With Bafang now shipping controllers with a built-in gear sensor cable and gear sensors now available from multiple U.S. resellers we now have a better option to consider to protect our drive train while shifting.
Now instead of figuring out how to shift safely under power by finding ways to manually cut the motor, we can add a shift sensor and let it and the BBSxx automatically handle it for us. We are now a step closer to those high-end factory ebikes that are so smooth but unlike our Bafang solutions are under-powered and over-hyped. They do still have one leg up on us though and that is PAS with torque sensing. That takes PAS to a whole new level but is not yet available on Bafang DIY mid-drive motors.
If you don't use PAS, you don't need the gear sensor unless you don't want to let off the throttle while shifting gears. To shift safely using the throttle just let off the throttle and shift. If you can't coordinate that then by all means get the gear sensor and it will lay off the throttle for you. Granted there are times riding single track that it is hard to lay off the throttle because of the grip you have on the bars. So mountain bikers that are using throttle only could definitely use a gear sensor.
If you use PAS and pride yourself in saving $50 by learning manual shift techniques like lightly squeezing the brake or back pedaling to cut the motor before shifting, you may not be a candidate for the new shift sensor. If you ever bought a car with a manual transmission for the pure joy of being in direct manual control, pressing in the clutch and moving the shift lever, you may also not be a candidate. However, you will not be doing something natural like pushing a clutch to shift, you will be unnaturally pulling the brake or back pedalling to shift which may not bring the same pure joy.
However, if you use PAS and like me don't like to do something unnatural like squeeze the brake or backpedal to shift and despite your best attempts to master these techniques find you occasionally shift before the motor cuts out and you hear and feel a hard 750W+ clunk go through your drive train then you are definitely a candidate for the gear sensor. The ability to shift gears just like you always have without worrying about what the motor is doing with the PAS system or whether you can even let off the throttle (think gripping the bars on a hill riding single track) has got to be nice.
If you use PAS without a gear sensor there is an often glossed over controller programming parameter under "Pedal Assist" called "Time of stop" that can make a huge difference. The "Time of stop" value is multiplied by 10 to get the number of milliseconds after you stop pedaling until the motor stops. Most vendors selling Bafang mid-drives are setting this to a value of 25 which introduces a full quarter second delay (250ms) between the time you stop pedaling and when the motor actually quits. This is why people use brakes and back pedaling to ensure that power to the motor is cut before shifting. If you set it to 5 (don't set it any lower or PAS will stop working) then it only waits 1/20th of a second (50ms) before cutting the motor so you can just stop pedaling and then shift and the timing works out. Karl Gesslein, in his "Hackers Guide to Programming the BBS02 and BBSHD" makes the statement, "I strongly recommend setting this to 5, especially if you want to use the PAS system without using e-brakes."
Even if you get a gear sensor you will want to adjust this value down to between 5 and 12 so the motor stops more quickly after you stop pedalling under PAS. Curiously, out of 5 vendors and experts with recommendations, all of them but Karl and now Luna recommend a value of 25. Trust me, changing this to 5 makes a HUGE difference. Unfortunately, a side effect of this setting is that at slow cadences the pulses from the PAS magnetic poles are coming slow enough that there is a greater than 50ms delay between pulses and the motor never engages. If this happens, you are probably in too high of a gear anyway.
The solution of course is to shift to a low enough gear that your cadence is high enough that it doesn't time out between PAS pulses or perhaps try a compromise value of 10 or 12. I set mine to 1o because I am sometimes lazy and don't always shift to a faster cadence. You can play with optimizing this for your own style by playing with numbers between 5 and 25 but it will be a compromise.
If you install a new gear sensor to cut the motor during shifting you will not need to worry about it. That's what I did and I really like the seamless shifting without thinking about it and potentially thrashing my drive train. You get smooth shifting without stopping pedaling or messing with the brakes to get the motor to stop. The motor cuts out as soon as I start to shift and comes back on right after shift completes. Timing is very smooth. Feels like a real pro set-up - sweet.
Have you made a decision on a gear sensor? Why or why not?
Gear sensor available here
Gear Sensor User Manual