Once you have got the bike and the battery, there's one last thing you need to make an electric bike: the motor
There are different kind of motors that you can find fitted on electric bikes. The majority of modern electric bike motors are placed at the bottom bracket (in between the cranks), while they can also be positioned within the hub of the rear wheel. This latter positioning is generally found on cheaper electric bikes.
Wherever the motor is located, it is always subject to UK and EU legislation, being limited to no more than 250w or 25kph.
As hub motors are very popular and produced extensively in China, they are commonly the cheapest ones one the market and, possibly, the most reliable. However, as the technology in this field has improved a lot in the last years, many of the top-performing have started to mount mid-drive motors located at the bottom bracket
A hub motor is an easy alternative to turn your standard bike into an electric bike, but before doing that, you should check that the motor’s specification would fit on your normal bike.
The first main difference between hub motors is that they can be mounted either on the front or rear wheel. As their weight is normally pretty high, having them on the back of your bike makes more sense, as the seatstays and chainstays provide better support for the additional support for the extra weight that would not be offered by the fork.
The other big difference between hub motors is that they can be geared or gearless. If the motor is in the hub of the front wheel, the gears at the back will normally be the same as on non-electric bikes. If the motor is in the real wheel then it may also contain the gearing system too.
There are however some downsides to positioning an electric bike motor in the hub. In general they do not perform as well as mid-drive motors, but also can imbalance the bike as the weight is positioned at either end rather than central, and also tend to transmit energy through the frame, making the ride unbalanced.
Mid-drive electric bike motors are mounted in the centre of the bike at the bottom bracket area (where the cranks are fixed into the frame). This positioning gives the bike better stability and balance, and while the motor is working and propelling the bike, the forces generated by it won’t be transmitted unevenly into the frame.
In terms of efficiency the mid-drive motors that are connected directly with the drivetrain offer better performance than hub motors are claimed to be more performing than the hub ones, as their power is transmitted from the centre of the bike.
Watch: what's it like to ride an electric bike
In general, mid-drive motors are very good on hills, as instead of just driving the wheel forward, they move the chain, giving the bike more power.
Like hub motors, mid-drive electric bike motors can either be geared or single-speed. When shifting with these motors you need to be as gentle as you would be on a normal bike, as the motor will generally be applying more force through the drivetrain than you would be through the pedals.
Another good thing about mid-drive motors is that they are positioned closer to the electric bike sensors which receive feedback from your pedals stroke, making sure the battery transmits the correct amount of power to the motor. Positioning the motor and the sensor in the same place makes the whole system more efficient.
Makes of mid-drive motors
There are two main manufacturers of mid-drive motors: Shimano and Bosch.
Shimano Steps is the first electric bike groupset from the Japanese giant, and features a mid-drive electric bike motor. The system also has an integrated electric shifter (Shimano Alfine 11 speed Di2 placed in the rear hub) and works with a torque sensor placed in the bottom bracket to make the ride smoother.
The Shimano Steps has got also a full automatic shifter: it analyses your pedal stroke and energy, and automatically changes the gear for you. If you stop at a traffic light, it changes automatically into a small ring to make the start efficient and not letting you (nor the system itself) to spend too much energy.
There's much more variety among Bosch mid-drive motors, depending on the sort of riding that you're doing and the depth of your pockets.
All Bosch motors feature four different levels of support, with those at the top of the range giving up to 300% support compared to your pedalling power, while cheaper models will only give 225% support. Models aimed at off-roaders and sport riders will be quicker off the line too, kicking in faster and with more power when going from a standing start.
There is also another way to turn your normal bike into an electric bike. Of course you can buy a kit and mount either a hub or a mid-drive motor, but you can also mount – directly to your seatpost – a friction motor. This system generate energy through a battery that makes a roller turning on your wheel and propel the bike forward.
They’re easy to mount and fast to use (and they work mostly on most kind of bikes), although the roller might not work well in wet conditions and can cause your tyre to wear down.
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Nick Busca is a freelance cycling and triathlon journalist. He is also a certified triathlon coach and personal trainer.