What is the maximum input torque on Sturmey-Archer internally geared hubs?

A pro cyclist putting out 1.4 kWatt at 60rpm at the starting sprint has a torque of almost 2000 lb-in or 222 N-m.

A recreational bicyclist may be normally only be capable of just 1/10th that but even an out of shape Clydesdale (heavy but strong rider) just getting off to a start on a hill may momentarily hit that level. If they weighed 200kg but jumped on the pedal and momentarily put 800 newtons of force on a 170mm crank, that’s 136 n-m, or still a lot of torque.

If I was an engineer, I’d want to make sure that my components could handle that level peak on an occasional basis. If you’re really worried, get a Rohloff as they’re really well designed.

I suggest you study what St. Sheldon had to say about Sturmey AW 3 speeds failing in top gear under heavy load.

I recall the old Sturmey company used to give a maximum rear cog / minimum front ring ratio limit, possibly a 24T? rear while assuming a 40T front as was common on roadsters in England. I don’t think they disclosed the assumed rider weight or crank length (IIRC a 6-1/2” or 165 mm crank was typical on roadsters). You’ll have to dig deep into Sturmey or Raleigh history to find this.