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Rear derailleurs explained

This post explains important facts about bicycle rear derailleurs. From construction and parts, to important characteristics such as chain wrap capacity etc.

1. How does the bicycle rear derailleur work?

Rear derailleurs (often refferd to as RD) are used on bicycles with external gearing and multiple sprockets at the rear hub. They work on a simple, brute force principle: moving the chain sideways, until it slips off a sprocket onto the adjacent one.

This derailleur movement is usually operated via shifters (shifter levers), that are located on the bars (or somewhere at hand). Shifter cable is attached to shifters at one and and at the derailleur at the other end.

It consists of a metal cage and a spring. The spring moves derailleur to one side when the shifter cable is released, while the shifter cable, when pulled, moves it to the opposite side. It also has jockey wheels on a cage, with a spring, used to pull extra chain when changing to different sprocket sizes, so it doesn’t fall off.

1. The upper area on the derailleur is the b-knuckle. 2. On Shimano and SRAM derailleurs, the b-tension adjuster fine-tunes the height of the guide pulley. On Campagnolo derailleurs, the adjustment is done via a screw near the p-knuckle. 3. To adjust cable tension, use the barrel adjuster. 4. The guide pulley keeps the chain in line as it moves from cog to cog during shifts. 5. The idler pulley helps hold tension on the chain regardless of gear choice. 6. The mounting bolt connects the derailleur to the frame. 7. The parallelogram linkage lets the chain move left and right and up and down the cassette while remaining parallel to the cogs. 8. High and low limit stops are usually found near the b-knuckle but sometimes on the front of the parallelogram. The low limit (often marked L) prevents shifts into the spokes; the high limit (marked H) prevents the chain from dropping off the smallest cog. 9. The p-knuckle holds both the guide pulley and a spring that keeps tension on the cage to hold the chain taut. Shimano’s XTR Shadow Plus (for mountain bikes) has a switch that increases spring tension and activates a friction stabilizer to keep the chain from bouncing in rough terrain. 10. The cable bolt pinches the shift cable in place. On most derailleurs, when the shift lever pulls on the cable, the derailleur moves up the cassette to a lower gear. The exception: Shimano’s low-normal derailleurs, on which it does the opposite. 11. The cage keeps the chain in line between the pulleys.
1. The upper area on the derailleur is the b-knuckle.
2. On Shimano and SRAM derailleurs, the b-tension adjuster fine-tunes the height of the guide pulley. On Campagnolo derailleurs, the adjustment is done via a screw near the p-knuckle.
3. To adjust cable tension, use the barrel adjuster.
4. The guide pulley keeps the chain in line as it moves from cog to cog during shifts.
5. The idler pulley helps hold tension on the chain regardless of gear choice.
6. The mounting bolt connects the derailleur to the frame.
7. The parallelogram linkage lets the chain move left and right and up and down the cassette while remaining parallel to the cogs.
8. High and low limit stops are usually found near the b-knuckle but sometimes on the front of the parallelogram. The low limit (often marked L) prevents shifts into the spokes; the high limit (marked H) prevents the chain from dropping off the smallest cog.
9. The p-knuckle holds both the guide pulley and a spring that keeps tension on the cage to hold the chain taut. 
10. The cable bolt pinches the shift cable in place. .
11. The cage keeps the chain in line between the pulleys.

2. Rear derailleur bicycle frame mounting

There are two ways of rear derailleur attachment:

  • Hook type (derailleur has a built in hook).
  • Direct mount (frame must have a point for screwing the derailleur onto).

Which one of these derailleur mount types is used, depends on the frame rear dropout type:

The way of mounting each derailleur type is shown in the pictures below:

3. Standard vs “rapid rise” rear derailleur

Most rear derailleurs have a spring pushing them outwards. When shifter cable is released, the derailleur moves outwards, towards the smaller sprockets. When shifter cable is pulled, movement is reversed – towards the wheel, bigger sprockets. This way of functioning is called “high-normal“, or “top-normal“.

Shimano Rapid Rise“, also called “low-normal” derailleurs work the opposite way: spring pushes them towards the wheel, while pulling shifter cable moves them towards smaller sprockets. Same shifter levers can be used for both standards. If a shifter lever has gear indicator, if a rapid-rise is put instead of standard (or vice-versa, for rapid rise shifter levers), indicator will show “upside-down”. But shifting will work correctly.

4. Derailleur capacity – cage length

Derailleur capacity is a number of tooth difference between front and rear sprockets that the derailleur can compensate. When chain is put on the largest chainring up front and the largest sprocket, the chain needs to be long enough to wrap. Then, when chain is placed on the smallest sprocket at the rear and smallest chainring, there will be a lot of “extra” chain length that rear derailleur pulleys must “gather”, to keep the chain tensioned. These are both extreme combinations, with worst cross chaining, so not recommended in practice.

So manufacturer chain capacity data can be taken with some liberty, if the user is experienced and careful, never using extreme combinations.

Derailleurs come in three sizes: short, medium and long cage. Derailleur capacity is proportional to cage length. For wide range MTB cassettes with three front chainrings, it is practical to have a long cage derailleur.  A typical narrow ranged road cassette and two front chainrings will make do with a short cage derailleur.

Shimano notes RDs as: SS (short cage), GS (medium cage) and SGS (long cage).

Example: if a bicycle has a crankset with 48-38-28 teeth, and a cassette that ranges from 11-32 teeth, then a needed RD capacity would be:

(48-28) + (32-11) = 20 + 21 = 41
This would take a long cage RD with over 40 teeth capacity

For a typical road bike setup with 53-39 crankset and a 11-25 cassette:

(53-39) + (25-11) = 14 + 14 = 28
A short cage RD with under 30 tooth capacity would suffice.

See here about: what if RD’s chain wrap capacity isn’t big enough?

5. Max sprocket size limit

Apart from RD capacity, an important characteristic of a RD is the maximum number of teeth of the biggest rear sprocket that RD can handle. For short cage road bike RDs, this is usually about 25, or 28, while long cage MTB ones often enable use of 32, 34, or even more teeth at the cassette.

6. Number of speeds, compatibility

A series of posts explaining what can be combined with what: Compatibility

MTB rear derailleurs from 6 to 9 speeds, and road RDs from 6 to 10 speeds are interchangeable and compatible. Slight difference in jockey wheel thickness is not a problem – a 6 speed RD will work on any number of speeds – up to 9 with a MTB and up to 10 with a road bike. It is the same ratio of derailleur cage movement per cable movement, so the shifter levers determine the movement and the number of “speeds”.

10 and 11 speed MTB derailleurs, as well as 11 speed road bike derailleur are not interchangeable. Different cable pull ratio and parallelogram movement for each. There are exceptions from these rules, so if some combining (mix-matchin) is planned, it is best to first read the post about bicycle rear derailleur compatibility.

If shifters used are indexed, shifter mechanism determines amount of cable pulled for each gear, with each shifter click. So rear shifter compatibility needs to be accounted for. There is a difference in cable pull ratio for Campagnolo, SRAM and Shimano. That is why a SRAM rear derailleur will not work properly with indexed Shimano shifters (and vice-versa). With friction shifters (those that don’t have a click for each gear), this is not an issue.

So, one could say that if using indexed shifters, they will work as long as the RD is by the same manufacturer as the shifters. Though, apart from Campagnolo and SRAM, most other manufacturers are compatible with Shimano.

14 thoughts on “Rear derailleurs explained”

  1. Hi there Relja
    I’m thinking about to upgrade my bike from Claris R2000 8s system to Tiagra 4700 10s starting from the RD and keep the rest.
    But i’m concerned about if the Tiagra 4700 pull ratio will work nice with Claris R2000 shifters and the 8s chain.
    Thanks.

    Reply
    • Tiagra 4700 derailleurs will not work nicely with Claris shifters. Goes for both front and the rear.

      What is the current system lacking in order to require “an upgrade”?

  2. The whole system still Claris, my thought is to change 8s to 10s in small steps, because here in Brazil buy the complete system is a little expensive.
    I didn’t bought any Tiagra part yet, i would start to do it if you said that Claris shifters and chain could work nice with Tiagra RD.

    Reply
    • Claris is a very good budget groupset. It’s worked for me flawlessly. With dirt-cheap chains and cassettes compared to 10 speed stuff (at least with prices in Serbia).

      Unless really lacking a gear combination in between the existing gears, I wouldn’t consider Tiagra much of an upgrade. Sure – slightly lighter, slightly faster rear shifting, but since I’m not racing – don’t really care.
      Claris works nicely and shifting is very precise. With nicely shaped levers (at least for my hands and taste).
      It is budget, but gets the job done.

  3. you will see alot of bicycle part makers including specs on most parts,these specs are a guide only but they may still not work on your bike setup,i have even came across incorrect specs on major bike parts,most specs are correct but some are total missprints,if you are doing custom bike building you basically just have to buy and try like we all do,some stuff works out and some does not.dont be worried to try different setups on bikes as home made custom setups often work better than than the factory spec originals.you only fail if you dont try.

    Reply
  4. Good afternoon!

    I am using a 1X set up. My crank is 34T and my cogs 11T-50T. My chain is 116 links.

    Is this good or bad? Thank you so much.

    Reply
  5. Hello Relja,

    Your site is honestly more like a (bicycle) bible! Very nice work (is it too much to say you’re the next Sheldon Brown? haha

    I was wondering if the tech specs of (Shimano) RD’s were set in stone, or more to be considered as guidelines? I have a 6/7 RD-TY300, of which I know will also work on a 9 to 10 speed cassette, yet the tech sheet (https://bike.shimano.com/en-EU/product/component/tourney/RD-TY300-SGS.html) states that max teeth count for the TY-300 is 34T, with a max difference of 20 between low and high.

    The cassette I can get right now (without having to wait eons) is a 11-36T, ie two teeth above max, and a difference of 25T instead of 20T. Will it totally not work, or will it just be a bit more “clunky”. I will be using a friction shifter BTW.

    Thanks in advance!

    Reply
    • Hi Andy,

      A short answer is: it will work, but be careful.

      A longer answer (with a proper explanation):

      There are three things to consider here:

      1) RD compatibility with the cassette and shifter combo
      – that’s fine in this case.

      2) Max. rear sprocket size
      – Shimano is conservative when it comes to this and, in my experience, on a vast majority of bicycles, RDs handle 2 extra teeth with no problems (though this depends on the RD hanger position and design). In other words: I’d give it a shot, but wouldn’t bet my life on it (only up to one beer bets 🙂 ).

      3) Max. chain wrap capacity
      This is usually a correct specification (not a pessimistic/conservative one). There are two paths to choose here:
      a) Leaving the chain long enough to cover the big-big combination – resulting in a chain that’s too long for the RD to wrap the excess in the small-small combination.
      b) Cutting the chain to be short enough for the RD to wrap the excess in the small-small combo, but that means leaving it too short to handle the big-big combo.

      Both options have their pros and cons. I prefer a), and think it’s safer. A detailed explanation of the scenarios with the pros and cons (and pictures) is given in the article:
      Chain length sizing for bicycles with derailleurs
      (Note to self: edit this article to include this link)

      P.S.
      Comparison with Sheldon Brown is very flattering, thank you. 🙂 He’s a role model and an inspiration, I wrote an article on Sheldon Brown’s legacy. Still, without any false modesty, cycling and mechanics was Sheldon’s life. For me, it’s still a hobby, I’m making a living by working with computers and I don’t think I’ll live long enough to get that much knowledge and practical experience as Sheldon had. But yes – this site was envisioned from the start as a cycling encyclopedia (with an idea to make it mobile/smartphone friendly), offering all the info in one place. I regularly use it as a reference and reminder (prefer writing things down to memorizing it all). 🙂

      Relja

  6. Hi Relja,

    What kind of lubrication do you generally use for pulley wheels with steel bearings?

    Thank you for all of the content on your website!

    Reply
    • Hi James,

      You’re welcome. 🙂

      Grease or oil can do the job.
      They both have their downsides.

      Grease offers longer-lasting lubrication (doesn’t have to be re-applied nearly as often), but it attracts a bit more dust and dirt.
      Oil is the opposite – requiring a more frequent re-application but accumulating less dirt.

      For greasing you have to disassemble the cage (freeing one jockey wheel at a time is fine – I’ll make a video on that), clean, lubricate, assemble.
      For applying oil, spray from the sides is good.

      If using grease, the “regular,” “general-purpose” grease is usually of NLGI2 hardness, and I haven’t had any problems with that, though for the application, a softer grease would be a better match (NLGI 1, or even NLGI 0 hardness).

      What I’ve been doing over the past decades has worked fine for me, though I’m sure some (many?) would not recommend it – still, it’s worked fine for me and that’s what I do with my bike(s):
      I disassemble, clean and put some grease, about twice a year.
      In between those “major overhauls,” I wipe off any dirt and dust from the outside (as best as I can) and spray some fine oil lubricant (MOTIP PTFE spray is my lube of choice – but any fine oil should do the job).

      The properties and “quality” of lubricant are of little importance. Regular cleaning and re-application is what makes the difference. Those systems aren’t sealed too well from dirt intrusion and it’s dirt that creates problems, not the lubricant imperfection.

      Having said that, spare jockey wheels (with the bushings) aren’t too expensive, and they are quite durable, even when not maintained properly. So I don’t sweat it too much – the bike(s) should serve me, not the other way round.

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