What does Nm really measure and is it applied the same by ebike manufacturers.

[scrambler]

A pro rider puts out 80 Nm and that is about the most a drivetrain can take, so how much to dislodge your elbow if he twisted it using his legs?

Small precision

A 76 kg rider (169Lb) standing on one pedal with a 160mm crank applies 750N on that pedal, creating a torque of 120Nm (750 x 0.16)
And that is just putting all its weight on, if he actually pushes back like someone jumping off the ground, he can probably apply more force on that crank.

The big difference is he can only do that for a fraction of a second...
So 80Nm may be a good number for sustained

 

[jharrell]

Yeah it has to be at the crank spindle, to many variables in play after that.
Yet this Emotorcycle motor is only rated at 116nm, yet it will outperform the ultra a million times at every level.
105HP @ 116nm

And this has been discussed before on EBR, but logically it doesn't make sense.
Somebody is going to come in and explain to us I am sure, as they have before but I forget how and why

Simply put torque isn't power, and focusing on torque is the wrong thing to do.

Power (kW) = Torque (Nm) x Speed (RPM) / 9.5488

Torque is one component of power, the other is rpm, one without the other is not very useful. Torque can be multiplied by gears and wheels, power cannot. Gears trade torque for rpm and vice versa.

It is good to measure power at the wheels because it will always be lower than at the motor due to drivetrain losses (inertial and frictional), quoting torque at the wheel is meaningless due to the torque multiplication of the drivetrain, you can achieve any number you want at the wheels by changing gearing or wheel size.

Nm isn't really a great way to compare motors, a lower power motor may have higher torque than a higher power motor due to motor design(turns) and internal gear ratios, however power input X motor efficiency = power output regardless you can't get more power out than put in.

 

[Ebiker33]

Simply put torque isn't power, and focusing on torque is the wrong thing to do.

Power (kW) = Torque (Nm) x Speed (RPM) / 9.5488

Torque is one component of power, the other is rpm, one without the other is not very useful. Torque can be multiplied by gears and wheels, power cannot. Gears trade torque for rpm and vice versa.

It is good to measure power at the wheels because it will always be lower than at the motor due to drivetrain losses (inertial and frictional), quoting torque at the wheel is meaningless due to the torque multiplication of the drivetrain, you can achieve any number you want at the wheels by changing gearing or wheel size.

Nm isn't really a great way to compare motors, a lower power motor may have higher torque than a higher power motor due to motor design(turns) and internal gear ratios, however power input X motor efficiency = power output regardless you can't get more power out than put in.

That makes a lot of sense, in the power tool world as you lower rpm on a drills you increase power and gross torque, so for example if you have the same company making drills and one is 7amps 120V and 2500rpm it will be much weaker in power and torque than the same drill that is gear reduced down to 450 rpm with the same amps and voltage. Mudmixers are a good example of this, my customers love this guy because if you need to you can dial it down to 300 rpm to do really heavy jobs and it's cordless, yet it gets the job done.
Cordless mud mixer

It seems your would need to see what RPM the motor is also at when turning the spindle crank arm running when pinned at full throttle. We do have a rating from Bafang I will post that below.
Something very interesting I noticed is that they increase the RPM slightly for the 52V version. Before I thought by going to 52V you sneak a little more power out of the system, the power in the VXA formula is true, more watts is being pushed into the motor, but it may be negated by the higher rpm.
Ultra Motor

 

[jharrell]

That makes a lot of sense, in the power tool world as you lower rpm on a drills you increase power and gross torque, so for example if you have the same company making drills and one is 7amps 120V and 2500rpm it will be much weaker in power and torque than the same drill that is gear reduced down to 450 rpm with the same amps and voltage. Mudmixers are a good example of this, my customers love this guy because if you need to you can dial it down to 300 rpm to do really heavy jobs and it's cordless, yet it gets the job done.

Be careful again torque is not power, gearing the drill down does not increase its power it increases its torque. 7 amp x 120v = 840w of power, the motor turns that power into torque and rpm along with gears. The speed control of the motor(variable speed trigger) adjust its power while a 2 speed gearbox will keep the same power but trade rpm for torque.

Before I thought by going to 52V you sneak a little more power out of the system, the power in the VXA formula is true, more watts is being pushed into the motor, but it may be negated by the higher rpm.
Ultra Motor

Voltage will determine max rpm based on the windings of the motor this is the Kv rating of the motor, so with the same windings a motor will have higher rpm at 52v vs 48v if the windings are changed you can get lower rpm but more torque, this is similar to gearing except on the electrical side. If you stick with the same windings but want more torque vs top speed with the increase in voltage then you can simply use lower gears, if you want both higher speed and more torque then get a transmission with a wider range of gear ratios.

 

[PedalUma]

Be careful again torque is not power, gearing the drill down does not increase its power it increases its torque. 7 amp x 120v = 840w of power, the motor turns that power into torque and rpm along with gears. The speed control of the motor(variable speed trigger) adjust its power while a 2 speed gearbox will keep the same power but trade rpm for torque.



Voltage will determine max rpm based on the windings of the motor this is the Kv rating of the motor, so with the same windings a motor will have higher rpm at 52v vs 48v if the windings are changed you can get lower rpm but more torque, this is similar to gearing except on the electrical side. If you stick with the same windings but want more torque vs top speed with the increase in voltage then you can simply use lower gears, if you want both higher speed and more torque then get a transmission with a wider range of gear ratios.

Two bikes may have similar specifications on paper, yet feel very different in practice. Alchemy is scoffed at. Yet, a certain amount of alchemy is at play in how a musical instrument feels, sounds, and plays. I would say the same goes with bikes. And this goes beyond the domain of the geek and into the realm of intuition and art, spirit and expression. One of the interesting things about alchemy is that new materials were created from desperate materials, from black powder, to blue glass, to the coatings on violins made from bee's wings that have lasted 500 years. The most important part about making a good electric bike is to get out of the way and let it express itself. I would say that most people cannot wrap their heads around this because it is not grasping, it is letting go that makes the best bikes.

 

[jharrell]

Two bikes may have similar specifications on paper, yet feel very different in practice. Alchemy is scoffed at. Yet, a certain amount of alchemy is at play in how a musical instrument feels, sounds, and plays. I would say the same goes with bikes. And this goes beyond the domain of the geek and into the realm of intuition and art, spirit and expression. One of the interesting things about alchemy is that new materials were created from desperate materials, from black powder, to blue glass, to the coatings on violins made from bee's wings that have lasted 500 years. The most important part about making a good electric bike is to get out of the way and let it express itself. I would say that most people cannot wrap their heads around this because it is not grasping, it is letting go that makes the best bikes.

Personally not one for imprecise artful language, I agree there are many subjective things about a bike that can make it feel better to one person vs another, however there is no alchemy involved there is applied science (geometry, physics, chemistry) to make this happen. The color, the shape, the material, the acceleration, top speed, the weight, the feel all a result of the technology developed through science, no medieval magic involved. The more you understand the science the more you can pick out a good product made by others who understand the science as well. Of course nothing beats touching it and using it in person, which unfortunately is hard to do with a Watt Wagon currently so one must rely on specifications and other media including attestations of other that have touched them.

 

[PedalUma]

@jharrell, I do both and was merely stating the unstated. I am all for science. I have enjoyed the informative discussion here. Numbers are fine and can only can only go so far. I will continue to use digital calipers and make calculations. The left brain would like to codify ballet. Ballet stats by Rain Man. It wants to grasp a butterfly, pin it, and tag it. But the best stuff happens when we create something new that is beyond its inanimate constituents, then animating it whole, setting it free. That is the "magic" that the left brain is tone deaf and color blind to but thinks it can control with categorization, and numbers. And that is where the art of making great electric bikes lies. Looking at a data table of an electric guitar is not the same as creating a superior one or playing a superior one in a soaring solo. I freely admit, I'm in the minority in thinking of electric bikes as akin to electric guitars. I ride and repair formulaic eBikes every week and they help inform the flowing creative juices of the Alchemic creative process that outlies the stats.

 

[EMGX]

A pro rider puts out 80 Nm and that is about the most a drivetrain can take, so how much to dislodge your elbow if he twisted it using his legs?
With 20V, "The EARTHQUAKE XT® Cordless Xtreme Torque Impact Wrench pounds out 1200 ft. lbs. of bolt breakaway torque to power off lug nuts and head bolts with ease. Combined with the EARTHQUAKE XT® Lithium battery system this impact wrench delivers high power, long run times and fast charging."
You are supposed to hold this thing in your hands. If true, that would shake and twist the meat off your bones.
From Harbor Freight: 1,200ft-lb= 1,626.982Nm

I don't care about the physics of advertised torque ratings, only if they are uniformly measured across different motors and brands - as much as possible anyway. And if one motor is rated at a much higher torque rating than another then I would expect a noticeably higher hill climbing assist performance. Maybe not exactly linear but close.
Same with the marketing hype of "breakaway" torque for torque impact wrenches.
BTW I just finished removing the blades from my mower with a Harbor Freight impact wrench, worked like a charm in a few seconds to rattle off the large bolts that I couldn't budge with a breaker bar.

 

[Ebiker33]

Be careful again torque is not power, gearing the drill down does not increase its power it increases its torque. 7 amp x 120v = 840w of power, the motor turns that power into torque and rpm along with gears. The speed control of the motor(variable speed trigger) adjust its power while a 2 speed gearbox will keep the same power but trade rpm for torque.



Voltage will determine max rpm based on the windings of the motor this is the Kv rating of the motor, so with the same windings a motor will have higher rpm at 52v vs 48v if the windings are changed you can get lower rpm but more torque, this is similar to gearing except on the electrical side. If you stick with the same windings but want more torque vs top speed with the increase in voltage then you can simply use lower gears, if you want both higher speed and more torque then get a transmission with a wider range of gear ratios.

True but in layman's terms they are used interchangeably, a drill press with it's pully system is another example, it can have 12 speeds (180 - 4,200 rpm) with 12.5 Amps. the torque required for doing a 1 inch hole in ferrous metal is much greater than doing a 1/16" hole in aluminum, and you adjust the rpm accordingly, some will translate that by saying "power" but the amps in stays the same if it fully draws on the full watts depends on how advanced the machine is, there are new drill presses that have done away with the pully system all together they have Digital Variable Reluctance (DVR) Direct Drive Technology with Adaptive Control (TM) Software. It's pretty cool actually, here is one DVR NOVA Drill Press

 

[Johnny]

I don't care about the physics of advertised torque ratings, only if they are uniformly measured across different motors and brands - as much as possible anyway. And if one motor is rated at a much higher torque rating than another then I would expect a noticeably higher hill climbing assist performance.

This is the problem, peak torque number at the crank does not say anything about climbing ability.

Take two mid drive motors, one which achieves peak torque of 200nm at 10 rpm but peak power is limited to 280W the other peaks at 50nm but can achieve that until 80rpm with a peak power of 420W. The second motor is a much better climber as long as you can keep your cadence around 80rpm.

Given how electric motors work a higher torque rating like going from 75 to 85nm while keeping peak power constant can only be achieved at lower cadences else the motor hits the power ceiling. For mountain biking this maybe of some help since one can not keep a stable cadence on the technical sections, for regular riders it doesn't mean much.

Companies use these numbers to deceive uninformed crowd of buyers and brand fanatics constantly brag about these meaningless numbers.

In the end only one thing matters and that is the amount of power you can transfer to the wheels.

 

[jharrell]

True but in layman's terms they are used interchangeably, a drill press with it's pully system is another example, it can have 12 speeds (180 - 4,200 rpm) with 12.5 Amps. the torque required for doing a 1 inch hole in ferrous metal is much greater than doing a 1/16" hole in aluminum, and you adjust the rpm accordingly, some will translate that by saying "power" but the amps in stays the same if it fully draws on the full watts depends on how advanced the machine is, there are new drill presses that have done away with the pully system all together they have Digital Variable Reluctance (DVR) Direct Drive Technology with Adaptive Control (TM) Software. It's pretty cool actually, here is one DVR NOVA Drill Press

They should not be used interchangeably they mean different things, they are related but very different. Laymans using torque for power is the source of much confusion and wasted time. Do you use distance (miles) interchangeably with speed (miles per hour)? One is a component of the other they are not the same.

Pulleys are the same as gears they trade torque for rpm, lower the drill speed by changing the pulley is increasing its max torque output at the bit. A variable speed drive controls rpm by limiting power through an inverter by controlling voltage and frequency rather than a normal a/c motor that will pull full power in order to achieve its max designed rpm so it must be pulleyed down to your desired rpm.

 

[PedalUma]

This is the problem, peak torque number at the crank does not say anything about climbing ability.

Take two mid drive motors, one which achieves peak torque of 200nm at 10 rpm but peak power is limited to 280W the other peaks at 50nm but can achieve that until 80rpm with a peak power of 420W. The second motor is a much better climber as long as you can keep your cadence around 80rpm.

Given how electric motors work a higher torque rating like going from 75 to 85nm while keeping peak power constant can only be achieved at lower cadences else the motor hits the power ceiling. For mountain biking this maybe of some help since one can not keep a stable cadence on the technical sections, for regular riders it doesn't mean much.

Companies use these numbers to deceive uninformed crowd of buyers and brand fanatics constantly brag about these meaningless numbers.

In the end only one thing matters and that is the amount of power you can transfer to the wheels.

This is why it is very easy for prospective customers to get obsessed with numbers and data sheets. They start thinking that more is better. Often less is more. Ride the bike and see how it feels! It does not matter what the data sheet says about a perfume. Hey, does anyone remember VCRs? They would advertise meaningless information such as Menus in 17 Languages. But will you ever use that?
I just saved a 2012 bike from the landfill today. It does 27.5Mph at 80Rpm on the test to the Nexus-8 and feels torquie at low speeds and technical situations. Thru-frame wires. 34.3 pounds.