Understanding Electric Bike Drivetrains

What is an electric bike drivetrain

An electric bike drivetrain is the system that transfers power from both the rider's pedaling and the electric motor to the wheels, enabling the ebike to move forward. It combines human and electric power, allowing the rider to pedal with assistance from the motor, reducing the physical effort needed for riding, especially on hills or long distances. The drivetrain determines how the bike accelerates, climbs, and responds to various terrains, providing a smoother, more efficient ride compared to traditional bicycles.

How do electric bicycle drivetrains work 

Pedal Assistance (Pedelec Mode):

• When the rider pedals, sensors (torque or cadence) detect the pedaling effort or speed and send signals to the controller.
• The controller adjusts the power from the battery to the motor, providing assistance based on the selected assistance level and sensor input.
• The motor adds power to the drivetrain, either through the chain (mid-drive motor) or directly to the wheel (hub motor), reducing the effort required from the rider.

Shifting and Power Management:

• The rider shifts gears just like on a regular bicycle using the derailleur and cassette. Lower gears provide easier pedaling, while higher gears offer more speed.
• The motor assists regardless of the gear, but in mid-drive systems, selecting the right gear is important for maximizing motor efficiency and minimizing strain on the drivetrain.

Braking and Regenerative Braking (if equipped):

• Some e-bikes, especially those with hub motors, feature regenerative braking, which captures energy during braking and returns it to the battery, extending range.
• When the rider applies the brakes, the motor may temporarily disengage to prevent unnecessary power usage.

Throttle Operation:

• If the e-bike has a throttle, the rider can engage the motor without pedaling by twisting or pressing the throttle. This provides instant power and is useful for quick acceleration or riding in stop-and-go traffic.

Types of ebike drivetrains

Mid-Drive Drivetrain:

A mid-drive motor is mounted near the bike’s bottom bracket (where the pedals attach), directly powering the electric bike’s chain and crankset.

Key Features:

• Direct Power to the Chain: The motor drives the crankset, which means power is applied through the bike’s existing drivetrain (chain, cassette, and derailleur). This allows the motor to take advantage of the bike's gearing system, making it more efficient in handling hills and varied terrain.
• Balanced Weight Distribution: The motor is centrally located, providing a lower center of gravity and better weight distribution, which improves handling and stability.
• Higher Torque: Mid-drive motors are known for producing higher torque, making them ideal for hilly or mountainous terrain, as well as for riders who need more power when climbing.
• More Wear on the Drivetrain: Since the motor drives the chain and gears directly, mid-drive systems can put more strain on the drivetrain components, such as the chain, cassette, and chainring.

Best For: 

• Riders tackling steep hills or varied terrain.
• Those seeking the best performance and efficiency in both off-road and on-road conditions.
• Commuters who need good range and torque for inclines.

Example Motors:

• Shimano STEPS, Bosch Performance Line, Brose, Yamaha, Bafang.

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2. Hub-Drive Drivetrain:

A hub motor is integrated into the wheel hub, either in the front or rear wheel, providing direct power to the wheel.

A. Rear Hub Motor:

• Direct Power to the Rear Wheel: The motor directly drives the rear wheel, pushing the bike forward. This provides a more natural "push" sensation.
• Easier to Install and Maintain: Rear hub motors tend to be simpler and less expensive to install and maintain compared to mid-drive systems.
• Less Stress on Drivetrain: Since the motor is independent of the bike's gears, it places less wear and tear on the chain, cassette, and derailleur.
• Less Efficient on Steep Hills: Rear hub motors may struggle to provide enough torque on steep inclines compared to mid-drive motors.

B. Front Hub Motor:

• Power to the Front Wheel: The motor drives the front wheel, while the rider’s pedaling powers the rear wheel, creating an all-wheel-drive effect.
• Simpler Setup: Front hub motors are usually simpler and less costly to install, and they provide decent power for flat or moderately hilly terrain.
• Poorer Traction on Slippery Surfaces: Front hub motors may struggle with traction, especially on wet or loose terrain, as the front wheel pulls the bike forward.
• Less Natural Ride Feel: Some riders feel that front hub motors provide a less natural ride feel compared to mid-drive or rear hub motors.

Best For:

• Commuters and urban riders looking for a simple, affordable electric assist for flat or moderate terrain.
• Riders who prefer a low-maintenance drivetrain.

Example Motors:

• Bafang hub motors, Shengyi, Rad Power Bikes’ rear hub motors.

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3. Belt-Drive Drivetrain:

A belt-drive system uses a carbon belt instead of a traditional chain, usually paired with an internally geared hub or mid-drive motor.

Key Features:

• Quiet and Low Maintenance: Belt drives are quieter and require less maintenance than chain drives, as they don’t need lubrication and don’t stretch over time.
• Durability: Belt drives last longer than chains and are more resistant to wear and tear.
• Clean and Efficient: Without the need for chain oil, belt drives are cleaner and more efficient for commuters who want a low-maintenance option.
• Compatibility with Internal Gearing: Belt drives are commonly paired with internally geared hubs (such as Shimano Nexus or Rohloff), which provide multiple gears in a sealed system.

Best For:

• Commuters who prioritize low maintenance, reliability, and cleanliness.
• Riders who want a quieter, smoother ride without dealing with chain lubrication and adjustments.

Example Systems:

• Gates Carbon Drive system.

Electric bike drivetrain vs traditional bike drivetrain

Power Source

• Electric Bike Drivetrain:
  • Uses a combination of the rider's pedaling power and an electric motor. The motor provides pedal-assist or throttle-based power, reducing the rider’s effort, especially on inclines or long distances. The motor is powered by a battery, and the level of assistance can often be adjusted through a control panel.
• Traditional Bike Drivetrain:
  • Powered solely by the rider's pedaling effort. All the force applied to the pedals is transferred through the drivetrain (chain, crankset, cassette) to move the bike. The rider must manually handle all terrain and resistance without motor assistance.

2. Components

• Electric Bike Drivetrain:

  • Motor: Central to the e-bike drivetrain, it can be located in the mid-drive (bottom bracket) or in the hub (front or rear). The motor provides power assistance.
  • Battery: Powers the motor and can be mounted on the frame or integrated into the frame itself.
  • Controller and Sensors: The controller manages the flow of electricity from the battery to the motor, while torque and cadence sensors adjust the motor’s output based on pedaling effort and speed.
  • Traditional Components: It still uses a chain, cassette, derailleur, and crankset, but these work alongside the motor and electrical components.

• Traditional Bike Drivetrain:

  • Chain: Transfers power from the rider’s pedaling to the rear wheel.
  • Cassette/Freewheel: A set of gears attached to the rear wheel, allowing the rider to change resistance and speed.
  • Crankset: Consists of the pedals and front chainring(s), transferring the rider’s pedaling force to the chain.
  • Derailleurs: Move the chain across gears (front and rear), allowing the rider to shift and adjust pedaling resistance.

3. Gear Shifting

• Electric Bike Drivetrain:
  • Shifting still functions much like a traditional bike, but with motor assistance. In mid-drive systems, the motor works in tandem with the bike’s gears, and shifting efficiently is important to maximize motor efficiency.
  • In hub-drive systems, the motor operates independently of the drivetrain, meaning the rider can shift as usual, but the motor assists regardless of gear.
• Traditional Bike Drivetrain:
  • Gear shifting is entirely mechanical, controlled by the rider's manual input. Shifting gears adjusts pedaling resistance and effort, especially on varied terrain like hills or flats.

4. Assistance and Pedaling Effort

• Electric Bike Drivetrain:

  • Pedal Assistance: In pedal-assist mode, the motor supplements the rider’s pedaling effort, reducing the amount of work needed. The rider can adjust the level of assistance (low, medium, high) using a control panel.
  • Throttle (if equipped): Some e-bikes allow the rider to engage the motor without pedaling, providing full motor power via throttle.
  • The motor takes care of some of the work, making the bike easier to ride over long distances, up hills, or into headwinds.

• Traditional Bike Drivetrain:

  • The rider must provide all the power through pedaling. Pedaling effort directly impacts speed and resistance, with no motor to help.
  • Shifting gears adjusts pedaling resistance, but the rider has to handle all inclines and resistance manually.

5. Maintenance

• Electric Bike Drivetrain:
  • Requires additional maintenance due to the motor, battery, and electrical components.
  • The traditional drivetrain parts (chain, cassette, derailleur) still need regular maintenance, but there’s less strain on them in hub-drive systems compared to traditional bikes.
  • Mid-drive systems may wear out chains and cassettes faster due to the increased torque provided by the motor.
• Traditional Bike Drivetrain:
  • Simpler and easier to maintain as it only consists of mechanical parts (chain, cassette, crankset, etc.).
  • Regular cleaning, lubrication, and adjustments are required for the chain, gears, and derailleur, but there are no electrical components to manage.

6. Weight

• Electric Bike Drivetrain:
  • E-bikes are generally heavier due to the motor and battery. The drivetrain has to handle the added weight, which can affect how the bike rides, especially when the battery is low or off.
• Traditional Bike Drivetrain:
  • Lighter, since there are no motor or battery components. Traditional bikes are generally more agile and easier to carry or move without assistance.

7. Cost

• Electric Bike Drivetrain:
  • More expensive due to the additional motor, battery, and electrical system. Replacing components like the battery or motor can also be costly.
• Traditional Bike Drivetrain:
  • Less expensive, with fewer components to maintain and replace. A mechanical drivetrain typically has lower upfront and maintenance costs compared to an e-bike.

8. Range and Efficiency

• Electric Bike Drivetrain:

  • E-bikes have a range (distance you can travel on a single charge) determined by the battery capacity and level of assistance used.
  • Efficient use of gears in mid-drive systems extends battery range, while hub motors provide constant assistance regardless of gear.
  • Once the battery runs out, the rider can still pedal, but the added weight of the motor and battery makes the bike harder to ride.

• Traditional Bike Drivetrain:

  • The range is determined solely by the rider's stamina and energy. There’s no motor assistance, so efficiency depends on the rider’s fitness and gear usage.

Benefits of e-bike drivetrains 

Efficient Power Assistance:

• E-bike drivetrains provide motorized help, reducing pedaling effort, especially on hills and long rides.

Increased Range and Speed:

• Riders can travel farther and faster with less physical exertion.

Customizable Assistance:

• Many e-bikes offer adjustable power levels, allowing for a tailored riding experience based on terrain or rider preference.

Improved Accessibility:

• E-bike drivetrains make cycling easier for riders of all fitness levels, including those with physical limitations.

Eco-Friendly Transportation:

• E-bikes provide an environmentally friendly alternative to cars, reducing emissions while offering an efficient, low-cost mode of transport.

Key components of drivetrain:

Motor:

• The motor is the heart of the e-bike drivetrain, providing electrical assistance to the rider's pedaling. It can be located in different places:
  • Mid-drive motor: Positioned near the bottom bracket, directly driving the bike’s crankset.
  • Hub motor: Integrated into the rear or front wheel hub, providing direct power to the wheel.
• The motor works in conjunction with the rest of the drivetrain to offer power when pedaling or via throttle.

Chain:

• The chain connects the front chainring (attached to the cranks) to the rear cassette, transferring the rider's pedaling effort and the motor's assistance to the rear wheel.
• The chain works with both mechanical and electric components to maintain smooth pedaling and shifting.

Crankset and Chainring:

• The crankset consists of the crank arms and pedals, which the rider uses to generate power.
• The chainring (or chainrings, if multiple) is the front gear attached to the crankset, which the chain wraps around.
• In mid-drive e-bikes, the motor powers the crankset, making it a critical part of the drivetrain.

Derailleur (Front and Rear):

• The rear derailleur shifts the chain across the gears (cogs) on the rear cassette, allowing the rider to change gears for different terrains.
• Some e-bikes also have a front derailleur, which shifts the chain between multiple chainrings in the front (though many modern e-bikes have only a single front chainring).
• The derailleur ensures smooth gear transitions and helps balance the rider’s effort with motor assistance.

Cassette/Freewheel:

• The cassette (or freewheel on older bikes) is a set of gears attached to the rear wheel hub. The chain moves across the cassette to change gears, affecting the bike’s resistance and speed.
• Shifting to lower gears (larger cogs) makes it easier to pedal uphill, while higher gears (smaller cogs) are for speed on flat terrain.

Pedals:

• The pedals allow the rider to apply physical effort, which is combined with the motor’s assistance to move the bike forward.

Sensors (Torque and Cadence):

• Torque sensors measure how hard the rider is pedaling and adjust the motor’s power output accordingly.
• Cadence sensors detect how fast the rider is pedaling and provide motor assistance based on pedaling speed.
• These sensors help the motor deliver the right amount of power, making the drivetrain more efficient and responsive.

Controller:

• The controller is the electronic brain of the e-bike that manages power distribution between the motor, battery, and drivetrain.
• It takes input from sensors and rider controls to determine how much assistance the motor should provide.

Battery:

• The battery powers the motor and is integral to the e-bike drivetrain. The battery’s capacity determines how long the motor can assist the rider before needing a recharge.

Can you upgrade the drivetrain on an electric bike

Upgrading the Cassette:

• You can replace the existing cassette with one that offers a wider gear range, which can improve performance on hilly terrain or provide more speed options.

Upgrading the Chainring:

• If you have a mid-drive motor, upgrading to a larger or smaller chainring can adjust the gearing for more speed or easier pedaling on hills.

Upgrading the Derailleur:

• You can switch to a more advanced or higher-end derailleur (such as upgrading from Shimano Altus to Deore or XT) for smoother and more precise shifting.

Switching to a Belt Drive:

• Some e-bikes can be converted from a chain-driven system to a belt drive for less maintenance and quieter operation, but this depends on the bike’s compatibility.

Upgrading the Shifters:

• Replacing basic shifters with more precise, higher-quality ones can improve the overall shifting experience, especially with a more advanced derailleur.

How to maintain an electric cycle drivetrain

1. Clean the Drivetrain Regularly:

• Frequency: Clean the chain, cassette, chainring, and derailleur regularly, especially after riding in wet, muddy, or dusty conditions.
• How: Use a bike-specific degreaser and a brush to remove dirt and grime from the chain, cassette, and other drivetrain components. Wipe the components dry with a clean rag.
• Why: Dirt and grime can wear down the drivetrain components, reduce efficiency, and affect motor performance.

2. Lubricate the Chain:

• Frequency: Lubricate the chain after every cleaning or at least once a week for regular riders.
• How: Apply a quality chain lubricant specifically designed for e-bikes or bikes in general. After applying, wipe off excess lube to prevent attracting more dirt.
• Why: Proper lubrication reduces friction, minimizes wear, and ensures smooth shifting and efficient pedaling.

3. Inspect the Chain for Wear:

• Frequency: Check the chain for wear every few months or after 500-1,000 km (or miles) of riding.
• How: Use a chain wear tool to measure chain stretch. If the chain is stretched beyond the recommended limit, replace it.
• Why: A worn chain can damage the cassette and chainring, leading to costly repairs. Replacing the chain when needed helps prevent premature wear on other components.

4. Check the Cassette and Chainring:

• Frequency: Inspect the cassette and chainring for wear whenever you check the chain.
• How: Look for worn or damaged teeth on the cassette and chainring. Replace them if they show significant wear (teeth that are sharp or "shark-toothed").
• Why: Worn cogs and chainrings can cause poor shifting and put unnecessary strain on the motor.

5. Adjust the Derailleur:

• Frequency: Check derailleur alignment and adjustment every few months or if you notice poor shifting performance.
• How: Ensure the derailleur is aligned correctly and that the limit screws, cable tension, and B-screw are properly adjusted. Fine-tune the cable tension using the barrel adjuster if necessary.
• Why: A properly adjusted derailleur ensures smooth and precise shifting, preventing wear on the drivetrain and motor.

6. Inspect the Derailleur Hanger:

• Frequency: Check after any crash or impact, or during routine maintenance.
• How: Ensure the derailleur hanger is straight and aligned with the bike frame. Use a derailleur hanger alignment tool if necessary.
• Why: A bent hanger can cause poor shifting and lead to drivetrain damage.

7. Monitor Chain Tension (For Belt-Drive Systems):

• Frequency: If your e-bike has a belt drive, regularly check the tension of the belt.
• How: Use a belt tension gauge to ensure the belt is not too loose or too tight.
• Why: Proper tension is crucial for smooth performance and to avoid unnecessary wear on the belt and pulleys.

8. Inspect and Maintain E-Bike Motor (Mid-Drive Only):

• Frequency: Check motor integration during regular drivetrain maintenance.
• How: Ensure the motor is functioning correctly and that it’s not causing excessive wear on the chain or drivetrain components. Consult the manufacturer’s recommendations for specific motor care.
• Why: In mid-drive systems, the motor works directly with the drivetrain, and any misalignment can wear down components faster.

9. Check the Bottom Bracket:

• Frequency: Every 6 months or if you notice any unusual noises or pedaling resistance.
• How: Spin the cranks and listen for noise or feel for roughness. Replace or service the bottom bracket if it feels worn or makes noise.
• Why: A well-maintained bottom bracket ensures smooth pedaling and power transfer.

10. Regularly Check for Loose Bolts:

• Frequency: Check bolts on the drivetrain components every month or after any long or rough rides.
• How: Use a torque wrench to ensure that the bolts securing the chainring, derailleur, and motor (in mid-drive systems) are properly tightened.
• Why: Loose bolts can lead to misalignment, noise, or damage to the drivetrain.

11. Avoid Cross-Chaining:

• What: Cross-chaining happens when you use extreme gear combinations, such as the largest chainring with the largest cog or the smallest chainring with the smallest cog.
• Why: This places excessive strain on the chain and drivetrain components, leading to premature wear and poor shifting performance.

12. Store the E-Bike Properly:

• What: Store your e-bike in a dry, sheltered place to avoid moisture and rust from affecting the drivetrain.
• Why: Exposure to moisture can lead to rust and corrosion, which will damage the drivetrain components.

Why is a drivetrain important in an e-bike

The drivetrain in an e-bike is crucial because it transfers power from both the rider's pedaling and the electric motor to the wheels, enabling smooth and efficient movement. It ensures proper gear shifting, optimizes motor performance, and helps manage different terrains, making the ride easier, especially on hills or long distances. A well-maintained drivetrain improves overall performance, battery efficiency, and the longevity of the bike's components.

Common problems with ebike drivetrains

Chain Wear and Stretch:

• E-bike drivetrains experience higher torque, leading to faster chain wear and stretching, which can cause poor shifting and drivetrain noise.

Misaligned Derailleur:

• A bent derailleur or hanger can result in skipping gears, slow shifting, or the chain coming off the cassette.

Dirty or Worn Components:

• Dirt and grime buildup on the chain, cassette, and chainring can lead to inefficient power transfer and shifting issues.

Chain Slippage:

• This occurs when the chain slips off the gears, often due to a worn cassette, chainring, or chain.

Motor-Related Strain:

• In mid-drive systems, the motor applies additional stress to the drivetrain, causing components like the chain and cassette to wear faster.

Incorrect Gear Shifting:

• Shifting under heavy load (especially on hills) can strain the drivetrain, causing damage or poor shifting performance.

Loose or Damaged Bolts:

• Bolts holding the drivetrain components can loosen over time, leading to misalignment or component damage.