Noob Qs Pedal sensor and wattage /efficiency question.

[Will]

Hi I'm looking at installing one of the e bike kits , first question is, assuming factory built pedelecs have a proportional assistance to pedal pressure, they must have some kind of torque sensor. I can see how these would be built into central (chain drive) kits but how are they done on front or rear hub kits, assuming they have them. I guess its technically possible to have a sensor in the rear hub motor but I can't see how it is sensed on a front motor.

2nd question, legallities aside, I see there are bigger than 250W motors available (750 etc) when used at low power (using mostly pedal power) are these less efficient than a smaller motor that is working harder?

Thanks all! Will.

 

[Deleted member 4366]

Good questions. Most cheaper kits use a pedal rotation sensor to detect pedalling. You adjust how much assistance you get via a LCD panel. For most people, that system is satisfactory. If you really want a torque sensor, you can get one included in a replacement bottom bracket. You need a matching controller to go with them or you can use one with a Cycle Analyst, which converts its output to a normal throttle signal. Some controllers can do "torque simulation" with a pedal speed sensor, which is somewhere between the two systems. Xofo, Falco, Bionx and others do kits where the torque sensor is in the motor. Soon to be released are crank-drive kits from Sunstar, Bafang and others with a torque sensor in the crank somewhere.

In my experience, larger motors are more efficient than smaller ones provided that they're able to work in their normal operating range. Big direct drive motors are better for constant high speed, while as geared motors are better for stop/start and variable speeds. The tests that I've done show that 500w geared motors are more efficient than crank motors.

I'm talking about the electrical drive system efficiency. We just had a guy on Endless Sphere claiming 3wh/mile. Anybody can get very low electrical power consumption by pedalling to provide a higher proportion of energy compared with the motor. If you have a torque control system, you have to pedal harder compared with a speed control system, so you provide a higher proportion of energy, which can distort perceptions about efficiency.

 

[Will]

Thankyou.

 

[trex]

just a quick note about Chinese CDs: the BBS0x have a very well designed controller, the Woosh CDs use run of the mill Lishui - the difference is very clear in the bike's response. The Sunstar is a premium product - the idea of varying power depending on the angle of the cranks looks promising.