The base of the SuperRobot has been mostly completed but I thought it would be better if each of the two driving DC motors had two wheels on rather than just the one as at present. This would then eliminate the 'bending' of the motors that happens at present as the wheel is offset from the line of support of the motor. By adding two wheels the pressure is evenly distributed, plus there is a greater surface area giving a better grip and less pressure on each tyre.
After looking on the internet there seemed plenty of the same yellow wheels available to I ordered some. Unfortunately it seems that not all yellow wheels are equal and the ones that arrived are slightly different from the ones I already have. They have a slightly different diameter (67 mm compared with the 65 mm of the original wheels I have) plus they have a different profile. My existing wheels are flat with a foam filling, whereas the new ones are curved with no filling, so are a bit squashy.
Two millimetres difference in diameter doesn't sound a great deal but it may well unbalance the motors. I will have to see whether two identical wheels on each motor works best, or a mix of wheels on each motor. I could of course just order two more of the same wheels and ten I would have four identical wheels.