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.
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.
WHAT DOES “TOTAL CAPACITY” OR “CHAIN CAPACITY” MEAN?
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.
CAGE LENGTH: SHORT, MEDIUM OR LONG
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.
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.
CASE 1. BINO AS A 1×10 BIKE
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.
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.
CASE 2. HYRO’S STOCK DRIVETRAIN
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.
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.
CASE 3. HYRO AND BINO WITH ROAD COMPACT CRANKS INSTALLED
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.
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.
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.
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.
CASE 4. REMEDYING HYRO’S ISSUES
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.
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.
I paired it with a basic Shimano CN-HG54 10-speed chain.
With the rear derailleur replaced, I installed the chain. It’s now at 112 links compared to the outgoing chain’s 108.
To check my handiwork, let’s shift to the smallest cog and small chainring combination.
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.
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.
CONCLUSION
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.
the accidental randonneur 