​The Often Misunderstood Bike Derailleur System Explained

The Often Misunderstood Bike Derailleur System Explained

​Even as a kid, I had always been interested in learning all about bikes and about how to fix bicycles. To me bikes have always been my way of getting away from things. Since I rode a lot, I always wanted my bikes in tip top shape so that I was never put into the predicament of having to walk my bike (no one wants that).

​If you are new to the world of bike maintenance then congratulations on taking the first step; bettering your understanding of how your bike components work.

​One component that had always made me nervous was the chain derailleur system. I always found it fascinating how it allowed you to change gears and at the same time adjusted for your ever changing chain dimensions. Let’s see what I mean by this.

How To Fix Bicycles – Derailleur’s 101

I have to say that in all my years of reading about chain derailleurs, the best explanation that I have found has to be from Sheldon Brown himself in his article Derailer Adjustment. In his explanation, he takes what most would deem a complicated subject and makes it understandable by everyone who reads it. This man knows how to fix bicycles.

What Does the Bicycle Derailleur System Do?

So in its simplest form, the function of your bike’s derailleur system is to “push” your bike chain off of one sprocket and onto another; that’s it. Obviously there is a bit more of a lead up than that (i.e. the rider moves the gear lever which moves the cables which in turn moves the derailleur arms), but that is its sole purpose.

Front Derailleur System

The task of your bike’s front derailleur is to move your bike chain between each of the front chain rings (on some bikes there are 2 and on others there are 3). To do this, the derailleur has what is called a derailleur cage that your bike chain moves within (see below).

To move the chain, the cage starts to push on the side of the chain and in doing so, starts to increase the angle of the chain with respect to the chain rings. As it pushes, the angle of the chain eventually gets too large and it is forced to jump over and onto the next chain ring. All that chattering when changing gears is your chain starting to slip off the cogs that it is currently riding on.

The process that the chain goes through when moving from a larger cog to a smaller one versus moving from a smaller to larger one is somewhat different. Let’s see the difference.

  • ​Chain moves from larger to smaller cog
    Chain moves from larger to smaller cog

As the chain is being pushed, it eventually gets to such an angle that it literally “drops” off the larger cog and onto the smaller one.

  • ​Chain moves from smaller to larger cog
    Chain moves from smaller to larger cog

​As the chain is being pushed it starts to ride up against the next larger cog beside it. As the chain angle gets bigger (i.e. pushing force increases), the chain eventually gets snagged by the next larger cog which pulls it up and onto the cog.

To learn more about the front derailleur and some of its common fixes see the article here.

Rear Derailleur System

The rear derailleur system looks a little bit more complicated but it really does act in the same way that the front derailleur does (i.e. it has an arm that forces the chain off of the cog that it is riding on, onto another one). As with the front derailleur, the rear derailleur pushes the chain sideways until such an angle is reached that it cannot ride on the cogs any longer and slips off and onto the next one.

The main difference with the rear derailleur is that it also has a moveable spring loaded pulley arm whose main function is to take up any slack that occurs on the bike chain as a result of moving either to larger cogs or smaller ones.

For a more detailed description of the rear derailleur system and how to maintain it.

See It Wasn’t That Complicated!

​So hopefully that has shed a bit more light on the sometimes misunderstood bike derailleur system. Remember that in order for you to enjoy your bike riding to its max you need to have at least a bare minimum knowledge of how to fix bicycles. Believe me, once you start to understand how all the components of your bike work together, you will start to enjoy your bike riding that much more as well; trust me on this!

​Do you have comments or questions about this article? Just submit them at the bottom and our community of members will definitely get back to you.

​The way I have always looked at questions is that you can be guaranteed that you are not the only person with that specific question in mind. By asking it, you are also helping out all those that were thinking it but never asking. We look forward to hearing from you. Until then ride safe!

Robert Flaherty

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