Motor Torque Specs Minefield!

[Sam Pinion]

Hello all,

I've been doing a little research into building my own mid drive bike, looking at different motors etc also looking to build a 3/4 speed gearbox to deal with a higher power motor. I have noticed that allot of manufacturers are quoting torque figures for their bikes that seem pretty far fetched, and maybe even misleading to the consumer.

For example the Specialized Turbo Levo FSR 36v 250w motor supposedly cranks out 90nm of torque, yet the Kranked Ego kit only musters 75nm with 2400kw peak from its motor(whats that like 2000kw cont.?). I also read that the Bafang 8fun 500w mid drive kit is rated at 160nm?! I have found no chinese motor anywhere online that can perform anywhere near that, and lets face it most of the stuff on the market is made in China. The one I've settled on puts out 6kw peak 3kw nominal, with 25nm of torque at peak and 10nm continuous, now that sounds pretty weak I know but that's more than the peak torque at the flywheel of my 250cc motorcycle. So the fact that Specialized is quoting 90nm from 250w motor starts sounding pretty ludacris right?

Can anyone shead some light on this?

 

[Ravi Kempaiah]

I have been amused by the exact thing you mentioned. I tried to discuss it here.

http://electricbikereview.com/community/threads/torque-rating-on-e-bikes-marketing-gimmick.2467/

 

[i-Zapp]

Most of the electric motors that these ebikes have are in the 350-700W range, and at least for the mid-drive motors they run up to 3000-4000 rpm, AT THE MOTOR. Using the power formula P=speed*torque, or in engineering units: W=rpm*Nm/9.554 it's pretty easy to see that these MOTORS produce torque in the single digits, like <10 Nm.

Further, you have to consider that DC motors with a typical linear torque curve have a peak power at half of max speed due to the parabolic shape of the power curve, so you need to realize that some people might quote "max torque" (at 0 rpm) and others might quote torque at peak power (half of peak torque).

But more to the point of your original question, if you add a gearbox to the output of the motor then since the power output of the assembly hasn't really changed, speed will go down, and torque will go up by virtue of the power equation. So for a mid-drive motor/drive combo that produces 500W, it's quite conceivable that even though the motor itself only puts out say 10 Nm, the drive will produce 50 Nm if the gearbox ratio is 5:1, which is realistically the only way to get any sort of power out of a mid-drive. Otherwise, the motor is stuck spinning too slow and not producing any POWER, which is what accelerates you, climbs hills, and achieves top speed. Not torque.

 

[Sam Pinion]

Most of the electric motors that these ebikes have are in the 350-700W range, and at least for the mid-drive motors they run up to 3000-4000 rpm, AT THE MOTOR. Using the power formula P=speed*torque, or in engineering units: W=rpm*Nm/9.554 it's pretty easy to see that these MOTORS produce torque in the single digits, like <10 Nm...

Such a good explanation thank you. I can see why Bagang quotes 160nm for it's mid drive now, I'm pretty sure the gear reduction on those kits is something like 13:1. It just seems like the main manufacturers should come to some sort of agreement that they quote the torque at the wheel, at peak torque (0 rpm), peak power, whatever. There just needs to be some continuity to the whole thing or else it just becomes a meaningless statistic.

 

[rick-n-ns]

Which mid drive ? I just got the 36v 500w, which was quoted as 80 nm, and at the crank, I'd say is a fairly accurate number.
I have to "stand on my pedals" quite hard (I know that's not a scientific measurement, but I weigh 160 pounds, bear with me) to get up the hill right outside my door. This BBS02 (36v 500w) with 80nm, or about 58 foot pounds takes me up no problem without turning the pedals. And yes I know the crank arms are about 7 inches long, so to equal 58 ft/lbs I'd be putting about 100 pounds on the pedals, which is about right. Not to mention the motor can probably do it a little faster than I prefer to.

So like I say not a real scientific comparison, but definitely in the ball park of what is claimed.

 

[Sam Pinion]

Which mid drive ? I just got the 36v 500w, which was quoted as 80 nm, and at the crank, I'd say is a fairly accurate number.
I have to "stand on my pedals" quite hard (I know that's not a scientific measurement, but I weigh 160 pounds, bear with me) to get up the hill right outside my door. This BBS02 (36v 500w) with 80nm, or about 58 foot pounds takes me up no problem without turning the pedals. And yes I know the crank arms are about 7 inches long, so to equal 58 ft/lbs I'd be putting about 100 pounds on the pedals, which is about right. Not to mention the motor can probably do it a little faster than I prefer to.

So like I say not a real scientific comparison, but definitely in the ball park of what is claimed.

Pretty sure it was the 1000w motor that they quoted 160nm for, which would make sense because that's double what the the 500w makes! Good to know a bit more about the 500w bbs02, was considering a kit like that for my road bike.

 

[rick-n-ns]

Pretty sure it was the 1000w motor that they quoted 160nm for, which would make sense because that's double what the the 500w makes! Good to know a bit more about the 500w bbs02, was considering a kit like that for my road bike.

FWIW I was quite impressed with the 500w version, with all you hear about "overstated claims etc."
Furthermore I think that is enough motor, especially with a mid drive using the gears, efficiency over a wide speed range etc. The hill I mentioned is an average of 7%, and some of it steeper. There are not many actual roads that are steeper, on the tour de France I believe the max. is about 10% that's in real mountains. Keep in mind I just got it, and only about 10km on it because of the cold here now, but it does real hills, and the legal speed with ease. I'm pretty happy with it, just looking forward to better temps. so I can get to play with it more.

I think if a person had a 1000w motor you would probably use it more than you should, or need, and just end up shortening the distance your battery "could" go. Also just came to mind is "if" the 500w is enough then the 1000w is just going to be harder on the internal nylon gear, and your bikes parts ? something to think about.

 

[dexey]

Has anybody got any idea how the Bafang mid drive 250W 36v BBSO1B performs, please.
If the 1000W motor is 160nm is the 250W a quarter of that?