There are so many misconceptions regarding PAS effectiveness. Some system do actually provide the ability to get exercise or clown pedaling. This regarding torque sensing bottom brackets.
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Torque Sensors
Torque sensing bottom brackets measure the force that the rider applies on the pedals in addition to their pedaling cadence. This enables pedal assist modes where the motor power varies in proportion with the rider's input power, something present in most of the higher-end ebike systems (eg Bosch, Shimano STEPs, BionX..) At the moment, nearly all aftermarket torque sensors are designed for threaded bottom bracket shells and use JIS square taper cranksets. If you have a press fit bottom bracket shell these won't work. Similarly if you have a splined hollow spindle crankset then you'll need to replace your crankset with one meant for a square taper interface.
NCTE (Thun): The original stand alone ebike torque sensor was made for THUN by NCTE. This sensor measures the actual torque across the spindle. When you pedal with the left crank it registers a full torque signal, but when you are pushing with the right pedal the force goes straight to the chainring, it doesn't couple through the spindle. This has some unique advantages for certain mid-drive and tandem bike applications, but for most riders it means a delay in power assistance coming in if you start pedaling with the right leg. The delay is further compounded by the fact that these sensors only have the equivalent of 8 cadence pulses per revolution.
On the plus side, this style of sensor is the very good at showing an accurate report of the human power in watts over any gear range and even at extreme pedal force, as it is a true linear torque measurement right up to 200 Nm. It is also easy to install, being mechanically no different that a standard sealed cartridge BB. There is also an ISIS splined option from NCTE, although we have had mechanical problems with them.
ERider: These sensors have a separate mount for the chainring spider that is independant of the right crank, allowing for accurate sensing of both left and right pedal contributions. They measure human torque and watts with good accuracy and the high pulse count 18 pole cadence sensor allows for a rapid PAS power engagement at the start of pedaling. They also feature a cable exit on the side bearing cap so there is no need to drill a hole through the frame for running the cable passage. These sensors are available for both 68mm and 73mm bottom bracket shells and are available with both 4 bolt 104 BCD and 5 bolt 130 BCD chainring standards.
With these ERider torque sensors you cannot reuse the original cranks from your bike as the right crank has no chainring spider. We include a zero 'Q' factor Miranda cranks with each package that works very well and helps to offset the additional spindle length of this sensor design . You need to move the chainrings from your original crankset to the appropriate 4 or 5 bolt spider mount that came with the kit.
Sempu: The Sempu sensor also responds to both left and right pedal torques, but unlike the TDCM or NCTE sensor it makes no attempt to isolate forces that put power to the wheel from all the other forces on the crank. Instead, it measures the general flex of the spindle and assumes that for the most part this will vary in proportion to how hard you are pedaling. In practice, this works surprisingly well. It also has 24 pole cadence sensor, 3 times more than the Thun / NCTE and twice the TDCM. These details make it by far the most responsive sensor for engaging immediately when you start pedaling from a standstill.
The Sempu installs almost as easily as a regular cartridge bottom bracket
(see video here) and is available in non-standard bottom bracket sizes, with adapters for 73mm BB shells and also models for 84mm and 100mm wide bottom brackets used in Fatbikes. On the downside we have seen a higher than normal issue rate (approx 20%) with the Sempu sensors throughout most of 2019 with the torque signal periodically dropping out, and are holding off ordering more of these until Sempu can assure that their QC problems are fixed.
TDCM: The TDCM sensor doesn't measure the torque on the spindle, instead it uses a clever arrangement with a slotted bearing support to measure how much the right side spindle flexes backwards due to chain tension. This allows the sensor to respond equally well to left and right pedal forces as they both put tension on the chain, while ignoring the downwards force which would only be present from the right pedal. It includes a higher 12 pulse cadence sensor helps it respond quickly to small pedal rotations.
Because the operation of this sensor requires an alignment in the direction of the chain tension, it involves a more detailed installation process taking care to read and follow the instructions. The output signal also saturates at the equivalent of about 50 Nm of pedal torque, which limits it's accuracy as a human power meter. But for the sake of providing proportional PAS assistance, this sensor works very well.
Availability Update: As of 2018 the TDCM has gotten out of the torque sensor business and these bottom bracket sensors are no longer available.
Spindle Length
All the torque sensing bottom brackets are available in a range of spindle length options. Even though the size of bottom bracket shells on bike frames is largely standardized to 68mm or 73mm, the exact location where the chainrings will line up can vary significantly between cranksets. Bicycle bottom brackets are produced in many different spindle lengths to achieve a desired chainline.
Best invention ever!
