The bike derailleur is one of the primary elements of most modern bikes. It is composed of a metal fork that pushes the bike chain to the sides, moving it from one sprocket or gear to another. This component is found on the vast majority of leisure, road, or mountain bikes.
The bike derailleur typically has a long life, but like any mechanical element, it is subject to wear and tear. Along with regular maintenance, you will also have to consider a periodical replacement. A worn derailleur is unable to shift the chain properly from one gear to another, causing performance issues.
On the contrary, a properly adjusted and maintained bike derailleur will offer a clean gear shifting even in the most demanding conditions. For this reason, it is essential to understand how to choose the right bike derailleur.
Most of the modern bikes available on the market have two types of derailleurs, front and rear one. Although they are both parts of a bike’s transmission system, there are slight differences between them. Let’s see what they are and what to consider when choosing any type of bike derailleur.
How To Choose The Front Bike Derailleur
As common sense suggests, the front bike derailleur shifts the chain allowing you to change the front gears. This will help you improve your riding performance and use less effort to cover a determined distance. The front derailleur was originally introduced in France in 1982 by Lucien Juy, and the basis of the modern type of front derailleur has been developed by the Italian manufacturer Campagnolo.
However, as it is easy to imagine, this essential transmission component has received numerous improvements over the year, many of them regarding the choice of materials and the operating efficiency.
The operation of the front bike derailleur is very simple and intuitive. The derailleur is mounted on the frame of the bike and it is positioned above the front gears. The chain must be pass inside the derailleur’s body which has a parallelogram shape, and it is important to check that the track of the derailleur is parallel to the gears.
To change the position of the chain, it is sufficient to act on the left handlebar. This will allow you to move the chain from the largest to the smallest gear and vice versa, controlling, in this way, your cycling performance and optimizing your efforts.
The chain shifting from one gear to the other is controlled through the transmission cables that transmit the impulse from the handlebar to the derailleur, triggering the movement. To guarantee maximum efficiency, you should consider the following aspects when choosing a front derailleur:
- Cage type: there are two types of front derailleurs, with double or triple cage. They fit either on bikes with two front gears or on bikes with three front gears. The vast majority of the bikes have a double chainring requiring a double cage derailleur but check your bike to make sure which type you need.
- Drivetrain compatibility: the front derailleur must also be compatible with the bike’s drivetrain. Most entry and mid-level bikes have a 9-speed drivetrain, although the actual number of the speeds can vary between seven or eight to 10 speeds or more. The reason why you have to consider the number of speeds is that the thickness of the chain may vary according to the size of your drivetrain, and it is essential to choose a derailleur that matches the thickness of the chain.
- Mounting: how the derailleur mounts to the bike is another thing to consider. Band derailleurs use a clamp system that secures the derailleur to the tube of the seat. In this case, you’ll only have to check the diameter of the clamp and that of the seat’s tube, making sure they fit. Braze on derailleurs mount directly to the frame and are most common on high-end carbon fiber bikes.
- Transmission: it is also important to consider how the transmission cable that actuates the derailleur is pulled. Conventional models use a bottom pull mechanism. The cable pulls downwards and is routed under the bottom bracket. This is common on full suspension mountain bikes, road bikes, and touring bikes. Unconventional derailleurs usually installed on certain types of mountain bikes use a top pull system. The cable pulls upwards and is routed under the top tube.
- Brand: it is recommended to stick to the derailleur brand that was installed on the bike from the beginning unless you’re a true expert in changing and choosing the front derailleur.
How To Choose The Rear Bike Derailleur
The rear derailleur consists of a cage that holds two pulleys. These pulleys guide the chain in an S-shaped pattern while an arm located above the cage guides the chain of the bike across the sprockets. The arm is attached to the bike’s frame and it is controlled by a spring mechanism connected to the transmission cable.
The rear bike derailleur also presents a number of adjuster screws that allow cyclists to control the tension of the derailleur’s spring.
The rear derailleur is actuated by the right-hand gear shifter and almost all types of multiple gear bikes are equipped with a rear derailleur. To choose the best one, you should consider:
- Cage type: the cages of the rear bike derailleurs have different lengths and usually a longer cage means that the derailleur can accommodate a larger drivetrain and set of gears. As a general rule, most types of mountain bikes need a long cage derailleur while most road bikes are equipped with short cage derailleurs.
- Clutch: on a rear derailleur, the clutch ensures that the tension is retained in the derailleur. Not all bike derailleurs have a clutch, although this feature is increasingly present on most models. Shimano derailleurs are among the most popular models with the clutch.
- Drivetrain: the rear derailleur must be compatible with the drivetrain of the bike. There are specific models that fit certain types of drivetrains and it is recommended to avoid switching derailleurs from one type of bike to another. For example, you should avoid mounting a mountain bike rear derailleur on a road bike.
- Compatibility: brand compatibility is as important as all the other characteristics. Like in the case of front derailleurs, the best strategy is to replace the old derailleur with one manufactured by the same brand as the original one. If you don’t have mechanic skills, it is recommended to ask for professional advice if in doubt.
A bike derailleur is one of the most important components of the transmission. Choosing the right one is crucial if you want to optimize your performance, improve your travel and minimize efforts.
Fortunately, choosing a bike derailleur is not rocket science and, by following these tips, you should be able to find the right types of front and rear derailleur for your bike. As a final tip, don’t forget to check the old bike derailleur and choose a similar model, ideally manufactured by the same brand.
As mentioned above, if in doubt, just ask your trusted mechanic for advice. Happy riding!