Why have a gear sensor? The purpose of a gear sensor is to protect your drive train from excessive power from a mid-drive motor while shifting a bike with a rear derailleur. A normal cyclist is around 150 watts of human power and most drive trains are designed to handle a little more than that much power.
But now add 450 to 750+ watts from a mid-drive motor to that which is equal to another 3-6 cyclists and you are way beyond what the drive train was intended be able to handle when the chain is across multiple cogs with only a few teeth on one of the gears. This is where a gear sensor comes in.
It detects that you are shifting and cuts power to the motor for about an .8 of a second. This is long enough for the shift to complete before the power comes back on.
How does a gear sensor work? To install the gear sensor you actually pass the shift cable through the gear sensor. When the gear sensor detects cable movement, it sends a signal to the motor to stop for 800ms. All Bafang mid-drive motors from Electrify Bike Co have a gear sensor cable coming from the motor controller so they are gear-sensor ready. If your motor does not have this cable you can hook the gear sensor up to a brake sensor cable instead. Although this does work brake sensors shut the motor off for a minimum of about 2 seconds instead of .8 seconds. This can be a bit of a nuisance to have the motor off for that long of a time.
I have a hub motor. Do I need a gear (shift) sensor? If you have a hub motor you do not need a shift sensor since the hub motor does not use the drive train.
I have a rear derailleur with a Bafang mid-drive motor. Do I need a gear (shift) sensor? Since you have to pedal through the shift on a rear derailleur drive train the motor will be on the whole time your are shifting. If you happen to be in a mid to high level of motor assistant you could be putting upwards of 450 watts through the drive train. With a BBS02 or BBSHD this could be more than 1000 watts. This is where the shift sensor is critical. If you have a mid-drive that is under 400 watts or you only shift in levels of assistance that are under 400 watts then you might get by without one.
I have an internal gear hub (IGH). Do I need a gear (shift) sensor with my mid-drive motor? If you have an internal gear hub like a 3-speed or 5-speed Shimano, you probably do not need a gear sensor since you are supposed to stop pedaling when you shift an IGH anyway. When you stop pedaling the motor will stop and you can shift. Start pedaling again and the motor will come back on. Some motors have a slight delay after you stop pedaling before the motor shuts off so you should wait until it stops before shifting. Electrify Bike Co. reprograms all of our mid-drive motors to cut off instantaneously when you stop pedaling so this is rarely an issue. Some still prefer the instantaneous cutoff of a shift sensor but there is lag before the motor comes back on so it is a trade-off.
I have a TSDZ2 mid-drive motor. Do I need a gear sensor? If you have a torque-sensing motor like the TSDZ2 then you do not need a gear sensor. Because the motor responds to the amount of torque you are applying to the pedals, you can simply ease off a bit when you pedal through the shift and the power will ease off as well. If you need to be able to shift when pedaling hard with a torque-sensing motor then you may still want one. Since the TSDZ2 does not have direct support for a gear sensor, it must be hooked up to a brake sensor. The problem with this is that instead of 800ms the brake sensor cuts power for 1500 - 2000ms which can feel like an excessive amount of time to lose power. But it does work!