The Bike Derailleur Explained: Everything You Need To Know

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Your rear bike derailleur is very exposed to the elements, so it should be no surprise that it can go wrong from time to time.

If your derailleur is bent, poorly adjusted, or just a bit old, you will not get precise and accurate shifting when you need it.

You can adjust your bike derailleur to fix most problems with shifting gear. You’ll find it much easier with a good understanding of how a bike derailleur works.

In this article, we’ll be covering:

  • What Is A Bike Derailleur?
  • What To Look For Before Adjusting A Bike Derailleur
  • How To Adjust The Front Derailleur
  • How To Adjust The Rear Derailleur

Ready to get into the details of the bike derailleur?

Let’s get started!

The Bike Derailleur Explained: Title Image

What Is A Bike Derailleur?

A bike derailleur is a device that moves the chain from one chainring or sprocket to another, allowing you to shift through the gears.

It’s actuated by a cable running from the gear shifters, usually mounted on the handlebars.

Some bikes have two derailleurs. The front derailleur shifts the chain between different chainrings located within the crankset. The rear derailleur allows you to change between the sprockets of the cassette on the rear wheel.

Photo of a silver rear derailleur against a white background.

Rear Derailleur

The rear derailleur tensions the chain, while also allowing you to shift through gears. It alters its position to keep the chain tight no matter which gear you’re riding in.

The rear derailleur shifts through gears by moving the bottom of the chain along the cassette. As you pedal, the top of your chain is pulled tight, transferring the force from the chainrings to the rear wheel and propelling you forward.

The rear derailleur has a spring-loaded arm fitted with two pulleys, allowing it to keep light tension in the bottom of the chain as it moves between different gear ratios. As this part of the chain doesn’t have much load, the derailleur can move it to a different gear.

Close-up of a silver front derailleur on a matt black bike.

Front Derailleur

If you have a bike with more than one chainring (the front set of gears), it will have a front derailleur. The front derailleur moves the top of the chain between each gear. It’s normally mounted to the seat tube, either with a clamp or a bracket.

The front derailleur has a box-shaped cage which the chain passes through. When the gear lever is pressed, the gear cable moves the cage via a linkage system, shifting the chain onto a different chainring.

What To Look For Before Adjusting A Bike Derailleur

A bike derailleur that isn’t working at its best can damage your bike or cause an accident. However, there are a few things you need to check before making any tweaks.

Chain Wear

Over time, your bike chain can stretch as the rollers and pins wear down. If you ride in bad weather often or don’t clean your chain, it can wear and stretch more quickly. Continuously riding with a worn chain will wear out the teeth on your cassette and chainrings.

Check your chain before making any adjustments to your derailleur if you want to solve any gear shifting issues. Your problem could be caused by your chain and not the derailleur, so you may need to get a new one.

Check out our Complete Guide To Removing and Replacing a Bike Chain here!

Damaged Housing or Cable

Another reason you may be experiencing shifting problems is that your gear cable could be frayed, or its housing could be damaged.

Look at where the cable connects to the shifter and the derailleur. If the cable housing has a kink or a sharp bend, it may be restricting the cable’s movement when you shift.

If you see any damage, it’s time to replace the cable and its housing. You may find that the cable is frayed where it clamps to the derailleur. If so, change the cable, but you should be able to use the existing housing.

Bikes with internal cable routing may have issues that you can’t see that affect your gear shifting. In this case, it’s best to change both the housing and cable to be sure.

Close-up photo of a rear derailleur hanger.

Check Your Derailleur Hanger

Many gear shifting problems are caused by a bent rear derailleur hanger. Sometimes it can be so bent that the derailleur shifts itself into the wheel spokes. This can destroy your derailleur, causing you to replace it, which can be expensive.

The derailleur hanger is made of soft metal that can bend easily. The hanger is designed to break. It’s a sacrificial failsafe that allows your derailleur to survive an impact, meaning you only need to buy a new hanger – which is much cheaper.

Your derailleur hanger should be in line with the gears on your cassette. You can have problems changing gear if it isn’t perfectly lined up.

How To Adjust Front Derailleur

There are three main ways your front derailleur may be causing gear shifting problems. We’ll walk you through how to adjust the front derailleur in each case to ensure your gear shifting is smooth and reliable.

Close-up of front derailleur mounting bolts.

#1. Your Front Derailleur Is Out Of Position

First, check the position of the front derailleur by shifting gear so the arm sits over the outer chainring. The long curved part of the derailleur that the chain runs through is known as the cage. This should be about 2 mm above the chainring’s teeth.

The outer part of the cage needs to be aligned with the chainring when viewed from above. You need to adjust your front derailleur if the cage looks out of place.

To adjust your front derailleur, loosen the bolt that mounts it to the frame. This will allow you to manually slide the derailleur up and down or rotate it into position. You may need to loosen the gear cable to do this.

When you have the derailleur in position, ensure you tighten the mounting bolt properly to prevent it from moving again.

You may have a bent or damaged derailleur if this doesn’t do the trick. It may be possible to bend it back into shape. If you’re unsure how to do this, take it to a bike shop.

Close-up of the limit screws on the front derailleur.

#2. Badly Adjusted Limit Screws

If your derailleur’s limit screws are not set correctly, they can cause inaccurate shifting.

As you shift through the gears, the derailleur case moves in and out from the derailleur’s body. To perform well, it needs to move within specific boundaries set by the limit screws.

Limit screws are usually located next to each other and control how far the derailleur can move in each direction. The outer limit screw dictates how far the derailleur can move away from the frame, while the inner screw determines how close it gets to the frame.

Tightening the screws limit the outer range. You should only tighten the screws if the chain comes off either end of your chainrings or off the end of your cassette.

Loosening the screws will give the derailleur more range. You should loosen them if your chain can’t reach the chainring you need.

It can take some trial and error to set the limit screws. You just need to be mindful of what you’re doing to make sure you don’t get yourself in a mess

Take your bike to a bike shop if you struggle to get it right or are concerned about your technical abilities. It can be dangerous if you drop a chain at the wrong moment.

#3. Adjusting the Barrel

If you have a modern bike, it may have barrel adjusters on the derailleur. These are small adjusters on the gear cable near the shifter or derailleur. They allow you to finely tweak the tension in your gear cable.

Not all bikes have barrel adjusters on the front derailleur, so you may not have this option to adjust front derailleur.

To tweak your derailleur with the barrel adjuster, start with your chain on the biggest chainring and the largest gear on the rear cassette. Then change down to the next chainring to see how close the chain is to the inside of the cage.

There should be about 0.5 mm between the two surfaces; close but not touching. Usually, turning the barrel adjuster clockwise will move the cage closer to the frame, and anticlockwise will move it outwards. However, the barrel adjusters might do the opposite depending on your bike. Therefore, when you turn the barrel adjuster, only turn it a quarter turn at a time, so you don’t lose track or turn it too far. 

Whichever way your bike is set up, you need to use the barrel adjuster to slightly reduce the cable tension, bringing the cage inward. Alternatively, you need to tighten the cable to move the cage outward.

You may have a more severe problem if you can’t get your bike shifting gear perfectly after all of these adjustments.

How To Adjust Rear Derailleur

Like your front bike derailleur, you can make three possible adjustments on your rear derailleur to ensure smooth shifting.

Close-up of a rear bike derailleur being adjusted with a screwdriver.

#1. Set Your Limit Screws

If the limit screws on your rear bike derailleur are incorrectly set, the derailleur can stray into your spokes, causing lots of damage. Alternatively, your chain can drop off the cassette onto your frame dropout.

You’ll need to adjust the limit screws if you can’t shift your chain to the smallest or largest gear. This is also the case if your chain keeps dropping off your cassette.

As with your front derailleur, tightening the limit screws will restrict the movement of the chain outward, but loosening them will give the derailleur more range.

#1a. Setting The Low Limit Screw

The low limit stops the derailleur from getting closer to the spokes. When set correctly, the chain will stay on the largest gear without dropping off or skipping.

When setting the low limit of your rear bike derailleur, first ensure that it isn’t so far out that the derailleur can hit the spokes. Then shift onto the smallest gear and push the derailleur toward the largest gear with your thumb.

The derailleur should stop over the largest gear. If it goes beyond it, you need to tighten the low-limit screw.

Then change to the second-largest gear on the cassette (if you can’t do this, your gear cable is probably too tight). Push the derailleur with your thumb to the largest gear while pedaling with your other hand.

If the chain jumps, you’ll need to loosen the low-limit screw. Only turn it a quarter of a turn at a time until the chain stops jumping.

#1b. Setting The High Limit Screw

The high-limit screw stops the chain from dropping off the smallest gear onto the dropout.

To set the high-limit screw, shift onto the smallest gear. If the derailleur isn’t under the smallest gear and too far inward, turn the high-limit screw anticlockwise. If it’s too close to the dropout, turn the screw clockwise.

While the chain is on the smallest gear, pedal forward to see how the chain behaves. If it’s making a noise or rubbing on the adjacent gear, loosen the high-limit screw gradually until it runs smoothly.

If the chain jumps, it may be trying to work its way off the cassette. This is because the high limit is set incorrectly, so you need to tighten it until the chain settles.

Pointing out the "B" screw on a rear bike derailleur.

#2. Set The “B” Screw

The “B” screw dictates the angle of the derailleur and can cause sloppy shifting if it isn’t set correctly.

To set the “B” screw, select the largest gear on the cassette. If the derailleur is close or touches it, you need to tighten the “B” screw. Turning it clockwise will move the derailleur away from the cassette.

But if you overtighten it, the derailleur will be too far from the cassette, negatively affecting your shifting.

Pointing out the barrel adjuster on the rear derailleur of a bike.

#3. Adjusting Cable Tension

You should only have to set your limit screws once. Still, you will probably need to tweak the cable tension occasionally. Over time, your gear cable can settle into its housing, affecting its tension. Therefore, you may have to tighten it to keep your shifting smooth.

You need to use the barrel adjuster near the derailleur to adjust the gear cable tension. Usually, turning the barrel adjuster clockwise will make the cable tighter, while turning it anticlockwise will loosen it.

Follow these six steps to adjust the cable tension:

  • Change gear so you’re on the smallest sprocket of the cassette.
  • Shift up to the next gear while pedaling. If it doesn’t shift or goes too slowly, tighten the barrel adjuster gradually until the chain goes up a gear.
  • The tension is too high if the chain goes up more than one gear or rubs on the next one. Therefore, you need to loosen the adjuster a quarter of a turn at a time until it runs smoothly on the desired gear.
  • Repeat for each gear on the cassette, adjusting the cable tension accordingly.
  • When you get to the biggest sprocket, change back down the cassette, paying attention to how the chain behaves. If shifting is sluggish or the chain doesn’t move, reduce the cable tension until it works perfectly.
  • Once you get to the largest cog, shift back into the smallest rings. If there is any hesitation or the chain will not move back onto the smaller rings, you need to remove cable tension until the shift is successful.
Close-up of a silver bike derailleur on the back wheel of a dark grey road bike.

Now You Know All About The Bike Derailleur…

This may be a lot of information to take in, but learning how to adjust and maintain your derailleurs will prove invaluable in the long run!

As with most aspects of bike maintenance and setup, it’s easier to do than to explain – so give it a go for yourself! The key to adjusting your bike derailleurs is taking your time and making incremental changes.

If you don’t feel confident about adjusting your derailleur, it’s best to take your bike to a shop. An experienced bike mechanic can get your gears running smoothly and reliably in no time.

Found this article helpful? Learn more from the BikeTips experts below!

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Tom is an experienced freelance cycling journalist and mountain biking expert who competed nationally in the junior ranks. Now based in the world-famous mountain biking destination of Morzine in the French Alps, Tom spends his summers shredding off-road trails by bike and his winters on the same mountains on a snowboard.

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