• The traction force equals Torque divided by wheel radius (F=T/R). 
• The more torque at the wheel gives you a higher grade-ability. 
• The smaller wheel diameter gives a higher traction force or climb-ability with same motor torque.

The torque at the wheel for mid-drive motors should be calculated like this:

• Max Torque at Wheel for crank motors = Max Motor Torque x Number of rear cog teeth / Number of Chain wheel teeth