Mr Ted

Brooks Flyer saddle, works really well! Brooks Flyer

Here's the geeky answer based on some assumptions! I can't find the exact figure but the maximum torque of your drive is probably around 40Nm, in order for the motor to produce 250W of useful power the cranks would need to be turning at 60rpm . Below this speed the controller probably operates in constant current mode to hold the torque at the same level, if it didn't do this then the torque would rise as the speed dropped which would make for rather brisk starts. Using our meat motor we tend to cycle at constant (ish) torque and use the gears to maintain a constant power output. Depending on the characteristics of the motor/gearbox the peak power will start to drop at some crank speed above 60rpm. So you need to look at your gearing based around having maximum assist at cadences of 60 and above, without seeing actual measurements we can only guess at what cadence the power output drops. (The Bosch CX produces maximum power at 85rpm and is about 10% down at 70rpm and 100rpm) If the maximum torque of 40Nm is held constant below 60rpm then at half the cadence you will get just half the power, 30rpm=125W Hope this helps!!