[Ryan_Todd]

Looks pretty solid!

1. Is that a 1.5" or 1" stroke on the piston? My gut instinct would be to balance the stroke so that the traction wheel contacts the ground at about 1/3 extension in the case of a flat game field, but this would of course need to be adjusted in case of ramps, etc.
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2. I agree that 1/4" thickness on the piston mounting plate (assuming aluminum?) is pretty much necessary the way you've mounted it, but consider what would happen if you switched the mounting plate to the inside of the chassis rail. This would of course require the mounting plate to bend maybe 10-15 degrees outwards to give enough clearance for the piston, but would place it roughly parallel with the piston's stroke; the result would therefore reduce the bending moment on the mounting plate, and let you switch to a thinner material or carve more weight out of it.
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3. Neat concept for attaching the piston bracket to the chassis; no welding required! Not sure how much benefit you're getting out of the bottom plate, though, considering how well the rest of it is already reinforced. Perhaps run some FEA, if you've got the capability?
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4. [EDIT] Just noticed that the bolt that you're using to transfer force between the piston and the module is rather small (1/4"?) for the distance that it's bridging. Even a grade 8 steel bolt might start to bend during a season under FRC conditions; at a minimum, I would reinforce that with aluminum or brass tubes as spacers (yes, that makes them structural; no clearance here, just tighten the heck out of them).
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5. [EDIT 2] The module side plates aught to be able to handle everything passed to them by the above bolts, but you might also consider filling in the far-left and upper-right corners (or simply increasing the corresponding fillet radii) of that far-left cutout to more directly transfer the force from the piston bolts to the traction wheels. I'd definitely do the far-left corner, at least, just to be safe.
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6. [EDIT 3] Plz also post the same set of images with the piston extended; I am trying to decide whether getting rammed from the side in traction mode could damage any part of this assembly. My guess is that the traction end of the module would flex and rest against the chassis rail or the inside edge of the bumpers before anything started permanently deforming, but having pics would help to confirm. I also wonder about retracting to mecanum mode while being pushed sideways; the open bolt heads and/or nuts may catch on the bottom of the chassis rail or bumper, and prevent the piston from completing its stroke until the opponent lets off.