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Observe these multi-axled vehicles:

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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?

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There are two basic reasons for this:

  1. In the back where you see the dual tires, these are the drive axles. The one with the singe wheel is not. The extra tire on the driven axle helps provides traction where it is needed.
  2. The axle with one tire in the back is there to support the extra load, but is also there to provide more comfort. With the second axle there, the roughness of uneven road can be taken up by both axles and thus distribute the bumps making them less jarring on the rest of the vehicle. The same can be said for the dual axles up front. The secondary axle is there to distribute the load and rough road.

Only the axles with the dual wheels are driven. The rest are just axles supporting the load and/or turning wheels.

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  • Good point about smoothing the ride.
    – Mobius
    Mar 21, 2017 at 0:26
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The "outie" axles in the rear are only single tire, where as the "innie" rear axles are dually. I suspect it is simply based on weight capacity. It is to much weight for just a single dually axle, but not enough to warrant the cost of 2 dually axles.

As for the front, I don't know of any vehicle that has multiple tires on each side of an axle, so if the weight is to much for one axle, the add another.

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  • Could it have something to do with whether or not there's brakes behind those wheels, too?
    – atraudes
    Mar 21, 2017 at 0:04
  • I would assume every axle in the pics has brakes. Edit. The dual steering setup in the middle pic might not have to have brakes on one of the steering axles
    – Mobius
    Mar 21, 2017 at 0:15

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