Now that I've taken bike.5 out caching, I don't think I can use it. With 8s (29.6 volts nominal, 33.6 newly charged) the motor overheated. So for the second circuit, I used 6s (22.2 volts nominal, 25.2 when newly charged). And even that led to overheating. And the overheating was such that the motor was too hot to comfortably touch. The main symptom is a grinding noise and a reluctance to run. The problem seems to only be there while the motor is hot, so I don't think I've wrecked anything. Still, not suitable for caching.
So I thought again, and I've decided to use bike.2, which is a Haro 26 inch folder just like bike.1. I've added a voltmeter, made the wiring neat and tidy with plastic curly-wurly. Tomorrow I'll add a second torque arm to the motor wheel, and I'll make a metal bracket to protect where the wires enter the wheel against being knocked. I'm also going to install a thermometer with a readout on the handlebars, so that I can see the motor temperature as I'm going along. I'm hoping to get this finished tomorrow (the field of rape has left me too worn out to go out tomorrow) and tested.