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A major ranking criterion for electric bikes is the torque rating of the motor. Generally speaking, bikes/motors with higher torques are sold at a higher price than bikes/motors with lower torques.

As a follow-up of this thread, I get that having a bigger torque helps in torque intensive situations, like hill climbing.

However torque can easily be multiplied/demultiplied using gears between the wheel axis and the motor axis. For example, these gears can be dedicated (embedded in the motor itself) or in the transmission system of the bike.
So the torque that matters is the final torque on the wheel axis, not the torque output on the motor axis.

So, this implies two questions:

  1. Use of dedicated gears to change output torque: Why is torque rating sold as a discriminatory constant of the bike (especially for the price), where it's easy to just add a gear on the motor itself to increase the torque, at the cost of increased RPMs of the motor axis (as it's done on electric cars)?
  2. Effect of the transmission system on the motor torque: Since motors can be hub motors or mid-drive motors, the motor is not at the same transmission step: meaning the same motor output torque will not have the same impact on the wheel torque.
    For example, a hub motor with a torque of 50 Nm will provide a final torque of 50 Nm to the wheel. However, a mid-drive motor with a torque of 50 Nm, with a gear ratio of 0.5, will provide a final torque of 25 Nm to the wheels.
    With this in mind, how to interpret the advertised torque for a bike with a mid-drive hub, since it's heavily dependant on the transmission?
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  • 2
    Because Torque rating is a single number that can be easily compared. Like speakers with "PMPO" wattage ratings, and computer CPUs in Megahertz or Gigahertz, the easiest comparison is that "more is better" Why buy a bike with 25 torkies when that one has 50.
    – Criggie
    Apr 22 at 11:13
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    1. "it's easy to just add a gear on the motor itself to increase the torque, at the cost of increased RPMs of the motor axis " from the engineering point of view / costs-profit-benefit it may be that it is easier to increase the torue rather than having additional cooling/wearing out due to increased RPM. And efficency (energy used over torque provided) varies with RPM of the motor, so increasing RPM may not be ideal in an absolute sense, too. 2. mid-drive torque are provided at the crank, transmission will not affect that measure, transmissions are the "standard" bicycle gears.
    – EarlGrey
    Apr 22 at 12:37
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    For electric motors, torque at the output shaft is constant over all rpm for which it is designed, so it sets the base for motor performance. Power varies with rpm but torque stays the same -- you reach max power at max rpm. More importantly in a bicycle application where speed and power are often constrained by legal rather than engineering limitations, torque will determine the acceleration possible.
    – R. Chung
    Apr 22 at 15:19
  • They make bicycles with dynamic field induction drives now? :)
    – Affe
    Apr 22 at 18:30
  • With a hub motor, the wheel axis and motor axis are the same, so the answer is self-evident there. With a mid-drive, the torque has to go through the same transmission as the rider’s pedaling. I believe current mid-drive motors are indeed geared.
    – MaplePanda
    Apr 23 at 4:38
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Addressing just the actual question as stated to avoid a 3,000 word essay:

With this in mind, how to interpret the advertised torque for a bike with a mid-drive hub, since it's heavily dependant on the transmission?

The advertised torque on a mid-drive bike will tell you over how large a band of cadences and how steep a hill the bike can actually provide the advertised assist level in watts. (generally 250W equivalent mechanical rider input at the crank.)

A low torque bosch active line motor might only provide 250W of equivalent mechanical input assistance at exactly 70rpm on flat ground, whereas a Performance CX can provide that assist level at much lower cadences and on much steeper inclines. The manufacturers try to reduce this to a simple number because it's consumer marketing. (and also the drive system manufacturers don't sell you a final assembled bike, so they don't know what final numbers will be at the drive wheel in the installed system.)

The marketing is targeted to consumers buying finished bicycles made out of mass market bicycle parts. In reality the transmission is always going to go from a bit over 1:1 to around 36:11 to 38:11 and the drive wheel will always be about 26" to 29" outer diameter, these things aren't just infinitely variable to get whatever outcome you want.

Certainly if it were all made out of infinitely ideal "high school physics" components you could use any combination of motors and gearings to get any torque number you like, but these things have to be made out of real materials that don't strip teeth or snap in half or overheat when spinning at the speeds that people's legs actually turn, so there are a lot of engineering compromises to be made.

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  • My interpersonal skills are twisted and withered from too many long days fixing utterly broken Yuba Spicy Curries, so I'm compelled to point out that there are a number of popular 20" mid-drive cargo and folding bikes, such as the Yuba Spicy Curry. Also Bosch chainrings can be teenier than one might assume. Apr 23 at 5:08
  • Haha, fair enough re: 20" wheels :) Those old "teenie" Bosch chain rings have a 2.5x gear between the cranks and the chain ring so from the 'human' perspective it's a 37.5 or 40, but you're right the drive system's output shaft sees a different ratio. I still did my best to address what the number means to the buyer without writing a book to cover every possible case!
    – Affe
    Apr 23 at 19:29
  • (Also with those Bosch systems you literally only have the options 15 or 16 teeth unless you machine your own replacements, so it does in a way reinforce the point of the paragraph that the circumstances of the overall drive system that the "torque number" is being considered in the context of are actually pretty constrained ;) )
    – Affe
    Apr 23 at 19:45

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