A derailleur system is something to behold. No really, if you stop and think about it a derailleur system is a very interesting bit of engineering … quite ingenious! But as is the case with just about everything they have their limitations. They can only handle so much. And that is what this posting is about … just what can they handle? How is their limitation determined? Well, people far more intelligent and understanding of all of this than I am have stepped up to the plate to explain it all.
But to start this off here is a video showing a rear derailleur doing it’s thing. Since the bicycle in this video is upside down I highly suggest that you stand on your head while viewing it in order to ensure you get the right perspective. (don’t worry, it is a short video) … Don’t cheat and turn your computer screen upside down.
And if you didn’t follow the instructions to stand on your head here is one where the bike is right side up:
By definition …Total Capacity: This is reference to a rear derailleur’s full range of motion; both vertically and horizontally. The larger the capacity, the wider range of gears that derailleur can handle. Your goal is match the derailleur and its capacity to the bike. If a derailleur doesn’t have the capacity to shift over the lateral range of a cassette, or to shift into the larger cassette cogs, those cogs will be unusable.
According to United Bicycle Institute:
Determine Maximum Chainring Difference by subracting the number of teeth in the smallest chainring from the number of teeth in the largest chainring
Determine Maximum Cassette Cog Difference by subtracting the number of teeth on the smallest cassette cog from the number of teeth on the largest cassette cog
Determine Total Drivetrain Capacity by adding Maximum Chainring Difference to the Maximum Cassette Cog Difference
Record the Maximum Cassette Cog (the number of teeth on the largest Cassette Cog)
SS – Short Cage Road Double – Maximum Cassette Cog is 27 and Total Capacity is 29
GS – Medium Cage MTB/Road Triple – Maximum Cassette Cog is 34(MTB)/27(Road) and Total Capacity is 33(MTB)/37(Road)
SGS – Long MTB – Maximum Cassette Cog is 34 and Total Capacity is 45
Short – Maximum Cassette Cog is 34(MTB)/28(Road) and Total Capacity is 32(MTB)/31(Road)
Medium – Maximum Cassette Cog is 34 and Total Capacity is 37
Long – Maximum Cassette Cog is 34 and Total Capacity is 45
Short – Maximum Cassette Cog is 26 and Total Capacity is 27
Medium – Maximum Cassette Cog is 29 and Total Capacity is 36
Long – Maximum Cassette Cog is 29 and Total Capacity is 39
NOTE – THIS INFORMATION IS SUBJECT TO CHANGE BY MANUFACTURER
And a great source for all of this is Sutherland’s 7th edition
The above is found online HERE.
Here are some more tables for reference:
These tables can be found HERE. There is also lots of other great and helpful information available on this website.
Did you follow all of that? It is really rather simple. Let’s use my Catrike Trail to illustrate this. I have 30/42/52 teeth chainrings (front sprockets) and 11-34 teeth cassette cogs (rear sprockets)(11 teeth is the smallest and 34 teeth is the largest). Only the smallest and largest sprockets are involved in this so ignore all the other sprockets. Now just do the math:
front: 52 – 30 = 22
rear: 34 – 11 = 23
22 + 23 = 45
I have a SRAM long cage derailleur which has a maximum capacity of 45 so I am okay with my current gearing. However, if I wanted to make any changes in my sprockets I would probably be in trouble as I am already at the maximum capacity of my rear derailleur. Sometimes we can get away with just a little bit of variation, but we shouldn’t press our luck as we might find ourselves in trouble.
The closer you get to the maximum capacity of a rear derailleur the more the derailleur will move about forwards and backwards. When this forward and backwards movement is taking place is where the derailleur will reach it’s limits of what it can handle. A cycle with a low number will not require the derailleur to move much at all in comparison to a cycle with a high number which is up around the limit of the derailleur. A derailleur can actually move back so far that the bottom return portion of the chain will rub against the top drive portion of the chain. And if the derailleur moves forward as far as it can and the chain is too short for the run between the sprockets front to back then the chain will simply get tight and could get too tight to where damage occurs to the derailleur. That is a scenario you want to avoid for sure. On a tadpole trike where the boom is adjusted in and out to accommodate different rider’s X-seams one must remember that the rear derailleur can only handle so much change in movement of the boom and the consequent change in the chain. Shortening the boom beyond what the derailleur can handle will only result in the chain hanging down (the derailleur can’t handle it all). Lengthening the boom beyond what the derailleur can handle is where you can have serious trouble and damage occur. Always be sure you don’t go beyond what the derailleur can handle. I wrote an ARTICLE about this in the past.
HERE is an article by Sheldon Brown.
Examples of the a rear derailleur when things aren’t right …
In the picture below the derailleur has ran out of forward travel and the chain is starting to get tight. This derailleur is definitely at it’s maximum capacity and perhaps might even be beyond. (This is when the chain is on the largest sprockets front and rear.)
In the picture below the rear derailleur has ran out of rearward travel and the chain is actually rubbing against itself. (This is when the chain is on the smallest sprockets front and rear.)
Neither of the situations are desirable. Now it is true that the second picture could be the result of the chain being too long and removing a link might cure it. However, it also could result in making the chain too short when going to the large sprocket to large sprocket combination. One must keep all this in mind so as not to experience damage to the rear derailleur.
HERE is ParkTool’s article on determining proper chain length.
So hopefully if you didn’t already understand this bit about the capacity of a rear derailleur this has helped you to grasp it. Having the chain, sprockets and derailleurs set up properly is very important. When everything is set up properly we should be able to …
ENJOY THE RIDE!