What Is a Derailleur on a Bike? A Plain Guide for Electric Bike Riders

If you have ever looked at the back wheel of a geared bike and wondered about the small, springy mechanism hanging below the chain, you have found the derailleur. It is one of the most important parts of a geared bike, and one of the least understood. On an electric bike it matters even more, because the motor and the gears work together to decide how easy or hard each ride feels.

This guide explains what a derailleur is, how it moves the chain, how to shift smoothly on an electric bike, which signs tell you something needs attention and what the gear specifications on EMotorad UK bikes mean. Product details were checked on 5 October 2026 and can change, so confirm them on the product page before you order.

A derailleur is a device that moves a bicycle chain from one sprocket to another. The word also describes the whole gear system on a multiple-speed bike that uses this mechanism. When you press the shifter, a cable moves the derailleur sideways, and the chain "derails" from one sprocket onto the next. That changes the gear you are in.

Most electric bikes with several gears use a rear derailleur. It sits near the rear wheel, below the cassette, which is the stack of sprockets on the rear hub.

According to the general description of derailleur gears, modern derailleurs have a movable chain guide that is operated remotely by a cable attached to a shifter, usually on the handlebar. When you move the lever while pedalling, the change in cable tension moves the chain guide from side to side. The chain then moves onto a different sprocket.

A rear derailleur has two jobs. It moves the chain between the rear sprockets, and it takes up the slack in the chain that appears when you move to a smaller sprocket. To do the second job, it sits in the lower, slack part of the chain and holds it under tension with a spring.

Most rear derailleurs share the same main parts.

• The cage. It holds two small wheels, called pulleys, that guide the chain in an S shape. The cage can rotate and is spring-loaded to take up slack.
• The upper pulley. It guides the chain toward the sprockets.
• The lower pulley. It keeps tension in the chain.
• The cable connection. The shifter cable pulls the derailleur against its spring, and the spring returns it when the cable is released.

Many mountain bike style derailleurs, including those often fitted to electric bikes, return the chain to the smallest sprocket when there is no cable tension. The shifter then pulls the chain toward larger sprockets, which feel easier for climbing.

Modern gear systems use indexed shifting. Each click of the shifter moves the derailleur by a set amount so that the chain lines up with the next sprocket. The shifter, derailleur, cassette, chain and cable are designed to work as a system, which is why parts need to be compatible and adjusted correctly.

This is also why a small stretch in the cable or a slight bend in the derailleur hanger can cause poor shifting. If the movement per click is off, the chain will not sit properly on the sprocket.

Many bikes with a lot of gears have two derailleurs. The rear one works with the cassette. The front one works with the chainrings by the pedals. A bike with three front rings and seven rear sprockets has 21 speeds.

Simpler bikes have a rear derailleur only. A 7-speed bike with one chainring has seven gears. Many electric bikes choose this design because it is simple and easy to use.

An electric motor does a lot, but it does not remove the need for gears. Gears let you keep your legs turning at a comfortable pace. Pedalling in too hard a gear makes your legs work hard and puts extra load on the drive system. Pedalling in too easy a gear can spin the pedals without much progress.

Think of the motor and the gears as a team. The motor adds power, and the gears let you and the motor work at a comfortable speed. As a general habit, shift to an easier gear before a hill, not halfway up, and keep your cadence steady.

These habits apply to most geared bikes, including electric ones.

1. Shift while pedalling. Derailleurs need the chain to be moving. The general description of derailleur gears notes that shifting usually requires the rider to be pedalling.
2. Ease the pressure. Pedal a little more lightly for a moment as you shift. It reduces stress on the chain and sprockets.
3. Shift early. Change down before you stop or start a climb, so you can pull away easily.
4. Avoid extreme combinations. On bikes with front and rear derailleurs, do not use the largest ring with the largest sprocket, or the smallest with the smallest. The same description notes that the diagonal chain line wears parts and works poorly.
5. Shift one gear at a time. It is gentler than slamming through several at once.
6. Stop pedalling hard while the motor is working hard. A sudden surge from the motor while you shift can make the change rough, so lower the assist level or ease off for a moment if you notice it.

Check the gears now and then, and look out for these signs.

• The chain jumps between gears by itself. The cable may have stretched or the adjustment has slipped.
• The chain is slow to move to a larger sprocket. Cable tension may be low.
• The chain will not go into the easiest or hardest gear. The limit adjustment may be wrong.
• A clicking or grinding noise. The chain may be rubbing, dirty or worn.
• Poor shifting after a fall or knock. The derailleur or hanger may be bent.
• The chain drops off. Check alignment and chain wear.

If any of these appear, a bike shop can adjust or replace the parts. Do not ignore a bent derailleur, because it can get caught in the spokes.

• Keep the chain clean and lubricated. A dirty chain wears the sprockets and shifts poorly.
• Wipe the derailleur and the pulleys. Dirt builds up quickly.
• Check the cable and housing. Frayed or rusty cable shifts badly.
• Look at the hanger. It is the small part that connects the derailleur to the frame, and it can bend.
• Shift through the gears after cleaning. Make sure each gear engages cleanly.
• Book a regular service. An electric bike works its drivetrain hard.

The EMotorad UK product pages list the drivetrain for each model. All details below come from the pages checked on 5 October 2026.

The T-Rex Air lists a Shimano TY300 7-speed drivetrain, a 40T alloy crankset and a 250 W rear hub motor.

The T-Rex Pro lists a Shimano Altus 7-speed derailleur, a 42T crankset with a chain guard and a 468 Wh battery.

The EMX lists a Shimano Altus 3x7, 21-speed drivetrain with a 24/34/42T crankset. That gives it three front chainrings and seven rear sprockets, so the extra range helps on steeper terrain.

A 7-speed bike has seven rear sprockets and a simple drivetrain. A 21-speed bike has three chainrings and seven sprockets, which gives more choice between easy and hard gears. More gears are not always better. The simpler setup has less to adjust, and the wider range suits hilly or mixed terrain.

The chain, the cassette and the derailleur wear as a set. A stretched chain wears the sprockets, and worn sprockets make shifting rough even when the derailleur is set up perfectly. This is why a bike shop will often check the chain first when a rider complains about skipping.

On an electric bike the chain carries both your effort and the motor's, so it is worth looking at more often than on a bike without assistance. A simple routine helps.

• Every few rides: wipe the chain with a clean cloth and add a little lubricant, then wipe off the excess.
• Every month or so: check the chain for rust, stiff links or a gritty feel.
• At each service: ask the mechanic to measure chain wear, so the chain is replaced before it damages the cassette.

Replacing a chain in time costs far less than replacing a chain, a cassette and sometimes a chainring together.

A few simple rules make the gears easier to use in day-to-day riding.

• Flat paths and cycle lanes: use a middle or higher rear gear, keep the assist level low to moderate, and pedal at a steady pace.
• Short climbs: shift to an easier gear before the slope starts. Keep pedalling so the chain moves freely.
• Long hills: pick an easy gear early and stay in it. Raise the assist level if you need to, but keep your cadence up.
• Stop-start city riding: shift down as you slow, so you are in an easy gear when you pull away at the lights.
• Carrying a load: use easier gears than you would without it, because the extra weight makes every start harder.

These habits protect the drivetrain and make the bike feel smoother. They cost nothing and become automatic after a few weeks.

In Great Britain, an electric bike that meets the electrically assisted pedal cycle rules can be ridden without a licence. The rules limit the motor to 250 W continuous rated power and cut assistance at 15.5 mph. Check the current rules on GOV.UK before you buy or modify a bike, and avoid changes to the drive system that move it outside those limits.

Not every bike uses a derailleur. Some bikes use gears inside the rear hub, and some use a single speed. A derailleur is light, simple and efficient, and it is easy to see and adjust. Its parts are exposed to dirt and knocks, so they need regular checks.

If you ride mostly on flat, short routes, a simple system may suit you. If you ride hills or carry loads, a derailleur with a good range of gears is more useful.

Book a ride through the EMotorad UK site and confirm the time before you travel. During the ride, check these points.

1. Does each click of the shifter change gear cleanly?
2. Can you reach the easiest gear on a hill without noise?
3. How does the bike behave when you shift under motor assistance?
4. Does the chain stay put over a rough patch?
5. Can you reach the shifter comfortably?

It moves the chain from one sprocket to another so you can change gear.

The rear derailleur hangs below the rear cassette. Some bikes also have a front derailleur near the pedals.

No. Some use hub gears or a single speed. The EMotorad UK models in this guide list Shimano derailleur drivetrains.

Derailleurs usually need the chain to be moving, so shift while pedalling.

Check them regularly and book a service according to the manufacturer's advice and how much you ride.

Common causes are a stretched cable, a worn chain or sprockets, or a bent derailleur. A bike shop can diagnose it.

Look at the drivetrain on any bike you plan to buy, and ride it through every gear. Keep the chain clean, shift smoothly and have a shop check anything that sounds or feels wrong.

A well-adjusted derailleur is quiet and easy to forget, which is exactly how it should be.

Product details were checked on 5 October 2026 and can change. The general description of how derailleurs work draws on the reference article on derailleur gears. Riding and care tips are general advice, not a substitute for the manufacturer's instructions or a qualified mechanic.

• GOV.UK electric bike rules: https://www.gov.uk/electric-bike-rules
• Derailleur gears reference: https://en.wikipedia.org/wiki/Derailleur_gears
• EMotorad UK range: https://www.emotorad.com/uk
• T-Rex Air: https://www.emotorad.com/uk/shop/t-rex-air
• T-Rex Pro: https://www.emotorad.com/uk/shop/t-rex-pro
• EMX: https://www.emotorad.com/uk/shop/emx-plus