is there any simple way of estimating amps/torque at different rpm from a single operating point?
Looking at performance curves for a motor of similar size and weight the torque values at various currents are:
130A = 13 ft lbs
260A = 33 ft lbs
390A = 57 ft lbs
so, it's not a linear relationship?
No, it's not. In theory there is an exponential relationship between current and torque (becuase both field and armature currents go up).
There is a mathmatical model for a series motor, but there are quite a lot of factors and the manufacturers don't publish them. If you have graphs, then you can calculate the factors and use them to determine figures outside the graph (different voltages, currents off the graph, etc.)
There is an EV calculator that with some care can give useful results and I has a motor calculator;
http://www.geocities.com/CapeCanaveral/ ... vcalc.html