Why are the rear wheels of buses not provided at the extreme rear, but at some distance from ends?


There are two parts of the question.
  1. Why wheels are not at extreme ends?
  2. Why rear wheels are more inside than the front wheels?
For a moment, forget the bus.
Imagine a beam 12 meters long and carrying a symmetrical uniformly distributed load, as shown in the figure below.
Now let's add some support to this beam. There are at least two cases of doing. The best one will give lesser shear forces and bending moments.
Case 1: Supports at extreme ends
The above configurations will give the following Shear forces and Bending moments.
Case 2: Supports away from extremes.
The above configurations will give the following Shear forces and Bending moments.
Okay now scroll back a bit, and check the values of both Shear forces and the moments. You will easily notice the values are lesser for the second case.
This implies the overall strength of the structure increases for the same load and the same beam. Also it implies, that for the same beam, you can load it more, in the second case. This answers the first query.
Edits: In response to one of the comments on if the uniform loading assumption true or not, this is the more accurate picture of the exact/typical loading.
2. Now depending upon the position of engine, seating arrangements, slight modification and shifting can be done, to increase the overall stability.
Rear Engine bus
Front engine bus

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