Torque - explanation please

[allen-uk]

Torque - can someone give me a brief clue on torque.

If a certain battery/motor/ebike configuration gives, say, 45Nm, is that by definition MUCH less powerful than an ebike giving, say, 80Nm, given the same heavy rider?

Speed isn't important in this equation, but raw power (hill climbing power) is important.

Is there such a thing as an ebike of 45Nm being AS powerful as an ebike of 80Nm, or is that a daft question.

Thanks for the education.


A

 

[Deleted member 4366]

That's a really good question with a not so good answer. It depends where you measure the torque. I think most crank motors talk about crank torque, but even that can't be right because Bosch went from a big chainwheel that went at low speed to a small one, which goes four times faster, so torque should be 1/2 of what it was before, but they gave an even higher figure.

The only torque that is meaningful is that which turns the back wheel. The higher the torque, the steeper the hill you can climb. All motors make the most torque at the lowest speed, i.e, when stationary, which doesn't help that much. What would be even more meaningful would be the torque at the back wheel at say 6 mph, which would determine how steep you could climb or whether you will accelerate to a higher speed.

In summary, you can more or less compare torque between hub-motors, but for crank motors, it's pretty meaningless, especially because you can alw2ays lower the gearing to get even more torque at the back wheel, where you're trading speed for torque.

 

[RobF]

Lots of definitions online, which may serve to confuse rather than inform.

Generally speaking, torque is twisting force which equates in an engine/transmission set up to pulling power.

Practically for ebikes, quoted torque figures cannot be reliably compared because there is no standard for measurement.

The only way to be certain which bike pulls better is to ride them side by side in the same conditions on the same hill.

 

[Will]

Torque is force x distance. If I have a 1 foot spanner, and I put 10 lbs of force on it to undo a rusty nut on a long thread , I've applied 10 FOOT POUNDS of torque.

POWER is torque times speed. So if it takes me 1 minute to undo the nut, and someone else does the same job in 30 seconds, using the same amount of torque, but at a higher moving speed, they have more power.

So power is a better measurement for bikes, and it should also be the same more or less whether measured at the crank or at the wheel because as either the torque or speed increases via gears, the other decreases making the power work out the same.

Its actually a similar thing when you think about it to the battery. Watt Hours is a good way to compare batts. Amps is your torque, (Amp hours is torque over a period of time) Volts is your speed of your electricity, together you know the power so you know how much work it will do. (10Ah x 48v on my bike is 480 Wh)