Explanation of torque

[Ambodach]

Can someone tell me - because the bike shop can't as yet - how one e-bike can have a power unit which is 250W and gives out 40Nm of torque and yet the next door bike, albeit a bit more expensive, is also 250W but has a torque rating of 80Nm. These are both Bosch powered machines.

I'm was an electronics engineeer for 45 years and know enough about engineering to know what torque is and also enough about electric motors to know what 250W means, but I am puzzled as to what element of technology allows a doubling of an output parameter for seemingly no increase in input power.

And again because I'm not a 'oily' mechanical engineer, what impact on cycling does this extra torque give ?
Thanks
Rob

 

[trex]

the 250W is the nominal power, only for the label. In physical terms, what determines the torque is how much current goes through the controller. If you have a 36V 15A controller, your crank drive gives about 50NM, 60NM on 18A controller, 70NM on 20A. I said 'about' because the highest torque is produced when the motor is stalled, no use in practice. You have to run the motor at some speed. I usually take 4mph as the minimum speed. The torque is further reduced through the gear ratio. So, a 70NM torque on 44T front, 34T rear becomes 54NM (when you are on gear 1 of a Shimano Megarange 14T-34T), roughly equivalent to a rear BPM on 700C wheels with a 20A controller.

 

[PH001]

The 250W is typically a figure that is completely derived from an acceleration test and bares little relation to the electrical input power.

Read all about it here....

http://www.pedelecs.co.uk/forum/threads/bit-confused-about-the-bosch-drive-unit-performance-cx.24405/

 

[Ambodach]

The 250W is typically a figure that is completely derived from an acceleration test and bares little relation to the electrical input power.

Read all about it here....

http://www.pedelecs.co.uk/forum/threads/bit-confused-about-the-bosch-drive-unit-performance-cx.24405/

Many thanks for that. I did go back over 10 pages of postings doing a CTRL F search for 'torque' and didn't find anything; that was of course in the post headers and not in the text. I couldn't find a forum search facility as that would have picked this thread up.

That solves my puzzlement. I'll pass the link onto the LBS who should have done some homework on this topic as they were inevitably going to get someone coming in with the query about 250W power limit but a 2 to 1 ratio on torque output.

 

[PH001]

No problem. You would think that both the 40 and 80nm motors are as close to the limit as possible when it comes to passing the acceleration test for approval. The difference must be in the way the torque curves are programmed vs rpm. I would imagine the 80nm motor feels much stronger off the line.