Torque

[Charliefox]

Can anyone tell me why Bosch and Yamaha centre motors quote torque figers of 75-80Nm for 250 watt motors and the average 250-350W rear hub is only around 45Nm. Where is the torque measured from in centre motors? At the engine sproket or at the back wheel with the benefit of the gears. Is there more gearing in centre motors viz hub ones? I notice that range using same size batteries is not so different.

 

[Deleted member 4366]

It's not measured, neither is it actual torque. It's a derived number based on a theoretical 1:1 gearing between the crank and the back wheel, which would be around 4 mph at a crank-speed of about 60 rpm, which is the sort of pedal speed you'd be at powering up a steep hill, but I would think peak torque would come at a lower speed than that - probably about 3 mph. Obviously, people wouldn't normally be using gearing that low.

Torque varies significantly with motor speed, so it's pretty useless giving a figure for torque without saying at what speed it applies. Maximum torque comes when the motor is stopped, which is even more meaningless.

 

[Woosh]

Can anyone tell me why Bosch and Yamaha centre motors quote torque figers of 75-80Nm for 250 watt motors and the average 250-350W rear hub is only around 45Nm. Where is the torque measured from in centre motors? At the engine sproket or at the back wheel with the benefit of the gears. Is there more gearing in centre motors viz hub ones? I notice that range using same size batteries is not so different.

To approximate the torque at the rear wheel, let's say you have a 16T front sprocket on your Bosch CX. The internal scaling factor is 2.5, that front sprocket is the same as a 16T * 2.5 = 40T chain ring on a normal bike.

with a CD motor, you can select a different sprocket at the rear, smaller sprocket gets less torque from the motor. The maximum motor torque at the rear wheel is thus:

@ 11T = 75NM * 11T/40T = 20.6NM
@13T: 24.4NM
@15T: 28NM
@18T: 33NM (match Bafang SWXH)
@ 21T: 39NM
@24T: 45NM (match Bafang SWX02)
@28T: 52NM (match Bafang BPM)
@32T: 60NM

For example, if you ride a CX up a long and steady hill against a similar bike with a BPM motor ridden by a similarly fit pedelecer, the gradient needs to be steep enough for you to stay on 28T or higher to win.