R6: Issues with drive system

I have head some real headaches with the drive system and the feet tipping under power. The front foot was also playing up and not lock off correctly.The solution to this was two wheeels instead of one but when I fitted the two 3" castors however I modified the foot always locked up in tight turns, Luckly my son had and old pair of roller blades whos wheels are exactly the correct size at 2 1/2". It was very simple to swap out the old 3" wheels with the new ones and now the front foot is perfect. The wheels are also much. much quieter being made of a higher quality materal.
The main feet tipping was a bit more of a problem. In the end I decided to fit second wheels into the main feet. These where free wheeling on axes spanned across the back of the foot. The wheels where the same 2 1/1" roller blade wheels as used in the front foot. The main feet where now sitting on two wheels, which instantly solved the foot tipping when moving forward. Because my main drive wheel is fairly centered on the foot when the droid moved backwards the foot tipped forwards and dragged. I considered fitting a third wheel at the front of the foot but in the end opted for wedges placed between the foot and ankle.
So far this has seemed to solve the problem and on smooth flat surfaces everything dirves very well. The only thing I am not sure about is how strong the smaller wheels in the main feet will be when driven on rougher surfaces, we will see when it stops raining.