How does the cadence sensor work?

BigNerd

Well-Known Member
Since @roshan is one of the few bike vendors here who actually interacts with us, I thought I would ask him this question as there is some confusion about how a cadence sensor works on a bike.

There are some that say it's just an on/off switch, once it detects pedaling, it tells the motor to run and that 100% power available at that PAS level will go to the motor.

Others, like myself, and the Evelo site, say that it's actually cadence based. The speed of the pedaling inversely tells the system how much power to give to the motor. The slower you pedal, the more power, the faster you pedal, less power.

Now, this could be brand/motor dependent so both are correct but what does Biktrix do on their bikes?

Thanks in advance.
 

Taylor57

Well-Known Member
Since @roshan is one of the few bike vendors here who actually interacts with us, I thought I would ask him this question as there is some confusion about how a cadence sensor works on a bike.

There are some that say it's just an on/off switch, once it detects pedaling, it tells the motor to run and that 100% power available at that PAS level will go to the motor.

Others, like myself, and the Evelo site, say that it's actually cadence based. The speed of the pedaling inversely tells the system how much power to give to the motor. The slower you pedal, the more power, the faster you pedal, less power.

Now, this could be brand/motor dependent so both are correct but what does Biktrix do on their bikes?

Thanks in advance.

;)
 

BigNerd

Well-Known Member
When I see that dead horse... I can't help but hit it a few more times. But this time I want professional assistance. :)
 

roshan

Well-Known Member
Great question. Here's how a cadence sensor actually works:

On regular bikes, there is a magnet (that is usually attached to a moving part like spoke/pedal) and a sensor (that is attached to the frame of the bike). In this case the sensor indeed works like a switch which turns on or off (depending on the model) when the magnet passes close to the sensor. This is typical of how hall-effect sensors work. This on/off signal can be used with respect to time, to figure out how fast something is moving. Ie. if the sensor was on the frame and the magnet on the pedal, this could measure how fast the person is pedaling by looking at x rotations/min. If this sensor was on the rear wheel, it could measure the RPM of the wheel. This RPM can be used with the circumference of the wheel to calculate wheel/bike speed.

On most ebikes with PAS (Pedal assist systems), there is a magnetic disc attached on the crank arm with multiple magnets and a sensor on the frame. Say for example, there is only 1 magnet. In this case, the crank arm needs to move an entire rotation (360 degrees) for the magnet to send a signal to the motor controller. This would result in a very bad user experience as there would be a huge lag in response. Due to this, ebikes have a PAS magnet ring that has usually 3-12 magnets. As you can imagine, if the ring has 3 magnets, then each magnet would pass the sensor at 360/3 = 120 degrees per rotation while if it had 12 magnets, it would pass at 360/12 = 30 deg rotation. Now you can see how, if the ebike controller was getting feedback every 30 degrees of crank movement, then the controller would have a lot more information to process, thereby potentially resulting in a much sooner engagement of assist and much smoother assist. On mid motors like BBS02, there are about 52 points of engagement per rotation - resulting in a much, much smoother pedal assist and almost immediate engagement.

Okay, now that the primer is out of the way, let's look at your questions:
There are some that say it's just an on/off switch, once it detects pedaling, it tells the motor to run and that 100% power available at that PAS level will go to the motor.
This really depends on 2 things:
a. the number of magnets in the PAS ring
b. the way the controller processes the data from the PAS
As explained above, obviously more the magnets, the better. However, if the data from the magents isn't being used properly by the controller, then even with a lot of magnets, you could have a pretty shitty riding experience as the controller could just look at if a magnet passed the sensor and push out full power to the motor. From my experience, most chinese controllers have settings like:
- number of poles on PAS (essentially the number of magnets)
- start current: what's the starting amps put out by the controller when PAS is engaged
- current ramp: how fast the controller puts out full current
- max current output: this is often limited by hardware. More mosfets in the controller, higher max current
- max speed (rpm of the motor): this is often limited by voltage of the battery. Motors have RPM that is a function of input voltage (called kv rating of a motor) and motor gearing (if internally geared).

Others, like myself, and the Evelo site, say that it's actually cadence based. The speed of the pedaling inversely tells the system how much power to give to the motor. The slower you pedal, the more power, the faster you pedal, less power.
The slower you pedal, more power, the faster you pedal, less power: you can see that this statement is more of a "feeling from the rider" because technically when you start pedaling from a standstill, the controller pushes power to the motor (based on the current ramp), and say you already reach full power in about 2 pedal strokes. After this point, no matter how much you pedal, you won't get additional assist as the motor is already at peak power. Thereby making you feel like the motor isn't doing any work or the motor is putting out less power.
what does Biktrix do on their bikes?
The answer is, it depends on the bike.
Juggernaut Classic, Stunner X, Stunner: BBS02 motor (highest resolution cadence sensor on the market). Our motors are optimized for response and natural feel. Max current 25A.
All Ultra bikes: Torque sensor - most natural assist with max current 30A.
Swift, Stunner LT: Bottom bracket torque sensor assist
Swift lite, Kutty: 8 magnet PAS

Hope that helps!
 

Afhill

New Member
Region
Canada
Great question. Here's how a cadence sensor actually works:

On regular bikes, there is a magnet (that is usually attached to a moving part like spoke/pedal) and a sensor (that is attached to the frame of the bike). In this case the sensor indeed works like a switch which turns on or off (depending on the model) when the magnet passes close to the sensor. This is typical of how hall-effect sensors work. This on/off signal can be used with respect to time, to figure out how fast something is moving. Ie. if the sensor was on the frame and the magnet on the pedal, this could measure how fast the person is pedaling by looking at x rotations/min. If this sensor was on the rear wheel, it could measure the RPM of the wheel. This RPM can be used with the circumference of the wheel to calculate wheel/bike speed.

On most ebikes with PAS (Pedal assist systems), there is a magnetic disc attached on the crank arm with multiple magnets and a sensor on the frame. Say for example, there is only 1 magnet. In this case, the crank arm needs to move an entire rotation (360 degrees) for the magnet to send a signal to the motor controller. This would result in a very bad user experience as there would be a huge lag in response. Due to this, ebikes have a PAS magnet ring that has usually 3-12 magnets. As you can imagine, if the ring has 3 magnets, then each magnet would pass the sensor at 360/3 = 120 degrees per rotation while if it had 12 magnets, it would pass at 360/12 = 30 deg rotation. Now you can see how, if the ebike controller was getting feedback every 30 degrees of crank movement, then the controller would have a lot more information to process, thereby potentially resulting in a much sooner engagement of assist and much smoother assist. On mid motors like BBS02, there are about 52 points of engagement per rotation - resulting in a much, much smoother pedal assist and almost immediate engagement.

Okay, now that the primer is out of the way, let's look at your questions:

This really depends on 2 things:
a. the number of magnets in the PAS ring
b. the way the controller processes the data from the PAS
As explained above, obviously more the magnets, the better. However, if the data from the magents isn't being used properly by the controller, then even with a lot of magnets, you could have a pretty shitty riding experience as the controller could just look at if a magnet passed the sensor and push out full power to the motor. From my experience, most chinese controllers have settings like:
- number of poles on PAS (essentially the number of magnets)
- start current: what's the starting amps put out by the controller when PAS is engaged
- current ramp: how fast the controller puts out full current
- max current output: this is often limited by hardware. More mosfets in the controller, higher max current
- max speed (rpm of the motor): this is often limited by voltage of the battery. Motors have RPM that is a function of input voltage (called kv rating of a motor) and motor gearing (if internally geared).


The slower you pedal, more power, the faster you pedal, less power: you can see that this statement is more of a "feeling from the rider" because technically when you start pedaling from a standstill, the controller pushes power to the motor (based on the current ramp), and say you already reach full power in about 2 pedal strokes. After this point, no matter how much you pedal, you won't get additional assist as the motor is already at peak power. Thereby making you feel like the motor isn't doing any work or the motor is putting out less power.

The answer is, it depends on the bike.
Juggernaut Classic, Stunner X, Stunner: BBS02 motor (highest resolution cadence sensor on the market). Our motors are optimized for response and natural feel. Max current 25A.
All Ultra bikes: Torque sensor - most natural assist with max current 30A.
Swift, Stunner LT: Bottom bracket torque sensor assist
Swift lite, Kutty: 8 magnet PAS

Hope that helps!
@roshan now I'd like to know why you do so many different things! Pros/cons and why you chose the different type of sensor for each bike. 😁
 

voidedwarranty

Active Member
Great question. Here's how a cadence sensor actually works:

On regular bikes, there is a magnet (that is usually attached to a moving part like spoke/pedal) and a sensor (that is attached to the frame of the bike). In this case the sensor indeed works like a switch which turns on or off (depending on the model) when the magnet passes close to the sensor. This is typical of how hall-effect sensors work. This on/off signal can be used with respect to time, to figure out how fast something is moving. Ie. if the sensor was on the frame and the magnet on the pedal, this could measure how fast the person is pedaling by looking at x rotations/min. If this sensor was on the rear wheel, it could measure the RPM of the wheel. This RPM can be used with the circumference of the wheel to calculate wheel/bike speed.

On most ebikes with PAS (Pedal assist systems), there is a magnetic disc attached on the crank arm with multiple magnets and a sensor on the frame. Say for example, there is only 1 magnet. In this case, the crank arm needs to move an entire rotation (360 degrees) for the magnet to send a signal to the motor controller. This would result in a very bad user experience as there would be a huge lag in response. Due to this, ebikes have a PAS magnet ring that has usually 3-12 magnets. As you can imagine, if the ring has 3 magnets, then each magnet would pass the sensor at 360/3 = 120 degrees per rotation while if it had 12 magnets, it would pass at 360/12 = 30 deg rotation. Now you can see how, if the ebike controller was getting feedback every 30 degrees of crank movement, then the controller would have a lot more information to process, thereby potentially resulting in a much sooner engagement of assist and much smoother assist. On mid motors like BBS02, there are about 52 points of engagement per rotation - resulting in a much, much smoother pedal assist and almost immediate engagement.

Okay, now that the primer is out of the way, let's look at your questions:

This really depends on 2 things:
a. the number of magnets in the PAS ring
b. the way the controller processes the data from the PAS
As explained above, obviously more the magnets, the better. However, if the data from the magents isn't being used properly by the controller, then even with a lot of magnets, you could have a pretty shitty riding experience as the controller could just look at if a magnet passed the sensor and push out full power to the motor. From my experience, most chinese controllers have settings like:
- number of poles on PAS (essentially the number of magnets)
- start current: what's the starting amps put out by the controller when PAS is engaged
- current ramp: how fast the controller puts out full current
- max current output: this is often limited by hardware. More mosfets in the controller, higher max current
- max speed (rpm of the motor): this is often limited by voltage of the battery. Motors have RPM that is a function of input voltage (called kv rating of a motor) and motor gearing (if internally geared).


The slower you pedal, more power, the faster you pedal, less power: you can see that this statement is more of a "feeling from the rider" because technically when you start pedaling from a standstill, the controller pushes power to the motor (based on the current ramp), and say you already reach full power in about 2 pedal strokes. After this point, no matter how much you pedal, you won't get additional assist as the motor is already at peak power. Thereby making you feel like the motor isn't doing any work or the motor is putting out less power.

The answer is, it depends on the bike.
Juggernaut Classic, Stunner X, Stunner: BBS02 motor (highest resolution cadence sensor on the market). Our motors are optimized for response and natural feel. Max current 25A.
All Ultra bikes: Torque sensor - most natural assist with max current 30A.
Swift, Stunner LT: Bottom bracket torque sensor assist
Swift lite, Kutty: 8 magnet PAS

Hope that helps!
@roshan Where does the Juggernaut HD duo fit in to this? I just purchased one a few hours ago
 
Last edited:

AHicks

Well-Known Member
Region
USA
City
Snow Bird - Summer S.E. Michigan, Winter Gulf Coast North Central Fl.
Where does the Juggernaut HD duo fit in to this? I just purchased one a few hours ago
You're going to need to identify your purchase as the hub drive version, or the mid drive. They are each goig to be WAY different PAS systems...