A guide to rear derailleur cage length: Short, medium or long?

Ahh, the good old rear derailleur. You already know that it shifts your chain up and down the cassette. But did you know it has a second job?

The funny looking thing dangling on its bottom with the two pulleys is called the rear derailleur cage. Apart from moving the chain to the next cog via the top guide pulley, it has to keep the chain in tension by taking up the slack – using the bottom tension pulley and the P-knuckle spring inside the derailleur itself.

Smallest cog + small chainring on the Dahon Vitesse T20.
The Tiagra rear derailleur’s medium-size cage is set all the way back.

This is most evident when you shift to your smallest rear cog and smallest front chainring. In this combination, your chain is the slackest it will get, with barely any tension in its links. The rear derailleur cage will be set as far back as it will go as the P-knuckle spring tries to keep the chain tensioned.

Conversely, if you shift to the largest cog and largest chainring, the cage will be set forward as the chain is under the most tension.

If you look at different bikes long enough, you’ll notice the cage varies in length from bike to bike. Ever wondered why this is so?

First let me tell you what it doesn’t mean.

  • The length of the rear derailleur cage doesn’t determine shift speed.
  • The length of the rear derailleur cage doesn’t determine how cool a cyclist you are.
  • The length of the rear derailleur cage doesn’t necessarily determine the largest cog it can accept.

Let me return to that last point – it does, sort of, determine the largest cog you can use. However, it’s not the whole story. The whole business about largest cogs is only part of a bigger, more important picture.

The service instructions bundled with your rear derailleur contain all sorts of instructions, specifications and numbers. One of the those that doesn’t seem immediately obvious or self-explanatory is called total capacity or chain capacity.


It’s the amount of potential slack chain the rear derailleur cage can handle — the discrete measurement of how well it can keep a chain in sufficient tension. Seeing as it’s expressed as a number of teeth, it’s derived by the following equation:

Chain capacity = (biggest cog teeth – smallest cog teeth) + (biggest chainring teeth – smallest chainring teeth)

Because it’s based the number of teeth of the entire drivetrain, the effective chain capacity varies from bike to bike. Drivetrain manufacturers therefore have to cater for these variations.


Shimano categorizes its rear derailleurs into three tiers depending on the cage length, and they’re denoted by the suffix on the model number.

  • “-SS” refers to the short-cage versions, with a maximum chain capacity of 33T or less. These are usually the domain of road bikes. On the mountain bike side, the few true short-cage rear derailleurs that exist are for downhill or gravity racing applications, such as the Saint RD-M820 unit, since this discipline usually does away with large cogs and multiple chainrings.
  • “-GS” refers to medium-cage rear derailleurs, built with a maximum chain capacity of 37T to 39T or less. This is usually the longest cage offered for road bike rear derailleurs, but also the default “short” option offered for their mountain bike counterparts.
  • “-SGS” marks the long-cage rear derailleurs and this is exclusively the domain of mountain bikes or any unit marketed for touring bikes. A typical unit sees chain capacity well into the 43-44T range.
Shimano Deore RD-M615-SGS, long-cage.
Rated max chain capacity is 43T.

Shimano Saint RD-M820-SS, short-cage for downhill MTB.
Rated max chain capacity is 25T.

In the real world, how does this all work? Let’s look at my bikes.


When Bino first received a drivetrain upgrade, his rear freehub body received a 12-30T cassette and a Tiagra RD-4600-GS medium-cage rear derailleur was bolted onto his frame. This has rated chain capacity of 39T or less.

Bino in Dahon Vitesse T10 form. 1×10 drivetrain on the largest 30T cog.

Computing chain capacity on a bike with a single chainring is pretty simple.

Chain capacity = (30 – 12) + (52 – 52)

Chain capacity = 18 + 0

Chain capacity = 18T

For a 1×10 bike, in terms of chain capacity, the medium-cage Tiagra rear derailleur is actually overkill. While there is technically nothing wrong with running a medium-cage rear derailleur, given Bino’s small wheel size and potential ground clearance issues, many people prefer a short-cage unit for their folding bikes.


Tiagra CS-4600 cassette, 12-30T + 105 RD-5701-SS short-cage rear derailleur
FSA Omega cyclocross crank, 46/36T

Hyro came stock with a RD-5701-SS rear derailleur from Shimano’s 105 5700 groupset, specified to accept a 28T largest cog and a total capacity of 33T or less. It was paired with a 12-30T cassette and a 46/36T cyclocross-specific crank. On paper, the rear derailleur isn’t supposed to work with a cassette with a 30T largest cog.

An excerpt from the service instructions for the 105 RD-5700 rear derailleur.

Or should it? It doesn’t seem to make sense for Giant to have built up the bike with these drivetrain parts. Let’s use the chain capacity equation and compute.

Chain capacity = (30 – 12) + (46 – 36)

Chain capacity = 18 + 10

Chain capacity = 28T

This combination of cassette and chainrings is actually quite comfortably below the 33T limit. On the stock gearing I never had any shifting problems, and could even cross-chain if necessary.


I completed the 105 groupset drivetrain parts by installing the FC-5750, a 50/34T road compact crankset. I used the exact same chain as before, and for a while I thought everything was peachy. Shortly after, I started getting poor chain retention and dropped chains when I used the two largest 27T and 30T cogs with the 50T chainring, which my chain catcher has been working overtime to combat.

Largest cog + large chainring. The rear derailleur cage is just too short, and the gear combination requires a longer chain as it’s no longer wrapping around the pulleys.

So what happened here? Let’s revisit the chain capacity equation.

Chain capacity = (30 – 12) + (50 – 34)

Chain capacity = 18 + 16

Chain capacity = 34T

Uh-oh. With increased gap between the 105 crank’s chainrings, my rear derailleur and chain are overwhelmed by the added chain capacity of the entire system. The effects are dramatic.

Largest cog + large chainring with the Tiagra RD-4600-GS medium-cage rear derailleur. The chain still makes an S-bend around the cage and pulleys, which means the rear derailleur handles this combination just fine.

Bino’s recently completed 2×10 drivetrain provides the answer. The cassette and chainrings are identical, but because the Tiagra rear derailleur has so much more chain capacity, used with an accordingly longer chain, I haven’t had any chain retention problems at all.

I don’t have the cyclocross crank any more, so solving Hyro’s problem requires a longer chain.

Smallest cog + smallest chainring with a short-cage 105 rear derailleur.

As seen in the above photo, retaining the short-cage rear derailleur while fitting a longer chain isn’t going to be enough, because its cage can’t handle any more slack. The chain is already close to rubbing on the rear derailleur’s guide pulley and the TCX’s chainstays.


I got a good deal on a second-hand RD-5700-GS medium-cage rear derailleur with 37T rated max chain capacity. It’s pretty rare, as most RD-5700 and RD-5701 units found in the Philippines are of the short-cage flavor.

Shimano 105 RD-5700-GS rear derailleur
The back side of the RD-5700-GS

I would then need to break the current chain and replace it.

With the chain broken and removed, off comes the stock RD-5701-SS short-cage 105 rear derailleur, and on goes its bigger brother.

Short-cage SS model on top, medium-cage GS model at the bottom

I paired it with a basic Shimano CN-HG54 10-speed chain.

Driving the joining pin into the chain.

With the rear derailleur replaced, I installed the chain. It’s now at 112 links compared to the outgoing chain’s 108.

Rear derailleur and chain swap complete.

To check my handiwork, let’s shift to the smallest cog and small chainring combination.

Smallest cog + small chainring on the medium-cage 105 rear derailleur.
A closer look.

The longer derailleur cage is doing a good job of keeping tension on the lower run of the chain. Let’s move the chain all the way to the other direction.

Largest cog + large chainring combination on the medium-cage 105 rear derailleur.

Things look good here too. The cage angle is nowhere near as horizontal as the short-cage derailleur was. Turning the pedals with the bike on the stand, I’m getting much better chain retention on the big chainring. The chain used to drop into the small chainring when I got within a sniff of the two largest cogs. Perhaps I could have even used all 114 links the chain came with.

While it’s still no good for the drivetrain to stay in a cross-chained state, with no optical gear indicators on the STI levers, it’s easy to lose track of gears when hustling the pace. Chain drop on such occasions can be catastrophic as forward drive is lost. With the 105 medium-cage rear derailleur and a suitably longer chain, my TCX can at least retain drive while remedying the cross-chained state.


With that, a quick summary of lessons:

  • A correctly sized chain is critical for a derailleur drivetrain with multiple gears.
  • A chain with insufficient length means there is too much tension to use certain gear combinations.
  • A rear derailleur is responsible for shifting the chain up and down the cassette, and maintaining tension on the slack links of the chain.
  • Rear derailleurs have different cage lengths in order to better handle the chain capacity of a drivetrain’s combination of chainrings and cassette.

31 thoughts on “A guide to rear derailleur cage length: Short, medium or long?

  1. Randonuer,
    Wish to ask some question. I have a Vitesse too, 52T Chainring, 8 speed 11-32T cogs. Currently on stock Altus RD-M310 ( looks longish cage to me). I intend to change the RD only, any issues if i use SS /GS Road RD like Tiagra / Sora 9 Spd.
    Based on ur chain capacity formula = 21T.
    It seems like single chairing is much less fuss on the choice of RD, right?
    Also, i intend to use back my stock Altus 8 speed shifters.

    Hopefully u can learn from you ..Thx.


  2. Vertigo, thx for the advice…successfully change to SS Shimano 105 RD-5700, and runs fine. Surprisingly the stock SL-M310 shifters works well enough with it too.


    1. Yes, any Shimano MTB shifter that’s 9-speed or less should work with most road bike rear derailleurs. Glad to know the swap worked!


  3. I have been given what i think is an Ultegra RD 6700 GS rear derailure, but how can i check – do you know the length of the cage between jockey wheels? I want to use it with a larger cassette.


    1. You can search your model of rear derailleur on si.shimano.com, where Shimano keeps most of its technical information on its parts.

      The model number of your rear derailleur is usually etched into the inboard side of the parallelogram linkage, but it won’t tell you cage length. Based on my RD-5700-GS unit, a medium-cage rear derailleur has a cage around 9 cm long between the axles of the pulleys/jockey wheels.


  4. Hi TypeVertigo,

    I have Tiagra GS RD, and plan to use 11 – 34 sprocket, with double crank 50 – 34.
    if i calculate with the formula then it comes to max 39t chain capacity.

    is it Tiagra GS can handle that situation?


    1. On paper it should be able to, but the only way to know for sure is to try it out. You may have to wind in the B-tension screw more to make it work. Also best to make sure you size your chain properly for your combination of cassette, chainrings, and your bike’s chainstay length.


  5. Any thoughts on running an 11-32 11-speed cassette with an Ultegra 6800 GS? Gearing up front is 46-36, possible 46-34. Chain Capacity would be (32-11)+(46-34) = 21+12 = 33. Right at the limit for GS, but the 32 would be bigger than the reported max cassette. Thoughts?


    1. The only real way of finding out is to test for yourself. No guarantees it will work fine on a short-cage derailleur, even though capacity checks out fine. The usual trick to making this work is to wind in as much B-tension screw as you can. If it still doesn’t work, you can swap in a longer B-tension screw.

      If you do proceed, make sure you run as long a chain as you can. Good luck


  6. Useful, but doesn’t help me. What are the various lengths? and how do I measure them?
    The Shimano derailleurs I have are not marked (SS, GS, SGS) so how do I work out which I have?
    Even the Shimano Spec sheet doesn’t say.


    1. You measure cage length from the axle of the guide pulley to the axle of the tension pulley.

      A little research on Google will point you the right way.


  7. SRAM XD drivers can actually handle a 9-tooth cog, therefore I’m surprised that neither SRAM or a third party has released a 9-36 XD cassette for small wheel bikes. This would give you a 400-percent overall gear range, and may let you run a short cage derailleur because there are only 27 teeth difference between the largest and smallest cog.


  8. Your article is particularly well written and clear. Also, great examples. Your efforts are appreciated.


  9. You left out single chainring cranksets here. I have a Giant MTB with a rear cassette of 42-11t which equals 31t. Because my largest chainring is 38t and my smallest chainring is 38t the formula works out like this for me (41-11)+(38-38)=31t. So I’m good to go with a short cage right?

    Thanks for the formula. This is the best expalnation I’ve found for my rear derailleur upgrade. I can’t stand my SLX gorup that came on a Giant’s carbon fibre 2018 XTC advanced 27.5+2


    1. Maybe read the post again. Case 1 literally talks about my folding bike which has a 1×10 drivetrain, and I computed rear derailleur capacity accordingly. “1x” is how many folks refer to drivetrains with just one chainring.

      I don’t know what rear derailleur you are using, but if it will accommodate 31T chain capacity then it is fine for your cassette.


      1. Ahhh, I skpped that one because it looked like it would have more than one chainring. My bad. Either way, out of all explanations I’ve come across on the internet for short, medium and long cage rear derailleurs.


  10. I have an old hybrid bike with six speed freewheel, 3 chainrings, Shimano Revoshift 6 & 3 speed shifters, Tourney RD35-TX RD. I damaged the RD but I found a Shimano Altus RD-M310-SGS at a very good price (it’s now discontinued I think). It has 43T chain capacity which is fine for my setup, but it specifies it is for 7/8 speed freewheel. Does this mean that the indexing ratio is different from a 6 speed RD? Or do they not mention 6 speed because they assume that no one will be fitting an Altus to an old six speed bike? I would love to give it a try but I don’t want to waste my money. Matt


    1. The indexing really happens in the shifter since that’s what’s doing the actual pulling and releasing of the cable. My best guess is the Altus RD should be fine to use with your bike, since it’s based around the same cable pull.


  11. I have some medium to hard climbs in my area But have a 10 speed 105 short cage with a compact crank 34/50 and a 11/30 cassette if I drop out my short cage rear cage plates and swap for a medium plates and a new chain would It be ok to run a 11/34-11/36 or what’s the max range limit I can go for ease on road bike climbs
    I would rather go down this route than fitting an wolf tooth bracket


    1. The cage swap is only half the equation. The medium-cage derailleur also has tighter angles in its knuckles. There’s no guarantee a short-cage derailleur modified with a longer cage will work to accommodate lower gearing.

      You’re better off running a smaller crank. There are 48/32 and 46/30 options that should get you the lower gearing you need for climbing, and the effect of a crank change is much more evident than any change at the cassette.


  12. Fantastic article, very well written and full of easy to understand explanation, thank you. I am hoping to install a 12-30 Ultegra cassette with a 53-39 which will come in just under the 33T limit, at 32T, for my current 10sp SS. Would you consider this to be workable or too close for the long term health of my drivetrain? Many thanks again.


    1. I’m guessing the rear derailleur is also an Ultegra RD-6700 unit? In that case I think it should work. Prepare a new chain just in case, if you don’t have one at hand already. You might need a slightly longer run of chain to accommodate your new gearing – about 2 or 4 links more is my guess. Good luck!


      1. Excellent. I’ll get right onto that.
        Thank you for such a speedy reply, I appreciate it the extra knowledge you have given me.


  13. Appreciate the article most helpful. My question is nearly the same as Nigel Nicholas’ but with a 52/39 crankset that I’ve been led to believe won’t accommodate a 36T small chainring. So I’m then left with either changing the whole crankset or getting the large cog up as high as possible. I run Ultegra 6600 rd with a short cage on a 10 speed (currently 11-23!) if i swapped it out for a medium cage (whole deraileur not just cage), could i get this up to 32 or even 34. I realise a new chain will probably be in order.


    1. I don’t have the details on the 6600 derailleurs. If you get a medium cage 6700 derailleur it should work and be a straight swap as it’s still 10-speed.


  14. Thanks for this helpful article. My question is just about the same as Nigel’s, trying to get a lower gearing without changing over the whole crankset. Currently running an Ultegra 52/39 crankset with 11/23 cassette on an ultegra RD6600 short cage. I can get a medium cage RD easily and trying to work out whether that will work with and HG500 11-32 or 11-34. Any insight or suggestions would be appreciated.


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