Well, that was what broke the camel's back. With the two standard AAA batteries that I used as a battery back while building and testing it, it weighed 12.3 oz, and ran perfectly, really sweet. But they were not rechargeable and two AAAs in series for 3V would only power it for about 15 minutes. I had always intended a bigger battery back and installed a pair of Li-Ion 18500 high capacity batteries in parallel to give it four times the energy storage - an hour of running before recharge. That, and an internal safety fuse ( I worry about shorting a Li-Ion battery this big) and it added about 3 ozs, and the steering would not function well. The added weight was only an additional 25% , but it changed the whole character, and not just the steergin. It ran like a pig: the motor/gearbox was struggling: it was running at 3.7V on the Li-Ion batteries vs 3 V with the AAAs and I expected it to be faster, but it wasn't - just a lot noisier (not a good sign).Seems like the favorites have been around for a while.
Bummer about the bus, it's interesting that a little weight threw the whole thing off. Was it friction in the steering that causes the issues?