Is it any wonder that the humble bicycle was voted greatest invention of all time by Radio 4 listeners? Such a simple design that not only provides one of life's greatest pleasure (I defy anyone to ride a bike on a nice day and not have a smile on their face) but quite literally transforms lives through projects such as World Bicycle Relief and the Qubeka Project. The advent of e-bikes has seen the original design complemented rather than reinvented. If it ain't broke don't fix it, right? The only real change to John Kemp Starley's original safety bicycle design is how you choose to use the electric motor to provide additional propulsion. The choices are using the motor to drive the front or rear wheel or to help you turn the cranks. Let's explore the benefits of each.
Front wheel driven e-bikes have their motor within the front hub. Since you are driving the rear wheel by pedaling this gives you in essence the bicycle equivalent of a 4x4. Like 4-wheel drive cars, your 2x2 e-bike has more grip on loose slippery surfaces since you can spread the amount of force that turns each wheel. Less force means less chance of the wheel spinning but, since you have more total force since both wheels are being driven, you can still get up that steep hill even when it's covered in gravel.
Front hub motors are usually controlled by a cadence sensor that detects how quickly you are pedaling and applying more power as your cadence slows down.
Rear hub motors provide additional drive through the rear wheel. Since you are also driving the rear wheel by pedaling you might think that it would be easy to lose traction. However, the design of the bicycle means most of your weight is above the rear wheel, pressing the tyre into the ground to aid traction. Rear hub motors can be controlled by a cadence sensor, like front hub motors, or by a torque sensor that measures the amount of force you are applying to the cranks. The more force you apply to the cranks (like when you are going uphill) the more power is delivered by the motor to assist you.
The third type of drive is a mid-mounted motor. This type of motor is particularly popular on mountain e-bikes designed to be ridden over very rough and steep terrain. With a mid-mounted motor the motor sits at the lowest point of the bike's frame and provides additional drive through the cranks as you pedal. Perhaps the greatest advantage of having a mid-mounted motor is that the additional weight of the motor is directly above the centre of gravity of the bike, allowing greater control over really rough terrain. This design is not limited to hard core mountain e-bikes and is becoming increasingly popular thanks to drive systems from Bosch and Shimano.
Which type of drive is best for you? That all depends on where you plan to ride. The best way to understand the difference between the different types of drive is to take a test ride on each.