overall friction penalty: 80% less top speed?

If one has a 13" hub-motor whose non-loaded max RPM would (theoretically) take a moped/scooter to 100kph, what would a reasonable best-guess calculation of the actual speed of such a moped/scooter on level ground be? 80% less? 75% less?

Thanx for any insight in advanced.
M

Loaded speed is typically ~80% of unloaded speed. This is a function of back-EMF, and tire friction, air resistance, etc are secondary losses on top of that.

neptronix said:

Loaded speed is typically ~80% of unloaded speed. This is a function of back-EMF, and tire friction, air resistance, etc are secondary losses on top of that.

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There's only true if the available power supports such speed. In this case, that's very doubtful.

It is a fact that motor efficiency tends to peak at about 80% of the unloaded speed. So matching your motor's output at 80% of free speed, to your vehicle's power required to maintain the same speed, is the ideal case. That makes the thing accelerate relatively strongly up to its most efficient cruising speed, and stay there under normal conditions.

What I see with many cheap hub motor kits is a free speed that's so high, the motors' maximum output can barely be expected to exceed half of the free speed. This is probably a good thing for maximizing top speed, but at the cost of never allowing the bike to reach its most efficient RPM range.

Chalo said:

There's only true if the available power supports such speed. In this case, that's very doubtful.

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So help me out here, why is it "in this case, very doubtful" that a well-designed eScooter's pack wouldn't be able to support 80kph if the unloaded max is 100kph?

M

MJSfoto1956 said:

Chalo said:

There's only true if the available power supports such speed. In this case, that's very doubtful.

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So help me out here, why is it "in this case, very doubtful" that a well-designed eScooter's pack wouldn't be able to support 80kph if the unloaded max is 100kph?

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First of all, 80kph will take about 5kW to the wheel, which is more than 6kW from the battery. So to attain that speed, the motor must be able to make that much power at 80% of its unloaded speed.

If you're familiar with motor output charts, you'll know that power output at 80% of the motor's maximum unloaded speed is much less than the motor's peak power (which occurs around 50% of the free speed). So to maintain 80kph, you would need a 5kW capable motor with a free speed of 160kph, or else a substantially more powerful motor than that.

If your motor can produce 10kW or so, then it just might make the 5kW at 80% speed which you need to sustain 80kph.

Chalo said:

If your motor can produce 10kW or so, then it just might make the 5kW at 80% speed which you need to sustain 80kph.

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The plan is to install a QS 273 8kW V3.
I guess the real question is what specs should I tell them so that they provide the proper winding for 80kph actual top speed?

M

MJSfoto1956 said:

The plan is to install a QS 273 8kW V3.
I guess the real question is what specs should I tell them so that they provide the proper winding for 80kph actual top speed?

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Figure out what wheel RPM corresponds to 80kph:
https://www.4qd.co.uk/road-speed-calculator/

You know you're looking for 5kW power out of the motor at whatever RPM that is. If you can get charts for the motor you have in mind, you can find out for yourself whether that's plausible.

Or you can contact the motor supplier and say, "I'm trying to get 5kW continuous power at XXX rpm". And they can make a recommendation.