Key Points

• An electric bike system generally consists of a similar set of components, primarily the motor, controller and battery
  
• The motor can be constructed in different ways and mounted in various locations; both these factors will impact the performance of the bike and give different advantages

System Components

Electric bike systems typically consist of the follow components mounted on a bike.

1. Battery, to provide power. Electric bike batteries are typically 36 volt lithium (Li) ion batteries, and need to have sufficient capacity (range) and power output - refer to battery notes for more information.
   
2. Motor, to drive the bike. This can be mounted in different ways as discussed below. In all cases however, riding an electric bike a 'joint effort': the pedals and chain work to drive the bike in the usual way for a bike, with the motor providing additional power on demand
   
3. Controller, which sits between the battery and the motor and regulates the system. The controller needs to be appropriate to the voltage and current of a given system and can limit performance otherwise. In some systems the controller is in a separate box; in some it is integrated into the motor unit, which generally gives a neater look with less visible wiring
   
4. Peripheral components required to operate the system. these may include some or all of the following
   

• On/off switch
  
• Torque / pedal sensor, which activates the motor when the pedals are turned. This may be referred to as pedal assist (PAS) i.e. activation of the motor via the pedals rather than a throttle
  
• Throttle (thumb, half twist or full twist), which activates the motor progressively
  
• Display / programming unit: while the controller runs the system, most setups will have an additional unit mounted on the handlebars which provides system information and may allow some degree of programming system parameters
  
• E-brakes (or variants); devices which cut the motor power automatically when a brake is applied or other action taken. Systems will generally run without these, but installation of at least one is recommended as good practice
  

Optionally, secondary systems that run off the battery may be added as required e.g. lighting.

Finally a charger: this is typically not mounted on the bike, but is a separate unit that plugs into the mains to charge the battery in between use

Motors

The motor consists of a set of coils of copper wire mounted within a set of magnets (as per photo - motor cover removed to show the copper coils and the ring of magnets circling them, with a small gap to allow rotation of the motor). When an electrical current from the battery flows through the coils, the magnetic field generates a force on the coils, which causes a rotor to turn. This is the basis of all electric bike propulsion. Note that the power is transferred via the magnetic field rather than by physical contact, so a simple motor does not need to have moving parts in contact and can be extremely robust as a result. Electric bike motors are general brushless construction, usually with Hall sensors (both being aspects of motor construction that give greater efficiency, typically up to 80% or more at optimum motor speed).

Types of Motor

Although the propulsion mechanism is the same, there are two types of construction of motor as follows.

Geared Motor

• This name can be misleading since, generally, neither type contains multiple gears in the sense of a car or bike having gears. Instead, a geared electric hub motor contains a reducing gear mechanism that allows the motor to turns at a (single fixed gear ratio) faster rate than the wheel or crank that is being drive. This makes the motor more efficient. 
  
• The gears may need greasing occasionally, but typically only every 10,000 miles
  
• A geared hub motor is likely to be lighter and more compact than a comparable non-geared motor.
  
• Geared motors contain a clutch that disconnects the gear when the motor is not operating, so a geared motor will provide almost zero additional resistance to turning when the motor is not in use. A bike with a geared motor will therefore likely feel very like a normal bike to ride when pedaling it without the motor.
  
• High quality, UK road legal electric bikes will most often be based on geared motors (whether front hub, rear hub, or crank drive mounted as below)
  

Direct Drive

• A direct drive (or non-geared) motor does not contain the reducing gears or clutch
  
• It is likely to be heavier, and will add significant resistance when pedaling without the motor (rather like having the tyres under-inflated)
  
• For very powerful motors that generate a lot of heat a direct drive motor is likely to be more appropriate. However, in general the extra weight of a direct drive motor makes them less desirable for an electric bike
  
• Direct drive motors do require less maintenance than a geared motor, which may require occasional greasing of the interlocking reducing gear mechanism.
  
• Finally, one advantage of direct drive motors is that they allow the possibility of regenerative braking (i.e. operating the motor as a generator) which cannot be done with a geared motor. The bike's momentum is converted by the motor back into electrical energy, simultaneously recharging the battery and providing resistance that slows down the bike (the exact reverse of the process of the motor pushing the bike). This is unlikely to add much to battery range even in fairly hilly terrain, but can be a useful additional braking mechanis 

Motor Locations

There are also several ways in which a given motor may be mounted on a bike, each of which introduces a specific set of considerations. These are as follows

• The motor forms the hub of the front bike wheel.
  
• Power from the motor is directly transferred to rotational force on the front wheel.
  
• Front wheel systems are relatively easy to install and give the benefit of "all wheel drive" (i.e. the front wheel is motorised, the back wheel pedal powered).
  
• Precautions must be taken to reinforce the front dropouts (the part of the frame/forks that holds the axle) with a torque arm or plate that resists rotation of the axle. All hub motors exert a significant force on the dropouts via the wheel axle when the motor operates, however front dropouts are usually less robust than rear ones and failure would be more serious compared to rear dropouts.
  
• Removal of the wheel (for example for fixing a puncture) is likely to be made more complicated due to wiring and torque arm / plate.

• The motor forms the hub of the rear bike wheel.
  
• Power from the motor is directly transferred to rotational force on the rear wheel.
  
• Rear hub installation is likely to be more complex than a front one, as the motor wheel must fit with the gears. The number of gears may be limited and of course it can't be used with internal gear hubs 
  
• Rear hub is more appropriate than front hub for more powerful motors due to lower risk from dropout failure.
  
• Removal of the wheel (for example for fixing a puncture) is likely to be made more complicated due to wiring and torque arm / plate