The motor of an ebike can be activated in two ways. Either there is something to push or turn, like a button; or there is a sensor which can tell whether the pedals are turning or not, and activate the motor accordingly; there can also be a combination of the two. In many countries, particularly in Europe, only the second kind of activation is legal: the motor can deliver its power only when the pedals are turning, and must be turned off immediately when the cyclist stops pedalling. This article is concerned exactly with the sensors which can tell whether you are pedalling or not.
There are two kinds of ebike sensors, and for a European-style ebike the choice between these two systems is often the most important one to make when choosing your ebike.
The first kind of ebike sensor is simpler, and just tells whether the pedals are turning or not. It works in a similar way to the simple bike computers which give information on your speed and distance travelled: there is a small disc, turning together with the pedals, on which a series of magnets is positioned; these magnets activate a sensor on the frame. The sensor activates the motor when the third magnet goes by it. The more magnets, and the closer together they are, the more reactive the ebike; in any case, in the first phase of the pedalling (from about a third to a half turn of the pedals) you’ll have to pedal without any assistance from the motor, taking charge entirely of the effort of the start. However, once the motor is activated, an ebike with this kind of sensor allows you to do what we may call a “symbolic pedalling”: it is possible to just turn the pedals, without any real effort, and let the motor push you along; in this way, the range of the ebike will be less. If you choose this kind of sensor, it is also of course possible to put some effort into your pedalling, to have a higher range and do some exercise. When going uphill, you will have to do so, especially if you have a European-style ebike with a 250W motor.
The second kind of ebike sensor, called torque sensor, is more advanced, because it can pick up the effort that you put into your pedalling. Basically, it can tell how much intensely you are pedalling, whether you are just strolling along or really pushing. It can do so by picking up the mechanical torsions which are generated when you pedal. This kind of sensors determines a more natural feeling for the combination of human and electric power: the motor is activated immediately at the start, as soon as the sensor picks up the pressure on the pedals; when you pedal with less intensity, the motor delivers a low amount of power; when you pedal more intensely, for instance when going uphill or because you want to go fast, the motor delivers a higher amount of power. The most widespread ebikes with this kind of sensor are those produced by Bosch, though the competition by Yamaha, Shimano, Sunstar and others gets stronger. With these ebikes it is impossibile to use the “symbolic pedalling”: you always have to really pedal, even at a very low level, just like a normal bike. For this reason, the range of this kind of ebikes is usually higher. Don’t underestimate the push of the motor, which can be very high anyway.
Even at the cost of simplifying a little, we can state that ebikes with a simpler pedalling sensor may be suited to those who are not used to pedalling: apart from the initial effort at the start, they will be able to rely on the “symbolic pedalling” and be pushed along by the motor. On the other hand, those who are more used to cycling will find an ebike with a torque sensor more natural to ride.
The best advice that we can give you is to find a shop and try the two kinds of ebike sensors for yourself, to see which is more suited to your needs and taste.