After the decision was made to use a common servo mounting bracket for both driving and steering Sawppy the Rover, the obvious next question is: why not use a common shaft design as well?
Consolidating to a common shaft design reduces the number of unique parts to keep track of, and has the potential to make fabrication easier. This reasoning made sense for servo brackets, who benefits from this commonality with no downsides.
Sadly, in the case of these steel shafts, there are functional downsides.
The servo side is not an issue. Since all servos use the same mounting bracket and coupler, the dimensions are obviously identical on that front.
The output side aren’t common and there are some minor reasons why. For the steering shaft, output section length matches the thickness of the steering knuckle. The driving shaft’s output section is longer, because it has to fit into the wheel hub as well as extending into the wheel itself for structural support. An experiment with a shortened shaft showed this length is an important guard against wheel torsional flex so we shouldn’t shorten it. Therefore a common shaft demands that the steering knuckle needs to be thicker where it meets the shaft. If we keep the existing mating location and extend shaft downwards, it reduces wheel clearance. If we move the mating location upwards, it increases the mechanical leverage of wheel load acting on the steering joint making its job harder. Both of these options are undesirable.
But the deal-breaker is the section in the middle – where the shaft is supported by a pair of bearings. The drive shaft has a short section in order to make room for the servo to be completely enclosed within the width of the wheel. Lengthening this section is not an option.
Neither is shortening the bearing support center section of the steering shaft. Forces coming in from wheel motion has great mechanical leverage on this joint and we want the bearings spaced apart to deal with this incoming torsional stress. There’s a chance the close spacing of the wheel shaft bearings might actually be sufficient to bear this stress. It might be worth exploring in a later version of the rover, but for the first edition we want to build it strong. Which means keeping shafts designs different with the steering shaft longer than the driving shaft.