After some inspiration and ideas from the likes of @TeamThrifty, @I_Igor, @nerdsforprez, @kennywest and many others, and several hundred hours sat on the living room floor, i have functioning skid unit for my 1:12 MF 6716S tractor.
The real thing: http://www.masseyferguson.co.uk/mf6700s.aspx#360
Disclaimers: it uses third party tyres - 2.2" RC4WD mud basher tractor tyres - and there's still a way to go on sorting cables, pipes, detailing etc but the functions i wanted to replicate are all... functioning
Full 4wd - no diff locks (no space!) from an XL motor which also drives the fake engine. From the rear diff the drive is split to two half-shafts on each side, driving z18s that engage inside the power miners wheels. This keeps the speed up and so torque down, and shares the torque going to the big rear wheels, as well as making a strong & balanced structure that (hopefully) can't pull itself apart under the torque.
Steering via an M motor & z24 friction clutch
"Quadlink" suspended front axle using 4 rubber figure of 8 donuts as the front mounts and two small pneumatic pistons as the springs
Working rear linkage & pto, plus working front linkage and pto
Working UK style automatic pick-up hitch - for those not 'in the know' this is standard fitment on most UK tractors and is basically a towbar that can be lowered and lifted with the rear link arms to attach trailers, and then locked into the raised position for safe towing or when not in use. The safety latch is operated from the cab.
Mounting position behind the fake engine for (future!) fitting a loader. Driven axles provide rotation to drive large LAs for the lift/lower and small LAs for the tip/crowd functions
Pneumatic system (incl tank) to provide pressure for the suspended front linkage, driving pneumatics on attachments (rear and hopefully on the loader) and for 'in-cab' operation of one of the changeover catches.
An L motor drives either the rear linkage or the loader lift/lower via a changeover catch (catch 1). The front linkage can be latched either to the rear linkage or to the loader drive via a second changeover (catch 2). This means the front linkage can either work simultaneously with the rear or, if there is no loader fitted, on its own. Putting catch 2 in neutral means the front linkage doesn't operate at all - for loader work.
An M motor drives either the rear PTO or the loader tip/crowd via changeover catch 3. The front pto can be connected, via changeover catch 4, either to the rear pto or to the loader tip/crowd. This means the front PTO can work simultaneously with the rear or on its own if no loader is fitted. Putting catch 4 in neutral means the front pto will not work at all - for loader work.
The changeover movements for catches 1 and 3 will be slaved and (hopefully) driven by a small pneumatic piston with valve lever in the cab - so this lever will control switching drive from the L & M motors between the rear linkage / pto and the loader. Movements for 2 and 4 will be manual.
The M motor also drives the pnuematic pump via a z24 friction clutch and changeover catch 5. The combination of on/off via the catch 5, friction clutch and air tank should hopefully give a full range of flexibility for keeping the air tank topped up without wearing out the friction clutch.
The plan is to use an SBrick to control everything, probably with two profiles - a 'loader' profile with joysticks to control drive & steering and the loader, and a 'field work' profile with joystick for drive & steering, sprung slider for linkage operation and latched slider for pto operation (if indeed such arrangement of sliders is possible - a future problem!) It could also be done with 2nr IR receivers and maybe a bit of mechanical jiggery-pokery to latch the PTO control on when required.
Some more pictures:
Oh, and this is the first time i've tried anything on this scale and complexity - my only previous moc experience of any note was a power harrow for the 8284 tractor... this Massey has made my head hurt!