub, I may have to go without say the last three sprockets (9T, 11T and 13T) which would leave me with an upper maximum of 54.21 gear inches, which may mean that I spin out quite a lot.
    The 38T and 9 to 50 cassette will give me a range of 22.03 to 122.33 gear inches. If I cut the chain short, it may just mean that I have to go without the top two sprockets (42T and 50T) which would still leave me with an lower maximum of 30.73 gear inches, which being a pretty good climber, I feel can live with.
    In this case am I right in thinking that my initial calculation would be (36-9) + (38-28) = 27 + 10 = 37? If so, How have you any idea how short I would cut the chain? 116 links (148cms) is what most chain length calculators advise for a 38T.
    The alternative would be to buy a 34T to close the differential a bit to account for the chain wrap capacity but without knowing what that exactly is for the RD I have, I think I may just opt for cutting the chain a little shorter and not changing up into the last two sprockets, when I have the 38T on.
    Thank you once again.
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      28/09/2022 at 08:27
      Thinking about it, I may be able to answer my own question of how short the chain would need to be;
      Using a chain length calculator, 28T on the front and 50T on the back will require 112 links.
      On a 38T at the front, according to the same chain length calculator 112 links will enable me to chain up till 42T on the back but no more.
      Am I missing something?
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      Relja Novović
      28/09/2022 at 11:27
      Hi Carlos,
      I usually take chain-length calculator results, print them out, and use the paper to start a barbecue. 🙂
      On a more serious note:
      I size the chains using the method explained in this article.
      It is the most accurate, and has given me the best results.
      If you intend to do the chainring swapping, you must decide which limitations you are happy with.
      There are basically two options:
      1) Not using the smallest few rear sprockets with the smaller chainring (28T).
      In that case, I’d size the chain as shown in this article using the larger chainring (the 38T one).
      2) Not using the largest few rear sprockets with the larger chainring (38T) (i.e. no chain slap with the smaller chainring).
      In that case, chain sizing would go a bit differently.
      I’d size the chain by using the smaller front chainring. But:
      I would shift into the smallest sprocket at the rear.
      Then, cut the chain to the longest length that allows the RD cage to wrap it
      (so NOT having the chain look like in either of the first two pictures in chapter 4 of this article).
      Alternativelly, for option 2), you could size the chain using the smaller chainring and the method described in this article.
      This will give you the optimal chain lenght for the smaller chainring.
      However, the method 2) explained in this comment will let you run with the longest possible chain for the smaller 28T chainring and your RD, so it might “buy” you a few more useful large rear sprockets when you are using the larger (38T) chainring. At the cost of having the chain length less-than-optimal for the 28T chainring.
      Not sure how well I’ve explained my reasoning, but I hope the procedures are clear enough at least.
      And, of course, you are free to use your prefered chain-sizing method, including the chain-lenght calculator, as long as you are aware of the limitations (it seems that you understand the problem correctly, as far as I figure based on your follow-up comments).
  1. Carlos
    28/09/2022 at 08:40
    Or alternatively, I would wrap the new chain around the 38T chainring and 42T sprocket (instead of the 50T) and add the 2 extra links that I usually do?
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    28/09/2022 at 08:43
    Or wrap the new chain around the 38T chainring and 42T sprocket (instead of the 50T sprocket) and add two links, in the normal way?
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    29/09/2022 at 07:21
    Hi Relja.
    Thanks for the confirmation.
    Best wishes and many thanks,
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    21/05/2023 at 22:42
    I have a specialized cross trail bike that I modded to be a 27 speed as the original 18 2x front and 9x rear was to easy for me to ride. What length chain would I need for that?
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      22/05/2023 at 10:18
      Hi Paul,
      It’ best to measure the chain on the bike – as explained in this article.
      So, wrap the chain around the largest front and the largest rear chainring, then add 1″ (one pair of outer and inner plates, or three pins – however it’s easier to visualize).
      As the chains are cut in 1″ (link pair) increments, in case you must choose between having 1.5″or 0.5″ longer, round up (to 1.5″, i.e. to three instead of two plates length, or 4 connecting pins).
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If you can't find the answer to your question in this thread, please open a separate thread with your question/problem, in an appropriate forum section (this is the Drivetrain section).

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