This isn't something I'm working on at the moment, but a thought occurred to me of an encounter against foes piloting an apparatus of the crab.
Let's assume the pilots have no real combat ability themselves (so we'll assume 1st level commoners for now. Putting a spellcaster or two inside the apparatus could presumably boost the encounter's DC, but we'll get to that later). What's the DC of an encounter against them while they're piloting the apparatus though? (I'm going based on the assumption that very little is capable of affecting the pilots while they're inside the apparatus. Maybe lightning or really intense fire/cold/acid damage, but little else.)
I'm not great yet with calculating the DC of non-standard encounters, so hopefully someone can help suggest a good DC to set this at for me.
--Edit: As an alternate idea, an animate Apparatus of the Crab. No pilots, and it can do 2 of its lever actions per round (aside from two attacks), in a sort-of move action & standard action kind of way. No pilots at all should allow for unbiased DC assignment, which isn't influenced by the class of the pilots.
(Yes, the title page of Ultimate Equipment's 5th chapter is what inspired this interest for me.)
I like this idea! I would recommend using the Animated Object entry in the Bestiary as a starting point. The problem is that the Apparatus doesn't really conform to those guidelines very well. It has far more hit points than most objects of that size (Large) and a formidable hardness, to boot. My gut tells me to go with a CR 8-10, depending upon the environment in which the encounter occurs (underwater, for instance). Its defensive abilities far outstrip its offensive capabilities, so expect a longer-than-normal fight with lots of slow, deliberate chipping away of HP. Consider giving it some novel abilities, like Trample or Grab, since it's now an honest-to-goodness creature and not just an extension of a pilot and it will break up the monotony of "I attack, you attack". I don't know what you plan to do with ability scores, but if you decide to make it intelligent/Awakened as well as animated, that opens up a whole new world of complications in terms of feats and skills.
I'm not sure if I would go with outright "Intelligent" (though if I did, Ultimate Magic's construct improvement rules would probably be a good reference point), so much as simply "capable of moving and attacking on its own"
For now I think I'm going to avoid anything that could potentially serve to increase the base CR of the crab (pilots, feats, special attacks, etc.) and try to figure out what it would be simply by itself. I can work on expanding the encounter afterwards.
...
Okay, here's what I have figured out so far, based on what's given of the apparatus of the crab's stats, and based on the Bestiary's rules for construct traits:
Size: Large
Hp 200 (31d10+30)
Str = 10
Dex = 10
Con = --
Int = --
Wis = --
Cha = --
BAB = +13 (based on its to-hit ratings it should be +13. Based on its hit-dice it should be +31. I can only assume the crab isn't following normal construct BAB progression.)
Fort = +10
Ref = +10
Will = +10
CR:
(based on hp): 14
(based on hd): 20
(based on attack damage): 3
(based on AC): 7
(based on saves): 11
Okay... So its potential CR is all over the place. The outright average of those five scores is CR 11, but I'm not sure that's the most effective method of calculation. Additionally, its hardness isn't taken into account by the bestiary's CR calculation charts.
Good point. CR 11 if used as a delaying tactic or shield in part of another encounter, probably CR... 9 or so if used on its own.
(Keep in mind though that it has low damage per-hit, but that its HP and hardness are so high that it likely gets a LOT of time to land hits before finally being defeated. Low damage can start to stack up when your opponent just won't go down.)