Torque: Measurement & Relevance to an ebikers' ability to enjoy the passing scenery

[jackhandy]

A question for those of a technical bent, not out enjoying a pastoral ride in this bootiful weather.....

I had a quick butchers at Torque on wiki If I follow it, Very basically,
Torque = Force applied x length of leverage (from the fulcrum) Have I got that right?
Hence it's measurement in Newton/Metres:

In ebike terms, how important a factor is Torque? Can it be used as a yardstick for deciding which system will provide most Grunt on hills?

My GnG 250w crank-drive kit is specified thus: Motor speed = 3,000rpm; gear reduction 9.78:1 ; Torque 0.8Nm .

The new Kudos crank-drive specs (on another thread): 60.0Nm @ 90rpm.

At first glance, the two sets of data don't have much in common but, on the GnG,
0.8Nm x 9.78 = 7.8Nm @ 307rpm at the drive sprocket? Does it work like this?

Obviously a meaningful comparison depends on knowing where in the drive train the measurement is taken.

I think I've just confused meself...

 

[flecc]

That's what matters, where the torque is measured. Gearing and motor torque are directly related, doubling the reduction ratio doubles the torque. I noticed one e-bike supplier quoting 120 Nm on a crank drive bike they sell, presumably in a low bottom gear and measured at the back wheel.

As with range, supplier figures are notoriously inaccurate and unspecified. One claimed their direct drive hub motor had been twice increased from it's original lowish 20 Nm torque, first to 40 Nm and finally to 60 Nm. In fact the true end figure was 30 Nm, half the claim!

 

[Deleted member 4366]

You got it just right.

Torque can be increased to anything you want by gearing. A 250w motor can pull you up vertically on a hoist. In fact it would be able pull up vertically every single forum member on their bikes at the same time, but it would be rather slow. How fast you can go up a hill depends on how fast you can apply the torque, which is power. The more power you have, the faster you can climb, so the amount of torque you have affects how steep you can climb and the amount of power you have affects how fast you climb. You could have all the power in the world, but if you don't have enough torque to drag you up a hill, you'll slide backwards down it. Power isn't affected by gearing, except that if you get your gearing wrong a motor might not be able to spin in its most powerful zone.
To summarise: for a bike to go up a hill nicely, it needs enough torque, enough power and gearing that gets the best out of it.

 

[Kudoscycles]

The supplier of my Kudos Ibex crank drive bikes claims 135 Nm of torque at max PAS setting,at the back wheel in lowest gear. I have,to be honest,and say that I have no means of checking that.
Dave
Kudoscycles

 

[Deleted member 4366]

The supplier of my Kudos Ibex crank drive bikes claims 135 Nm of torque at max PAS setting,at the back wheel in lowest gear. I have,to be honest,and say that I have no means of checking that.
Dave
Kudoscycles

A motor with output power of 250w producing 135NM torque at the back wheel will travel at a speed of 1.368mph.

 

[jackhandy]

I guess, at some point, without a "Testing House" of some kind or at least an accepted & adhered to methodology; We have to accept whatever figures are quoted by manufacturers.

It would be nice if they all measured in like manner: Perhaps at the back wheel, quoting the wheel nominal diameter; front & rear sprocket tooth count e.g. 26"/44t/28t.

Is it a bird? Is it a plane? NO - It's a flying Pig

 

[Kudoscycles]

Dave,that would stack up with actual performance. In the lowest gear,the bike climbs hills with little effort but is very slow-my leg power is still quite strong so I tend to go up a couple of cogs to achieve a slower cadence and more road speed.
Dave
Kudoscycles

 

[Deleted member 4366]

At that speed it should be able to clinb a 40% hill, which would be more than you'd encounter on any road. I think you'd flip on that hill with that much torque though.

 

[Jeremy]

A motor with output power of 250w producing 135NM torque at the back wheel will travel at a speed of 1.368mph.

??? A bit of basic physics can determine rpm from torque and power, but not speed, I'm afraid. In this case 250W and 135N-m gives 17.68rpm. To get speed you need to know the wheel rolling diameter, which hasn't been stated, AFAICS, so without this you cannot determine speed.

For example, for my bike would be doing 1.052mph for a wheel torque of 135N-m and a power of 250W. My wife's bike would be doing 1.447 mph with the same torque and motor power.

 

[Deleted member 4366]

??? A bit of basic physics can determine rpm from torque and power, but not speed, I'm afraid. In this case 250W and 135N-m gives 17.68rpm. To get speed you need to know the wheel rolling diameter, which hasn't been stated, AFAICS, so without this you cannot determine speed.

For example, for my bike would be doing 1.052mph for a wheel torque of 135N-m and a power of 250W. My wife's bike would be doing 1.447 mph with the same torque and motor power.

It's a 26" wheel.

 

[hoppy]

None of us knows the real power output of that motor at that torque,do we?One thing we can be pretty sure of,it's not 250 watts!My guess would be more like 700 watts.

 

[jackhandy]

A motor with output power of 250w producing 135NM torque at the back wheel will travel at a speed of 1.368mph.

Perhaps we should introduce a new factor into the usually quoted range figures, then....

Elapsed Time

 

[Jeremy]

It's a 26" wheel.

That makes all the difference then! Without knowing that originally the numbers were baseless and potentially misleading, hence my observation. It always helps to have all the data..................

As already pointed out in this thread, though, the true power will be greater than 250W by a fair margin, I suspect, given the latitude allowed in the power assessment test procedure, so in practice the speed could be around a slow walking pace at that torque.

 

[OldBob1]

So my 20" rear wheel with hub motor should give me better torque then a larger wheel?

 

[Deleted member 4366]

So my 20" rear wheel with hub motor should give me better torque then a larger wheel?

Yes, but only with the same motor. The motor manufacurers can infuence the torque by having fast and slow windings in the motor and, in some cases, different internal gearing.

 

[Jeremy]

So my 20" rear wheel with hub motor should give me better torque then a larger wheel?

No, the torque would be exactly the same, as that is a function of the motor and drive train. What would change is the force available to move the bike forwards.

I find it easier to forget about torque, as it's only an intermediate parameter that doesn't relate directly to the way the bike performs. It's the force available to move the bike at any particular speed that is important as it is this that you feel directly as assistance.

Changing the wheel size changes the overall gearing from the motor to the road, so changes both the speed and the available force. Reducing the wheel size is exactly the same as increasing the size of the rear sprocket, or decreasing the size of the front sprocket, it reduces the overall gearing. In the case of a hub motor, it also increases the reduction ratio between the motor and the ground, so increasing the propulsive force available for a given motor torque and reducing the maximum speed for a given motor voltage.

 

[flecc]

So my 20" rear wheel with hub motor should give me better torque then a larger wheel?

It will climb and accelerate 1.3 times better than the equivalent 26" wheel e-bike with the same motor etc.