Cadence sensor inherent issues?

[Gee_Whiz]

Has anyone who uses a cadence sensing bike, noticed an "issue" where if you have your foot on the pedal at a stop, the bike may lurch forward slightly? If so, any idea how to keep this from occuring?

 

[HCooke]

Keep one of your brakes lightly engaged.

 

[harryS]

Shouldn't happen on a cadence sensor as most need 1/4 turn of the pedals to recognize pedal movement. I've read comments where that has happened with a torque sensor.

The work around in either case, if you have brake cutoffs, which I believe are required in many states and also by the Consumer Safety Product commission, is to keep a brake lever depressed. Another is to shut off the assist level til you're ready to go. The latter is probably best for novices. My first ebike, I always shut off assist for any low speed maneuver.

 

[AHicks]

USUALLY, at least 2 magnets will have to pass by the PAS sensor before the motor receives power. How far the crank has to move to do that depends on how many magnets in the PAS sensor. For instance, all else being equal, to get 2 magnets past the PAS sensor, a crank attached to a PAS sensor with 5 magnets is going to have to move twice as far as a crank using a 10 magnet sensor. That's the only practical difference between the 2 types by the way (5 magnet vs. 10). Not necessary to assume more magnets = bigger/better/faster. That's not the case at all.

SOMETIMES though, the controllers are set up with a "sensitivity" adjustment, or a setting allowing you to set the number of magnets that must pass the sensor to your 'druthers. Different strokes you know? Somebody looking for a sensitive set up, or maybe just one magnet passing the sensor, might be somebody riding tight trails where immediate response to the crank is a good thing! Somebody else, saying somebody that might be taken off guard by a sudden/accidental lurch forward, may not care for that sensitive/1 magnet setting at all......

The aftermarket KT controllers, and the Bafang mid drives (UART based), are 2 examples where the sensitivity/number of magnets can be tailored to the riders wishes.

And oh ya. Climbing on board a hyper powerful mid drive with torque sensing may have your attention right quick like. The SECOND time that happens, you WILL have a brake lever pulled when climbing on AND off! Just like a dog only burns it's nose once. That lesson still fresh in my mind.... -Al

 

[Gee_Whiz]

USUALLY, at least 2 magnets will have to pass by the PAS sensor before the motor receives power. How far the crank has to move to do that depends on how many magnets in the PAS sensor. For instance, all else being equal, to get 2 magnets past the PAS sensor, a crank attached to a PAS sensor with 5 magnets is going to have to move twice as far as a crank using a 10 magnet sensor. That's the only practical difference between the 2 types by the way (5 magnet vs. 10). Not necessary to assume more magnets = bigger/better/faster. That's not the case at all.

SOMETIMES though, the controllers are set up with a "sensitivity" adjustment, or a setting allowing you to set the number of magnets that must pass the sensor to your 'druthers. Different strokes you know? Somebody looking for a sensitive set up, or maybe just one magnet passing the sensor, might be somebody riding tight trails where immediate response to the crank is a good thing! Somebody else, saying somebody that might be taken off guard by a sudden/accidental lurch forward, may not care for that sensitive/1 magnet setting at all......

The aftermarket KT controllers, and the Bafang mid drives (UART based), are 2 examples where the sensitivity/number of magnets can be tailored to the riders wishes.

And oh ya. Climbing on board a hyper powerful mid drive with torque sensing may have your attention right quick like. The SECOND time that happens, you WILL have a brake lever pulled when climbing on AND off! Just like a dog only burns it's nose once. That lesson still fresh in my mind.... -Al

This was wonderfully laid out.. thank you! Looks like modifying the PAS is probably a good idea if possible too as Harry mentioned and still being mindful of keeping the brakes steady.. I think some bikes, if the pedal comes to a stop near the sensor, is ready to jump forward once it moves slightly past the sensor

 

[indianajo]

If you have either a throttle or torque input, one of those could be surging temporarily. You can see it with an analog meter or oscilloscope on the sense wire of that sensor, to negative. Dual Slope integrator DVM (digital volt meter) are too slow to see surges like that. They average over 1/2 to 1 second.
Even if nothing is plugged into those inputs, I would monitor the bare connectors to see if something was going on inside the controller.

 

[rich c]

If your freewheel is stiff, just moving the bike can rotate the pedal and power up the bike. Pull the brake so the motor inhibitor switch shuts off the current to the motor.

 

[AHicks]

This was wonderfully laid out.. thank you! Looks like modifying the PAS is probably a good idea if possible too as Harry mentioned and still being mindful of keeping the brakes steady.. I think some bikes, if the pedal comes to a stop near the sensor, is ready to jump forward once it moves slightly past the sensor

You have it. Exactly. That's generally the downside of a sensitive/one magnet setting. Wonderful for quick response, with the down side being the potential for "false starts". Thankfully, those false starts are usually very short lived, causing the bike to lurch forward an inch or two. It is something that will get the attention of even experienced riders.

If you set up for 2 magnets (or more), that situation will be avoided completely.

A lot of mystery around PAS sensors. I like to explain them once in a while, removing the smoke and mirrors, exposing them for their wonderful simplicity. -Al