This might be a stupid question but… I know that wide bases turn much better than long bases, how does the amount of wheels affect turning? For example,what is the difference between a 6w tank and a 8w tank?
The logic is the same. Imagine an 8WD like 3 wide 4WD wheelbases put together. Because the center to center distance between each set of wheels is smaller (for two drivetrains of equal length), the 8WD turns better than the 6WD. Things like center of gravity and wheel type affect this, but that’s the gist of it.
Edit: This assumes a center dropped drivetrain, see EricH’s post below for more detail
That depends a lot on the wheels. I would even go so far as to say that the tread choice would have more variation. But…
It also depends on the drivetrain. Confining ourselves to tank drivetrains, the single biggest factor is going to be the ratio of trackwidth (the distance between the left and right wheels) to wheelbase (distance between front and back wheels that are in contact with the floor). Bigger is better (unless your robot is falling over due to its own motion) for this ratio, as it relates to turning (but there are other design considerations as well, which I won’t go into here as they’re game-dependent).
A 6WD flat drivetrain will probably not turn as well as an 8WD flat drivetrain, but for different reasons than a 6WD drop vs. and 8WD drop. In this specific case, the 8WD has a slightly lower frictional force from the far corner wheels (less normal force on those wheels) with which to resist turning. A 4WD tank will be a nuisance to turn smoothly.
But when a dropped center wheel is introduced in a 6WD, the effective wheelbase is shortened. An 8WD has two dropped center wheels on each side, which will further shorten the wheelbase (and tends to remove the rocking motion of a 6WD). The shorter wheelbase improves turning, assuming the trackwidth stays the same.
Unless a robot is really long, 8 wheels are not required for it to turn properly. 8 or more wheels are most commonly used to prevent a high-center when climbing an obstruction or less commonly when trying to stabilize the pitching motion of shifting from the front to rear wheel pair. This is more important if you have a long arm or are throwing into a small goal.