We calculated that we’d want about 90Nm of torque on each drive wheel to take the vehicle loaded with tools for a total estimated mass of 500lbs/220kg up a 30 degree slope.
If we don’t use hub motors we have to transmit torque to the wheel somehow, and I’ve not found a cleaner way of doing that.
For steering we have an absolute rotation sensor that can sense multiple rotations, so the wheels are allowed about 3 full rotations before we start throwing safety errors. But with the current control code they never move more than +-90 degrees. The vehicle is capable of driving completely sideways, and if you also steer while driving sideways you’d want to exceed +/- 90 degree rotation. That’s not implemented however.
Our specs are guesses until we get some tools mounted and really start doing work. But in my experience this level of torque seems warranted. One thing that’s interesting about the farm is that the ground can sometimes get very uneven. Like after it rains and everything turns to mud, then the tractor drives through and makes a huge rut in the mud, and then the sun comes out and hardens it and you have a 1 foot deep 1 foot wide rut that’s now solid. Other challenges come when acorn needs to drive over a 4” irrigation pipe up a hill. Or when you drive up a curved part of ground and one front wheel goes in the air while the opposite wheel bounces off the ground, so now you only have two wheels with traction to keep you moving and you need enough torque with just those two.
It’s not clear to me how much “off roading” will be required in practical use, so we could relax these requirements if it’s only getting used on level ground. But things are pretty uneven at our farm.