[pengyou]

I understand the theory behind the efficiency rating of a motor but is it a useful measurement? If one motor is 80% efficient and another motor is 90% efficient will the second motor go 12% faster than the first? Use 12% less electricity?...or is this a "paper" measurement, i.e. something that never makes any difference?

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[JeanCoutu]

It's useful to know eff if you're designing an electric bike piece by piece, since your parts have to be matched for the bike to do what you want. Beyond that point it's e-penis measurement.

[cerewa]

I think in practice, manufacturers' claims about efficiency are of very little value. They will leave out information that doesn't suit them (such as stating motor efficiency without accounting for the losses in a reduction gear [without which the motor wouldn't really do anything]) and give you ideal-conditions efficiency rather than efficiency at normal RPMs/ normal power levels.

If one motor were actually 10% more efficient than another, then you'd be able to get about 10% more range out of the same battery or use a battery 10% smaller to power it.* Which is really not that impressive ... when it comes down to it, a 10% increase or decrease in the cost and weight of my battery wouldn't matter a whole lot to me.

[pengyou]

Thanks! I am shopping for a folding bike now and want to put a motor on it. I am thinking about a 400 watt +- hooked up to a 3 speed gear hub to enable me to take hills more effectively. I am encounter hills a couple of months each year but when I do they are big ones.