More talk about torque

E-bikes have an electric motor. The regulation is measured in power (Watts) but its performance, particularly for mountain bikes, depends on torque (Nm). Making sense of this is not easy. Everybody gets confused. Some understanding will help with your e-bike shopping.

The simple answer is that for flat country torque is not so important but the steeper it gets the more torque counts. At maximum torque on a hill, the bike will stall, and from that point, you will be pushing. That’s why cargo bikes and mountain bikes are sold with motors with the highest torque.

The manufacturer websites are not that helpful in providing detailed information about the motors. Bosch is the best for explaining this sort of thing. The new Active Line motors are also very quiet.

The question regarding the relationship between power and torque often comes up in forums. Here is one.

Photo by Yogendra Singh on Pexels.com

Question

In the UK, e-bikes are currently limited to a maximum of 250W (ed. true for the ACT, too) assistance if you wish to use them on public roads. However, it seems that the motors do vary in terms of the available torque – from around 40Nm on some of the lighter e-bikes to 80Nm on those that are described as more “powerful”.

Answer

I will explain:

  • The maximum wattage isn’t really a technical limitation but a legal limitation.
  • Customers want maximum power that fits the (legal) bill.
  • So, we’re in the very case of “customer getting more than what’s on the paper” because “whoa, that drivetrain is more powerful than that other one” even if, on paper, they’re both 250W.
  • There is no standardised method to determine the 250W legal limit.
  • Sooo…, most, if not all, manufacturers provide “continuous max power: 250W” that may be much, much lower than the “peak power” (which could easily be 36v*20A=720W).
  • While the torque (specified) on paper is, indeed, max torque. (50-80NM in most cases).

So, with that said, what now?

  • You’re on a hill, at max 50NM torque, with the engine running at 100RPM = 250W.
  • Same hill, 50NM torque but at 200RPM = 500W.
  • Ergo: more watt, you go faster at same torque.

Source: Understanding e-bike power/torque, Bicycles Stack Exchange, accessed 20/8/2020

Photo by Simon Migaj on Pexels.com

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