Chain-drive versus belt-and-pulley. I used belt-and-pulley mainly because the washing machine already had a pulley, and I needed to convert from rotation in a vertical plane to a horizontal plan. That conversion is easily accomplished with a belt – it’ll survive a half twist.

Grain mills versus washing machines. I’d imagine turning grain mill is quite different from turning a washing machine. When you spin a washer, getting it from a standstill to those first couple of turns is pretty tough. There’s sometimes some slippage of the tire on the training roller. But after it develops some momentum, you settle in and maintaining the rotation is not too hard. With a grain mill, you’re not going to have any momentum at all, right? It’ll constantly be maximum resistance, and it won’t ever get any easier.

So, for a grain mill, you’ll just want as tiny a ratio as you can cobble together. The strategy of tapping the rotation of the roller on a bike trainer works against you here — it’s a relatively large wheel spinning against a small cylinder, which increases your ratio dramatically. If you can figure out a way to drive the grain mill directly off a rear cog (for example, by using a flip-flop fixed gear rear wheel hub, which has one cog on each side) that would take that unnecessary multiplier out of the equation. I think you might be able to source a granny ring for the front chain ring of 16 teeth. That would be a good start to achieving a smaller gear ratio.