How do I choose a rear derailleur? In addition to price, performance and weight an apparently “uninteresting” detail also exists on the spec sheet: derailleur cage. What role does it have and in which way can it help you while you’re pedaling?
In this article we will show you, step by step, the advantages and the disadvantages of using long and short cage derailleurs.
Long cage derailleur
It is most often used for 27 or 30 gears combinations and is recommended especially for Cross Country and Marathon competitions to pick up excess chain when using small ratios (small chainrings – small sprockets). Basically you will never use these ratios, but the cage keeps the chain under tension regardless of the chosen ratio. Its disadvantage is that even if it functions on all ratios, it may slack and hit the frame. Also, shifting performance on strongly irregular terrain decreases, due to the cage unwanted movements. A practical advantage is that it supports the chain tension, irrespective of the choice.
Disadvantages: Although it works in all situations, the chain may dropt and hit the frame. Also strongly irregular terrain changes may be inaccurate due to derailleur movement..
Short cage derailleur
This gives you the advantage of a better chain tension, providing more precise shifting, and the chain no longer hits the frame. Shifting gears also become more silent.
However, a short cage is not to be used for 27 gear ratios bicycles. As we (should) know, a chain must be long enough to fit the largest chainring – largest sprocket ratio, plus two links. If we follow this rule and install a short cage derailleur, if we utilize a small chainring – small sprocket, we will drop the chain.
Why a short cage offers a better tension and more precise shifts when the chain length stays the same?
According to the laws of mechanics, the cage acts as a lever. The longer the lever, the easier it is to move it against a fixed force, in our case, the derailleur’s spring which keeps the same tension regardless the cage’s length. A chain that slacks can be represented as the force acting on the lever (derailleur’s cage in our case) and puts tension on the spring. Long cage = easier to move / short cage = harder to move.
Given the fact that usually mountain bikes use cassettes with the widest ratios (11-34), we get to the following rule:
- 1 chainring – SHORT cage
- 2 chainrings – MEDIUM cage
- 3 chainrings – LONG cage
If you use only one chainring you can use a short cage derailleur. One chainring, regardless its size, does not affect the derailleur rated capacity.
Instead, what influences derailleur’s rated capacity is excess chain resulting from two different chainrings, therefore, in this case, you do need a longer cage.
What rated capacity means
Depending on chosen ratio, derailleurs have the ability to take up excess chain. Basically, to pedal using a large chainring – large sprocket combination you need all the chain’s length.
When using small chainring – small sprocket, a few extra links remain, basically without any use, but they must be somehow taken by the derailleur. Hence its “rated capacity”, marked as 30T, 45T etc.
Basically you will never use the aforementioned ratios as it should result in a fast chain wear.
Using these reports is an abuse, the chain alignment resulting in rapid wear of the chain due to its diagonal positioning.
Let’s take for example, a typical mountain bike with 27 gears ratios, featuring a 42-32-22 teeth crank and 11-34 tooth sprockets. To find out the derailleur’s rated capacity (T) we apply a simple formula: add up teeth on the biggest chainring and sprocket and then deduct the sum of the smallest chainring and smallest sprocket.
(Big Chainring + Big sprocket) – (Small Chainring + Small sprocket) = rated capacity (T)
T = (44 + 34) – (22 + 11) = 78-33 = 45T
According to Shimano, their derailleur’s rated capacities are 45T for long cages and 30T for short cages.