It is often asked: what is an optimal chain length for a bicycle with derailleurs? Should a new chain be shortened and how much? That is answered in this post. For single speed bicycles (or bikes with internal gear hubs), see: Determining optimal single speed chain length.
A new bicycle chain usually comes with 116 links. This is long enough for the biggest chainrings and for most distances of rear wheel from front chainrings. So for optimal length a new chain is usually shortened from the 116 links that come in the box. In rare occasions – touring frames for example, with extremely long chainstays, 116 links might not be long enough, so a few links off another new chain need to be added to reach optimal chain length.
Too long chain will increase risk of chain dropping, while too short chain puts too much stress on the drivetrain, or can damage it, if it can be put and closed over the chainrings in the first place. Chain that is too short can also damage rear derailleur.
There are several methods for getting the chain at the correct length. Method explained here is certain to result in a chain long enough not to cause any drivetrain damage. Method is called “big-big”.
In order to make absolutely sure the chain is long enough, it should be put around the biggest chainring in the front, the biggest sprocket at the rear and then add one more inch, or one link because of the derailleur. When measuring, the chain is not put through the rear derailleur, the derailleur is moved so it doesn’t get in the way.
For bicycles with rear suspension, the suspension should be totally bottomed – compressed when using this method. See the position of travel where the rear wheel is furthest away from the cranks. To achieve this with air shocks just let the air out, while for ones with spring one has to remove the shock, remove the spring, then put the shock back. Simpler, though not 100% reliable method is adding one extra pair of inner and outer links to the length determined by the method explained below.
With modern, more and more popular systems that have one chainring in the front and multiple chainrings in the back (“1x” systems), the optimal length is achieved when adding two extra links, instead of just one, after wrapping the chain around the only front chainring and the largest rear sprocket.
That’s all. A simple method. What to look out for?
Rear derailleur chain wrap capacity may not be big enough.
The explanation of what RD capacity is can be found under “Derailleur capacity” in this page: Rear derailleur.
This results in a situation that in a small-small chainring combination, the chain is too slack and can even fall off chainrings on bumpy roads.
This problem can be solved in three ways:
- Getting a rear derailleur with a longer cage.
- Avoiding smallest few sprockets at the cassette when the chain is on the smallest chainring in the front.
- Shortening the chain.
The third method is potentially dangerous, because in a big-big combination the chain will no longer be long enough. This can result in tearing the rear derailleur when changing to the biggest sprocket at the rear if the big chainring is used in the front.
Small-small and big-big combinations are combinations to avoid anyway, because the chain is cross-chained in those combos, same gear ratio can be achieved with middle-middle combination easily. Still, it can happen that in a moment of lesser caution, one does shift into one of those combinations. That is why, if method 1 is not possible for any reason (financial or some other), it is safer to use method 2 than method 3.