I’ve noticed that on most, if not all, large trucks and buses, the front wheels’ hubcaps stick out, whereas the rear wheels go inward. Like the front wheels are convex, and the back are concave. I assume it has something to do with the drive wheels, but what?
The rear wheels are tandems, that is two tires side by side. The inner one by itself bulges out just like the steering tire, the outer one is reversed so the bulge faces inward.
Time for some crappy ASCII art…
Front wheels
(------)
Rear Wheels
)(----)(
And a single spare tire will fit on the front or the rear. You just have to flip it around.
Also, the front wheels have to steer, so their pivot points are the little patches of ground where they grip the road. Therefore the steering gear has to end in the centre of the wheel, directly above the tire, and the wheel structure is dished out to accomodate it.
Or so I guess.
On most coach buses [those used in intercity travel and tours - e.g. NY to Atlantic City runs], only one of the 2 rear axles has dual wheels - the other is single wheeled, and the hubs reflect this (in/out). This perplexed me (‘why don’t the rear axle hubs match?’) until I realized this fact.