The derailleur is an important part of the bike to ride smoothly, which primary function is to move the chain correctly to the sprockets on demand. Derailleur replacement can be an expensive process, whereas can we use any derailleur on our bikes? Or, are bike derailleurs universal?

Replacing a bike’s derailleur can cost around $110. But if you can do it yourself, you can replace the derailleur for just $50. And as an enthusiast or mid to pro rider, you must have expert knowledge of this important part of the bike as well as replacing the derailleur.

In this article, I will present some important information about bicycle derailleurs including their types and parts. And, I really believe, this will help you to know more about bicycle derailleurs.

What Is Bike Derailleur?

A derailleur is a part of the bike that allows you to change gears while riding the bike. It is attached to the frame with a mounting bolt. And, these usually consist of a movable chain guide that easily moves the chain from one sprocket to another.

  • If the bike has multiple chainrings at the front and multiple sprockets at the rear, it is necessary to use a derailleur.

A derailleur looks like a collection of folded pulleys and springs. They may have different designs or models, but the basic functionality is the same.

  • The main purpose of using derailleurs is to change gears.
  • Plus, they maintain a balanced tension of the chain between the chainring and the sprocket. Because of this, the chain does not slip off the sprocket or chainring while riding.

Types Of Bike Derailleur

Generally, 4 types of rear derailleurs are used for bikes which are determined according to convenience, considering the type of bike, drivetrain, and others. Below I am going to explain these.

  1. Short Cage with Hanger: This category of derailleurs is generally suitable for bikes that are available with low gear ratios. Thus a biker can use them on a road bike or hybrid bike with two chainrings. These types of derailleurs also include a small hangar attached.
  2. Short Cage Bolt-On: These are suitable for bikes with low gear ratios too, but do not have separate hangers with derailleurs.
  3. Long Cage with Hanger: If your bike has a high gear ratio, then you can select the derailleur of this category.
  4. Long Cage Bolt-On: These derailleurs are also suitable for bikes with higher gear ratios such as mountain bikes. But they also have no hangers.

Bike Derailleur’s Parts & Their Proficiency

A derailleur has several parts and knowing about these parts will give you a better understanding of how the derailleur works. Below I’ll describe the parts of a rear derailleur briefly.

  • T-Pulley: Acts to maintain a balanced tension on the lower part of the chain.
  • Guide Pulley: The guide pulley serves to transfer the chain from one sprocket to another.
  • Cage: This helps keep the chain in line between the guide pulley and the T-pulley.
  • Barrel Adjuster: Helps adjust cable tension, which is connected to the shifter.
  • Mounting Bolt: This keeps the derailleur firmly attached to the bike frame.
  • Parallelogram Linkage: Acts to move the chain left or right to the correct position.
  • B-Knuckle: The upper part of the derailleur.
  • High and Low Limit Stops: Prevents the chain from slipping into the spokes or dropping to a smaller cog than the specified one.
  • P-Knuckle: It holds both the spring and the guide pulley.
  • Cable Bolt: Mechanical derailleurs have this type of bolt that connects the shifter cable to the derailleur.

But Are Bike Derailleurs Universal?

No, bike derailleurs are not universal at all. In most cases, the derailleur of one model bike does not fit another model bike. Although they work in the same way, the size varies according to the type of bike and drivetrain. And, this different size makes them inconvenient for use on all bikes.

You may notice that the rear derailleurs have different cage lengths (the distance between the upper and lower pulleys).

  • The cage length of bike derailleurs is determined based on the gear ratio. In most cases, the cage length of mountain bikes is more than the cage length of road bikes. This is because mountain bikes have larger cassettes and increased gear ratios.

Besides, road bikes use derailleurs of slightly shorter cage lengths because of their relatively low gear ratio.

  • But if the road bike has three chainrings, then you should go for derailleurs with a slightly larger cage length.

Another reason one bike’s derailleur cannot be used on another bike is the shifter.

So, if you need to replace your bike’s derailleur, you certainly don’t want to spend extra and change the shifter as well. In most cases, a particular gear shifter does not fit the derailleur with a different gear ratio.

And, Are Derailleurs Interchangeable?

Usually, the front derailleurs on a bike are interchangeable. However, most rear derailleurs are not interchangeable. The category of the derailleur is determined based on the gear ratio of the bike. So, rear derailleurs are not interchangeable unless they are of the same model or type of bike.

For the same reason, any derailleur will not fit with all bikes.

  • For example, a short cage length derailleur may fit on a road bike, but you may not fit it easily on a mountain bike.
  • Additionally, as mentioned earlier, the shifter on the bike will not fit all derailleurs.

Therefore, if there is ever a need to replace the derailleur, be sure to select the correct model of the derailleur based on your bike and gear ratio.

Final Thought Regarding Bike Derailleurs

There are different types and sizes of bike derailleurs in the market but any of them are not compatible with all types of bikes. Where a bike with more gears will need a larger derailleur, on the other hand- bikes with less gear ratio require a derailleur of small cage length as well.

Therefore, bike derailleurs are not universal for their structural reasons. Some derailleurs are only compatible with certain types of shifters. But, you’ll also need to make sure that the derailleur is compatible with the type of shifters you have. Thus while choosing a derailleur, pay attention to whether it is compatible with your bike or not. And, only after that move on to doing the job to do like derailleurs installing or replacing.