I put an extra heavy capacity rheostat in my e12 m (with an e-20
driveline). I also put a latching relay circuit in. Every time I want
field weakening I push a button and adjust the rheostat to the speed I
want with my eye on the power meter. Every time I put the control in off
or reverse in un latches the relay. This way I don't forget end leave it
on. Before I put a Curtis speed control I did not allow weakening until
the voltage to the armature was close to 36 volts. I have used this set
up for about 10 years.
It works well for finding a good speed at the right power level and using
regen going down hill. You don't get a free lunch however. At reduced
field you get less torque and the max starting torque is very reduced.