The cadence sensor is the small, black, plastic disc that sits just behind the crankset; one of the magnets is visible at the top.
As inventions go, e-bikes rank as one of the better uses for an electric motor man has yet devised. Seriously, we climb on an e-bike, begin pedaling and the motor kicks into gear, zooming us up to the speed of fun. And the motor turns on without any elaborate effort on our part, just pedal and whee.
E-bike makers use one of two different kinds of sensors to switch the motor on and off. The most common type is a cadence sensor. The other type is a torque sensor. They work in fundamentally different ways and until recently appeared on e-bikes in very different price ranges, with cadence sensors being spec’d on more value-oriented e-bikes and torque sensors appearing on more performance-minded models.
And because the sensor is an essentially unseen component, it can be easily overlooked, even though the type of sensor used determines a fundamental part of the e-bike’s personality. It’s fair to ask: Which type is better? To do that, the Best Electric Bikes teams first will break down how they function.
Cadence sensor
A cadence sensor features a small sensor mounted to the e-bike frame that sends out a small pulse of electricity to the controller every time a magnet passes it. The crank arm (what the pedals mount to) sports a small disc with 12 magnets mounted in it. As the pedals turn, each magnet passes the sensor, sending out a stream of pulses that tell the motor to go.
A cadence sensor operates in a binary fashion; it either tells the motor to run or not. When it tells the motor to run, the controller determines how much juice to send to the motor based on what PAS level the rider has selected.
Cadence sensors have a few pros as well as a few cons.
Pros
- Inexpensive. They help to keep the cost of an e-bike down by using simple technology in an ingenious fashion.
- Clever. By using the rider’s pedal action to turn the motor on or off, the rider doesn’t have to fuss with a switch (something that was common in early e-bikes).
- Ghost pedaling. Not everyone wants to get on and pedal to the point that they start sweating. In between going full Tour de France and using the throttle, there’s a zone where riders can turn the pedals without making much effort and based on the PAS level selected, the e-bike will cruise up to that speed and keep going as long as the rider turns the pedals continuously. y
Cons
- Slow to act. Cadence sensors generally need between ½ and ¾ of a pedal stroke to decide that the rider is pedaling enough to warrant turning the motor on. The lag also causes the controller to continue to send power to the rear hub for a second or so after the rider has stopped pedaling. Many e-bikes include a motor cutoff switch in the brake levers so that the brakes don’t have to fight the motor when it is time to stop.
- Lack sensitivity. Cadence sensors function as a pedal-activated on/off switch. They don’t take into account the level of effort on the part of the rider.
Torque sensors
While cadence sensors all work in the same way, torque sensors come in three different flavors, though all three share the same basic input. Torque sensors use strain gauges to measure how much force the rider is delivering to the pedals.
The strain gauges can be integrated into one of three different components. The most common type of torque sensor places strain gauges in the bottom bracket. The bottom bracket is the set of bearings and the axle (or spindle) to which the cranks are mounted. As the rider pedals, the strain gauges can tell if the rider is going for an easy cruise or sprinting up a hill.
A torque sensor incorporated into the freehub mechanism for a rear wheel.
The second kind of torque sensor incorporates the strain gauges into the rear hub. It measures how much force is transferred from the chain to the cogs mounted to the rear wheel. This is a little bit down-stream from the bottom bracket, but still offers remarkable accuracy.
A torque sensor that mounts to the frame where the rear wheel attaches.
The third kind of torque sensor features strain gauges mounted to an aluminum plate that is mounted to the rear of the frame where the rear wheel bolts on, and may include the hanger for the rear derailleur. That such an arrangement can accurately measure how hard a rider is pedaling is nothing short of amazing; it seems like magic.
Torque sensors, for all their sophistication, come with pros as well as cons.
Pros
- No delay. The moment a rider begins pedaling the strain gauge sends a signal to the controller. The sensors samples at a rate of more than 1000 times per second, so there’s no lag between when the rider starts and when the motor starts. Instant power.
- Proportional power output. In addition to producing power the moment a rider puts weight on a pedal, the strain gauges in a torque sensor measure how much effort the rider is producing. The greater the effort the rider produces, the more power the controller sends to the motor. This is the aspect of an e-bike equipped with a torque sensor that makes the experience so seamless. By synchronizing the motor to the rider’s pedal force, an e-bike equipped with a torque sensor goes harder when the rider does and backs off the instant the rider does.
- More range. The presence of a torque sensor in an e-bike tailors the power output to the rider’s effort, which results in more efficient usage of the battery. As a result, e-bikes with torque sensors generally see more range for a given battery size than e-bikes built with cadence sensors.
Cons
- Expensive. Torque sensors cost more to purchase and the factory passes that cost onto the brand, who then passes that expense onto the consumer. Until recently, we only saw torque sensors used in e-bikes with mid-drive motors. Thanks to newer designs that incorporate the torque sensor in the hub motor or the rear dropout, we’ve begun seeing e-bikes in the sub-$2000 price range that include these sensors, so this isn’t the con that it used to be.
- Make for more work. Some people want to ride and pedal, but may not want to make much of an effort. Riders who want as much help as possible short of using a throttle may be frustrated that every time they back off the pedal stroke the e-bike slows down. Getting up to top speed will require real work.
- No ghost pedaling. Related to the previous point is the fact that ghost pedaling—turning the pedals only enough to keep the motor running—isn’t possible on e-bikes with torque sensors. To go, a rider will need to make some effort.
PAS Levels
Because cadence sensors and torque sensors send fundamentally different signals to the controller, how the controller deals with those signals is just as different.
As we mentioned previously, a cadence sensor sends an on signal to the controller and when that signal ends, the motor shuts off. With cadence sensors, the PAS levels correspond to levels of power output. PAS 1 may be 20 percent of the hub motor’s nominal power rating. PAS 2 may be 40 percent, and so on, until PAS 5 produces 100 percent of nominal power.
Practically speaking, what the rider may perceive is that in PAS 5, they begin pedaling and when the motor kicks in the e-bike accelerates up to 20 mph and stays at that speed provided the e-bike doesn’t encounter a steep hill or stop pedaling.
With a torque sensor, the PAS selector acts as a governor, limiting the amount of power the motor can generate. In the case of a Class 3 e-bike with a torque sensor, to reach 28 mph the PAS selector will need to be set to the highest output level (such as PAS 5 or “turbo”) and the rider will have to pedal with some real effort, otherwise the e-bike is unlikely to go much faster than 20 mph.
Which one is right?
For people shopping for a new e-bike the question of whether to pick an e-bike with a torque sensor vs. a cadence sensor isn’t much of a question in most price ranges.
People shopping in the $1500 and less price range are unlikely to encounter an e-bike equipped with a torque sensor. The technology is still expensive enough that we don’t see torque sensors on e-bikes in that price range. Riders who are willing to spend $3000 or more and want an e-bike with a mid-drive motor are unlikely to run across an e-bike with a cadence sensor.
The $1500 to $3000 price range is where we see e-bikes that combine hub motors with torque sensors. This is the price tier in which buyers can choose based on preference rather than their budget driving the outcome.
While a torque sensor is a more sophisticated piece of electronics, not everyone will prefer the experience of riding an e-bike equipped with one. The difference can be compared to power walking vs. an easy stroll. Anyone who wants to maximize the zoom is likely to prefer a torque sensor and the person who would rather not find themselves breathless as they go up a hill will prefer a cadence sensor.
Our suggestion? Visit some e-bike dealers and take a test ride on e-bikes featuring the two different sensors. One will appeal more than the other.
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