[Joe G.]

This is what the innards of a Ball Shifter look like



You have a series of steel ball bearings which ride in holes in the shaft. The gears, which all freespin on the shaft by default just like in a dog shifter, have circular pockets cut into their bore, which line up with the holes in the shaft. A plunger with a variable diameter rides inside of this shaft. As it moves, the wide part of the plunger forces the ball bearings upward, such that they poke out of the holes and engage the pockets of the gear.

Here's a video that shows how it works: https://www.youtube.com/watch?v=D7ng4OWilUY

Ball shifters can be more compact than Dog shifters in certain circumstances, because they do not require space between gears for the dog to ride. Good dog shifter design can minimize this though, with a first stage reduction that rides in between the second stage reductions. Ball shifters are also generally the best choice for transmissions with more than two speeds, since one multi-position plunger can control any number of ratios. In my experience, Ball shifters can be less reliable, since they depend on exact positioning of the plunger, whereas a dog shifter has a hard stop at each end of the dog's travel. Properly used, the VEX ball shifter components shouldn't give you any trouble here. Also, ball shifters are somewhat more daunting to custom fabricate the components for, as there are a whole bunch of factors to consider in order to get proper engagement and proper free-spinning behavior on demand. Tolerances are critical. I'd definitely recommend playing with the VEX ball shifter to get a feel for how it works.