bottom bracket compared to rear drop out torque sensors

[ADKdadro4]

I am curious if anyone has more information to compare and contrast the types of torque sensors used on some hub motor bikes. I have seen lots of comparisons for cadence versus torque, but not much on the different types of torque sensors.

It appears as though the rear drop-out ones also measure cadence in addition to torque and could therefore give the controller more data and could allow for a better experience. It does appear that they can measure cadence to a finer granularity because the rear wheel is rotating at faster rpm than the crank arm. Do the bottom bracket style sensors also measure cadence? is one more sensitive or accurate than the other? Is one style more reliable over the long term than the other?

I see Juiced and the 2020 version of some Surface604 bikes use the rear dropout style sensor( there are probably others too) and I have seen past surface604 models and the new Biktrix swift use the bottom bracket style sensor.

example pictures:

 

[Scott Adams]

Some manufacturers build the torque sensor into the motor.



The small, flat gold structure surrounded by white just to the right of the thumb is the strain gauge which measures the deformation of the axle of the hub motor which is the partly threaded cylinder running along the right side of the image.

*edit for clarity - in the photo I am peeling the strain gauge away from the axle, normally it is glued tightly to the axle*

In order to convert the torque measurement to "work" or energy being exerted by the rider, it is necessary to measure the RPM of the same part where the torque is being measured (work = torque x rpm).

So, the bottom bracket is measuring the RPM / cadence of the cranks, while the rear sensor is measuring the RPM of the rear wheel.

Like you, I'd be interested to hear the thoughts of various forum members on the trade-offs between these two types.

 

[Rossnewport]

I have a older Surface 604 Shred with a Bottom Bracket Sensor and a newer Shred with the drop out sensor. Both are very responsive although the drop out sensor has the edge. The true advantage is in maintenance. It is challenging to remove, clean or repair the bottom bracket sensor without damaging and needing to replace it. If moisture gets in the bottom bracket, it is prone to problems.