Observe these multi-axled vehicles:
See how their wheels are either "innie" (stick inwards) or "outies" (prodtrude outwards).
Why is this?
I note that in 3-axle (6-wheel) vehicles in particular, such as the third picture below, it's often the 2nd (middle) axle that has hubs distinct from the very front and very rear wheels.
I wondered if "innie" wheels are cheaper to make and less sturdy, so they're only used for non-driving wheels, while "outie" wheels are stronger and used on the driving wheels?
... but then in the 2nd photo below the bus has 3 "outie" axles and only 1 "innie" axle - so is it really a six-wheel-drive vehicle?