Mid motor torque ratings

[raylo32]

While I understand that electric motors generate a lot of torque for their weight I wonder by what method and how accurately these mid motor kit and OEM bike mfgs rate these things. You see them rated 50NM, 85NM, even up to 150NM. But these numbers seem unreasonably high. Consider a motorcycle for comparison, for instance the Triumph Tiger 900's Engine Power is 93.9 bhp @ 8,750 rpm and the Engine Torque is 87 Nm @ 7,250 rpm. I have a Tiger 800 with similar spec and I don't feel anything like that kind of grunt from my e-bikes.

 

[amberwolf]

The rating by itself doesn't mean much.

Some possible additional info that would be required, or helpful, for a complete OEM bike:
What's the gearing to get that torque? Is it a peak rating, or one it can continuously produce all day long? Can it do that torque at stall, without system damage? Or only for a moment and if it doesn't get going does it shutdown, or just limit the current to the motor to what it can handle in this state without damage? Etc.

For a "bare motor", you'd have to know the rest of the system--for instance, how many phase amps does it take to generate that torque? (because if you don't duplicate the test conditions / equipment, you may not get the "rated" output).

 

[raylo32]

That's my question. I understand they are peak ratings. But they seem way too high. it would be good if the mfgs would agree on some sort of standard that we all could see and understand. As it is all these ratings may be good for is roughly comparing one to another, but not a gauge of actual performance. I say "may" because it isn't clear all mfgs rate them the same way.

 

[amberwolf]

They aren't even good for comparing to each other, because unless they state the test conditons/etc, you don't know what the comparison actually is.

There *are* standards for testing things, but these companies aren't using them, and aren't likely to. I wish they would...but I can't imagine it will ever happen. They won't even always use wiring standards that have existed in various types of systems for...decades.

They're going to do what everyone in marketing does that doesn't have to follow a set of laws / regulations in how they say what they say: They'll use the set of numbers that make their stuff sound better than anyone else's, even if one of the numbers in the set can't be used to compare with a different number in the set because each one is from a different set of test conditions.

Like saying you can go 150mph and 400 miles range on something...it's very unlikely you could go anywhere near 400 miles *at* 150mph. But marketing writes things in a way that implies you could....and that's what the buyers want to hear, so that's what marketers say, even if it's a lie.

 

[richj8990]

While I understand that electric motors generate a lot of torque for their weight I wonder by what method and how accurately these mid motor kit and OEM bike mfgs rate these things. You see them rated 50NM, 85NM, even up to 150NM. But these numbers seem unreasonably high. Consider a motorcycle for comparison, for instance the Triumph Tiger 900's Engine Power is 93.9 bhp @ 8,750 rpm and the Engine Torque is 87 Nm @ 7,250 rpm. I have a Tiger 800 with similar spec and I don't feel anything like that kind of grunt from my e-bikes.

If you look more in the 'fine' print for some of these motors, or complete e-bikes, you'll see more like 1 Nm, or 5 N, for the end product (the realistic torque). The calculated torque of for example 40 Nm peak is theoretical for gearing multipliers inside or outside the motor.

But to get to the real equation you need to convert Nm to foot pounds. You also need to know what the rpm motor is normally operating at and what the normal watt input is. Say in this example it's exactly 800 watts. That's one horsepower. And let's also say the motor is normally at 1500 rpm.

(Horsepower x RPM) / 5252 = ft lbs of torque

1.0 x 1500 / 5252 = 0.286 ft lbs

0.286 ft lbs x 1.32 = 0.377 nM (not very much)



Let's now look at peak power at say 6000 rpm and 1200 watts

1.5 HP x 6000 = 9000

9000 / 5252 = 1.71 ft lbs x 1.32 = 2.26 nM




This would be the real torque if there was no gearing at all. But to go from 2.26 to 40, you'd need some gearing inside or outside the motor that multiplies the initial torque production about 17.7x. The bottom line is that an electric bike with 40 Nm of torque still can't pull 500 lbs of weight. Horsepower or watts is still limiting how much it can pull. And if a bike has 1/3 of a horsepower, all the calculated torque in the world is not going to pull more than a few hundred lbs up a hill.

 

[DanGT86]

Torque is simply turning force. It doesn't tell you anything about the power output. Motorcycle manufacturers are listing the torque output at the motor crank. Ebike manufacturers are listing torque after gear reductions. If you listed torque output at a motorcycles front sprocket it would be way higher than the motor crankshaft. So the ebike manufacturers are not lying they are just providing an unhelpful spec other than to compare between models of the same brand rated the same way.

A good way to visualize this is to think about how easy it is to kill a motorcycle engine by letting the clutch out too fast. There is not enough torque there to overcome the load. When properly slipping the clutch the motor has time to get into its highest torque output RPM. A mid-drive ebike with the same torque would not stall and would easily just start the bike moving very slowly across the ground.

Years ago I was working on a toyota pickup and torqueing the crank pulley. The torque spec was high like 250ft/lbs. I had the truck parked in gear and the ebrake on. When turning the wrench the truck kept overpowering the ebrake and inching forward. So here is me a measly human able to input more torque than the motor which would not overpower the ebrake without stalling. I can produce more twisting force than that gas motor can but definitely not more HP which is why I can't push that truck down the road at 60mph.

Similar to my wrench analogy, a 200lb rider standing up on a 175mm bike crank arm with all their weight on one foot is producing at minium115 ft/lbs of torque just standing. It peaks way higher when they jump hard on it with their weight. This tells you nothing about the total power or how it would compare to a motorcycle with an engine rated at 115ft/lbs

I like to think of torque as a simple yes or no measurement of "can I turn this against this load in this particular instant?" Gas engines use lots of other methods like clutches and torque converters to stretch out the "instant" into a longer period of time to mask their relatively low torque.

I guess in theory if you could instantly clamp an 88Nm load on your triumph motor at 7250rpm you would kill the engine where you wouldn't stop whatever component is making 150Nm on the ebike despite the huge difference in power.

 

[Ebike rider]

If you look more in the 'fine' print for some of these motors, or complete e-bikes, you'll see more like 1 Nm, or 5 N, for the end product (the realistic torque). The calculated torque of for example 40 Nm peak is theoretical for gearing multipliers inside or outside the motor.

But to get to the real equation you need to convert Nm to foot pounds. You also need to know what the rpm motor is normally operating at and what the normal watt input is. Say in this example it's exactly 800 watts. That's one horsepower. And let's also say the motor is normally at 1500 rpm.

(Horsepower x RPM) / 5252 = ft lbs of torque

1.0 x 1500 / 5252 = 0.286 ft lbs

0.286 ft lbs x 1.32 = 0.377 nM (not very much)



Let's now look at peak power at say 6000 rpm and 1200 watts

1.5 HP x 6000 = 9000

9000 / 5252 = 1.71 ft lbs x 1.32 = 2.26 nM




This would be the real torque if there was no gearing at all. But to go from 2.26 to 40, you'd need some gearing inside or outside the motor that multiplies the initial torque production about 17.7x. The bottom line is that an electric bike with 40 Nm of torque still can't pull 500 lbs of weight. Horsepower or watts is still limiting how much it can pull. And if a bike has 1/3 of a horsepower, all the calculated torque in the world is not going to pull more than a few hundred lbs up a hill.

Thanks sir for your lesson.
Please
48v , 1000w , 3100rpm, torque 2.7Nm( as written but I do not know for sure ! ) . and I will connect it with outer gear to make the 3100rpm be 1200 wheel revolutions , or I can make it 750 wheel R.
Is that mean I will obtain an enough torque ( in general ) for mountain streets ?