Hub or Crank Drive Motor for your Electric Bike

There isn’t just one way to translate power from the drive unit into your ride. Hub motors and crank drive motors represent the two most common varieties of motor to provide the assistance which defines the boosted power of electric bikes. There is a lot of information and confusion out there. As specialists in electric bikes the aim of this article is to try to clarify matters for you without being too technical.

You’ll find both varieties on our bikes, and while we’d happily recommend bikes that have either motors, there are important differences worth considering before purchasing your bike.

Hub Motor

So called because of their location on the hub of the front or rear wheel, the hub motor creates the sensation of being “pushed” or “pulled” by directly powering the wheel it is attached to. This motor features on some of our folding bikes and generally speaking on the less expensive urban bikes  that we stock. Because of their position on the wheel as opposed to the frame, very little is altered on the overall silhouette of the bike outside of the battery.

There are some benefits of the motor being on the wheel rather than in the crank drive.

Firstly, it places no added strain, and in no way depends, on the strength of the chain or sprocket of the bike. Some e-bikes (but not all) are equipped with special chains because of the additional torque that can be applied to the chain from a crank drive motor. However, in our experience, the standard chains on crank motor bikes do not break or stretch any more when compared to conventional bikes.

Bafang rear hub motor for electric bike

Bafang Rear hub motor

Secondly it can benefit from enhanced efficiency due to its direct positioning on the wheel it powers. This is true whether the motor is on or off – it efficiently targets its power onto the wheel it is attached to when switched on, and when switched off provides minimal resistance to powering the bike manually.

Front hub motors have the added advantage of creating an “all wheel drive”, as your pedalling powers the backwheel, while the motor powers the front. This is particularly helpful when riding over snow, sand or sticky mud , as it shares the benefits of a four-wheel drive vehicle. Rear wheel hubs meanwhile benefit from reduced chance of spin, as the added weight of the rider over the rear wheel keeps it firmly planted to the road.

We have noticed that some people who come to the shop are concerned that a front hub motor could lead to less control – in the same way as a front drive car.  We would suggest that there is little to be concerned about here. This is because, as mentioned above some balance to where the drive is coming from is retained because the cyclist is driving the rear wheel.

An important difference between the hub motor and a crank drive motor is that most of the hub motors that you are likely to come across are not torque driven. A torque driven motor delivers power dependent on the torque applied by the cyclist. In simple terms a torque motor will measure the force applied by the cyclist and then multiply this, so the harder the cyclist pedals the more assistance will be delivered. A non-torque motor on the other hand will simply run at a set speed so long as the pedals are moving round. If the level of assistance is increased then the motor will run faster, regardless of the effort made by the cyclist.

Crank Drive Motor

Crank drive motors that feature on our bikes are made by Bosch, Shimano and Brose.

The majority of our bikes have a crank drive motor and this is the only motor to be found on our mountain or trail bikes. The Crank Drive Motor is typically found between the pedals. The motor powers the crank, in exactly the same way the cyclist does via the pedals. Because of this position, the extra weight of the motor is kept low and centralised, and so although it may increase the overall bulk of the design of the bike, it keeps the weight in a place that feels natural. Your wheels are also easier to detach than hub motor variants, as all electrical components are on the frame.

Bosch PerformanceLine CX motor

The Bosch Performanceline CX crank drive motor

The motor works in direct concert with the gears and the cyclist’s pedalling, and so it is able to achieve a smooth and reliable drive that feels more controllable and natural than the power assistance provided by a hub motor. Working in concert with the gears means that the motor has more success powering the rider up hills, which is why it is seen on more mountain bikes than hub motors. Building on this relationship, crank drive’s are typically built to be more powerful than hub motors, the norm being a torque of between 50-80Nm. Batteries built for crank drives also benefit from this increase in quality – Bosch powerpacks feature enormously high energy density in order that the rider benefits from the increased power of the motor during the more demanding usage that they have been built for. However, because of the efficient relationship between gears and motor, even low powered variants will assist very considerably when climbing hills.

As described above the crank drive motors are torque driven and this adds some complexity to the motor. They also tend to benefit from more sophisticated engineering. For example, fanned casing is used to keeps the motor cool and the components are built to survive the added pressure of the torque strength. Many crank drives, including Bosch motors, also feature an in-built diagnostic capability. We at Electric Bikes Guildford have the equipment and training to quickly perform a “health check” on your motor, so we can find out what’s wrong and the best way to fix it. When a bike with a crank drive motor comes in for service we carry out a diagnosis of the motor, the battery and the display unit. We also update the software on each of these parts.

If you would like to find out a bit more about the Bosch Motor please read our blog that seeks to explain the different Bosch motors that are available.

Conclusion

Although there are some advantages of a hub motor the advantages of the crank drive motor generally outweigh these and we believe that the best place for the motor is in the crank. The reasons for this are :

  • The motor is in the centre of the bike and is low down – better for the overall balance of the bike
  • Because they are torque driven it gives the cyclist a more realistic cycling experience. It also means that the motor is more efficient and can be up to 40% more efficient than a hub motor.
  • Hub motors can put the rim and spokes of the wheel under enormous pressure
  • In general the motors are of a better quality than the hub drive motors. As a result these bikes invariably command a higher price.

Of course the best way to decide which is right for you is to come and test ride the bikes yourself, which we’re always happy to encourage either at the shop or by arranging to meet up with you. We think both varieties of motor have great models that should suit any cyclist, but a lot will depend on what feels most natural to you, as well as what the bike will be used for. Hopefully this has given you some ideas about what type of motor might fit you best, but if you have any questions, please do not hesitate to ask!