I received a question a few weeks ago, and I decided I wanted to answer it. It’s off-season after all. If you've got a question, send it to me and perhaps I'll make a video/article about it.
Yeah, there is a bit of talk about these two types of sensors. In most cases, on a factory built ebike, both sensor types are used. So, it’s rather a question of sensor bias on a modern emtb motor.
Just mounting the motor, controller, display and battery on the bike left me with no way to activate the motor. I could install the handlebar throttle which legally would turn the ebike into an extremely weak electric moped, or I could install the PAS kit. PAS is short for Pedal Assist Sensor; it’s more typically referred to as a cadence sensor.
But the motor system doesn’t only rely on the cadence sensor, I also had to install the speed sensor that measures the rotation of a wheel. Now my bike had turned into a street legal ebike and the speedometer started working.
And that’s how a basic ebike with a cadence sensor works. But it’s not a very pleasant riding experience. Once the motor starts powering up, it has no idea how hard you’re pedaling or how fast you want to go. The motor tries to spin up to its native rpm speed which is determined by the motor design and the battery voltage.
On my second DIY emtb build I was using the Bafang BBS01, a 250 W nominal crank motor powered by a 36 V battery. It behaved just like my first bike. You had to pedal a bit, and the motor was trying to hit about 80 rpm on the chainring. If the battery was fully charged, at about 42 V, the motor would want to spin at over 90 rpm. Unless the speed limiter was prohibiting it. If I wanted to ride slower, I had to reduce the assistance so the motor wouldn’t be strong enough to reach its target rpm.
This kind of motor behavior is fine for going straight ahead on endless straights. But it makes it more difficult riding on challenging, technical trails. If only there was a way for the motor system to determine how hard the rider was pedaling. That way the motor would know when to ease off.
But every factory built emtb that I have reviewed, which is over 90, has used the torque-, cadence- and speed-sensors in combination. But they rely on these sensors to a varying degree. The first Specialized/Brose and Bosch motors from around 2017 needed the bike and cranks to move a bit before the motor would kick in. This felt like a cadence sensor motor. But if you just pedal slowly and barely put any force into the pedals, the motor wouldn’t output much power. So, there was a cadence sensor there too. In my opinion, these motors were cadence sensor biased.
The Yamaha motor would rely much more on the torque sensor. You could rest one foot on the pedal and feel the motor twitching from the weight of your leg. The motor was activating to be ready, but it wouldn’t take off unless there was some motion of the cranks. Putting one foot on the pedal and immediately start riding would leave you with the impression of a torque-sensor-only bike. I call these torque sensor biased motors.
Not even the Rocky Mountain Dyname motors relied solely on the torque sensor. Its design could lead you to believe you only had to push the torque sensor arm to get going. But we tried pushing the sensor with the bike in a stand and nothing happened.
It makes less sense talking about a torque biased and cadence biased motor these days. But there are still some differences that can be attributed to how the motor sensors are being used. Some motors will twitch if you rest your foot on the pedal while stationary. Some motors need more pedal motion than others before deciding to kick in. But pretty much all motors are now easy to control riding with high assistance on technical trails.
Yeah, there is a bit of talk about these two types of sensors. In most cases, on a factory built ebike, both sensor types are used. So, it’s rather a question of sensor bias on a modern emtb motor.
The cadence sensor
There are motors that rely on cadence sensors only. My first DIY emtb I built using the Q100 hub motor. It was the 201 rpm version that would do 25 kph with a 26” wheel and 36 V battery voltage.Just mounting the motor, controller, display and battery on the bike left me with no way to activate the motor. I could install the handlebar throttle which legally would turn the ebike into an extremely weak electric moped, or I could install the PAS kit. PAS is short for Pedal Assist Sensor; it’s more typically referred to as a cadence sensor.
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But the motor system doesn’t only rely on the cadence sensor, I also had to install the speed sensor that measures the rotation of a wheel. Now my bike had turned into a street legal ebike and the speedometer started working.
Riding with just a cadence sensor
When starting to pedal, the motor system had no idea I was about to ride until the cranks had moved enough for the PAS-magnets to be picked up by the cadence sensor. Also, the motor system needed to know the rear wheel magnet was registered by its sensor. Once the system decided the cranks were being pedaled and the bike was moving, it would activate the motor.|
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And that’s how a basic ebike with a cadence sensor works. But it’s not a very pleasant riding experience. Once the motor starts powering up, it has no idea how hard you’re pedaling or how fast you want to go. The motor tries to spin up to its native rpm speed which is determined by the motor design and the battery voltage.
On my second DIY emtb build I was using the Bafang BBS01, a 250 W nominal crank motor powered by a 36 V battery. It behaved just like my first bike. You had to pedal a bit, and the motor was trying to hit about 80 rpm on the chainring. If the battery was fully charged, at about 42 V, the motor would want to spin at over 90 rpm. Unless the speed limiter was prohibiting it. If I wanted to ride slower, I had to reduce the assistance so the motor wouldn’t be strong enough to reach its target rpm.
This kind of motor behavior is fine for going straight ahead on endless straights. But it makes it more difficult riding on challenging, technical trails. If only there was a way for the motor system to determine how hard the rider was pedaling. That way the motor would know when to ease off.
Cue the torque sensor
I guess there are ebike motors out there that only relies on the combination torque- and speed sensors. As far as I remember, BH had a rear hub powered ebike with a sensor that was measuring chain vibrations. That might not sound like a torque sensor, but it was indirectly measuring how hard the rider was pedaling. The chain would vibrate sideways to a varying degree, depending on how hard I pedaled. I rode this bike in about 2015 and it was quite pleasant to ride.But every factory built emtb that I have reviewed, which is over 90, has used the torque-, cadence- and speed-sensors in combination. But they rely on these sensors to a varying degree. The first Specialized/Brose and Bosch motors from around 2017 needed the bike and cranks to move a bit before the motor would kick in. This felt like a cadence sensor motor. But if you just pedal slowly and barely put any force into the pedals, the motor wouldn’t output much power. So, there was a cadence sensor there too. In my opinion, these motors were cadence sensor biased.
The Yamaha motor would rely much more on the torque sensor. You could rest one foot on the pedal and feel the motor twitching from the weight of your leg. The motor was activating to be ready, but it wouldn’t take off unless there was some motion of the cranks. Putting one foot on the pedal and immediately start riding would leave you with the impression of a torque-sensor-only bike. I call these torque sensor biased motors.
Not even the Rocky Mountain Dyname motors relied solely on the torque sensor. Its design could lead you to believe you only had to push the torque sensor arm to get going. But we tried pushing the sensor with the bike in a stand and nothing happened.
Modern emtb motors
We haven’t really got cadence sensor biased motors of the early days anymore. New motors are faster at transmitting and computing data. Also, the sensors are becoming more advanced with a much higher sample rate.It makes less sense talking about a torque biased and cadence biased motor these days. But there are still some differences that can be attributed to how the motor sensors are being used. Some motors will twitch if you rest your foot on the pedal while stationary. Some motors need more pedal motion than others before deciding to kick in. But pretty much all motors are now easy to control riding with high assistance on technical trails.
